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In the journals…

February 22, 2011
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Here are some recent and soon-to-be published articles from the journals that have caught my eye this month. I’ve concentrated on articles covering objects, mechanisms and areas of astrophysics that I’ve recently blogged about:

Brüns, R., & Kroupa, P. (2011). A NEW FORMATION SCENARIO FOR THE MILKY WAY CLUSTER NGC 2419 The Astrophysical Journal, 729 (1) DOI: 10.1088/0004-637X/729/1/69

Jithesh, V., Jeena, K., Misra, R., Ravindranath, S., Dewangan, G., Ravikumar, C., & Babu, B. (2011). BLACK HOLE MASS LIMITS FOR OPTICALLY DARK X-RAY BRIGHT SOURCES IN ELLIPTICAL GALAXIES The Astrophysical Journal, 729 (1) DOI: 10.1088/0004-637X/729/1/67

Marscher, A., & Jorstad, S. (2011). THE MEGAPARSEC-SCALE X-RAY JET OF THE BL Lac OBJECT OJ287 The Astrophysical Journal, 729 (1) DOI: 10.1088/0004-637X/729/1/26

King, A., Miller, J., Cackett, E., Fabian, A., Markoff, S., Nowak, M., Rupen, M., Gültekin, K., & Reynolds, M. (2011). A DISTINCTIVE DISK-JET COUPLING IN THE SEYFERT-1 ACTIVE GALACTIC NUCLEUS NGC 4051 The Astrophysical Journal, 729 (1) DOI: 10.1088/0004-637X/729/1/19

Touhami, Y., Gies, D., & Schaefer, G. (2011). THE INFRARED CONTINUUM SIZES OF Be STAR DISKS The Astrophysical Journal, 729 (1) DOI: 10.1088/0004-637X/729/1/17

Ma, J., Wang, S., Wu, Z., Fan, Z., Yang, Y., Zhang, T., Wu, J., Zhou, X., Jiang, Z., & Chen, J. (2011). AGE AND MASS CONSTRAINTS FOR A YOUNG MASSIVE CLUSTER IN M31 BASED ON SPECTRAL ENERGY DISTRIBUTION FITTING The Astronomical Journal, 141 (3) DOI: 10.1088/0004-6256/141/3/86

Preite Martinez, A. (2011). The Cygnus Loop: a weak core-collapse SN in our Galaxy Astronomy & Astrophysics, 527 DOI: 10.1051/0004-6361/201015213

Zabalza, V., Paredes, J., & Bosch-Ramon, V. (2010). On the origin of correlated X-ray/VHE emission from LSI+61303 Astronomy & Astrophysics DOI: 10.1051/0004-6361/201015373

Astrophysics 102: Extragalactic Globular Clusters

February 19, 2011

Christine over at Cosmic Rays has a very interesting post on new research into the globular cluster system of one of our nearest neighbouring galaxies, M31, the Andromeda Galaxy:

M31 (Image: Tony Hallas)

In our own Milky Way, globular clusters are found in a halo surrounding our galaxy:

Globular Clusters in our Galaxy (Image: Atlas of the Universe)

And unsurprisingly, as Catherine reports, the same situation exists in other galaxies, although the numbers of globular clusters per galaxy varies widely (and appears to be a function of the physical laws of the universe, galaxy mass and galactic evolution).

M13, a galactic Globular Cluster (Image: Yuugi Kitahara)

Leaving aside their obvious beauty and their impressive physical properties, unlike a lot of the objects I’ve been blogging about, globular clusters can be seen in instruments as small as binoculars, and even in small telescopes they put on a phenomenal show. Furthermore, some of M31’s globular clusters can also (just) be viewed in amateur telescopes.

But did you know that some of our own galaxy’s globular clusters are actually invaders from another galaxy? Studies of cluster kinematics & metallicity have found that the galactic globular cluster M54 actually belongs to the Sagittarius dwarf galaxy:

The Sagittarius dwarf galaxy (Image: NASA)

The Sagittarius dwarf galaxy is one of the closest (and faintest – it wasn’t discovered until 1994!) satellite galaxies of our own, and M54 is thought to lie in the nucleus of said galaxy; there are also 3 other “galactic” globular clusters associated with the Sagittarius dwarf galaxy(Layden & Sarajedini 1997, ADS/arXiv); indeed it has also been suggested (e.g. Caretta et al. 2010, ADS/arXiv) that certain other galactic globulars are the “stripped” remains of former galaxies devoured by the Milky Way in the past, although the jury is still out on their claims, to say the least.

But in the meantime, in a few months time (if you’re in the Northern Hemisphere), if you have binoculars or a small telescope, and a good southern horizon, take a look out for M54, sitting just inside the handle of the “teapot” of Sagittarius and remember that the stars therein originated in another galaxy altogether.

The location of M54 within Sagittarius (Image: Torsten Bronger)

References:

Layden, A., & Sarajedini, A. (1997). The Globular Cluster M54 and the Star Formation History of the Sagittarius Dwarf Galaxy The Astrophysical Journal, 486 (2) DOI: 10.1086/310848

Carretta, E., Bragaglia, A., Gratton, R., Lucatello, S., Bellazzini, M., Catanzaro, G., Leone, F., Momany, Y., Piotto, G., & D’Orazi, V. (2010). Detailed abundances of a large sample of giant stars in M 54 and in the Sagittarius nucleus Astronomy and Astrophysics, 520 DOI: 10.1051/0004-6361/201014924

When a standard candle flickers: What happened when the Crab Nebula had a fit?

February 18, 2011

This post was chosen as an Editor's Selection for ResearchBlogging.org

I’ve previously blogged about extreme particle acceleration producing gamma-rays in many different astrophysical contexts, including galactic binary systems & blazars, but I haven’t talked in any great depth about another source of extremely high energy particles: supernova remnants.

The Crab Nebula: a typical supernova remnant (Image: NASA/STScI)

A supernova remnant is the remains of a supernova, and as well as the aforementioned types of astrophysical objects, they are one of the chief sources of gamma-rays in our galaxy, as can be seen from the following image by our old friend Fermi/LAT:

The Gamma-ray Sky (image: NASA, DOE, Fermi LAT Collaboration)

On the extreme right of this skymap we can see the gamma-ray output from the Crab Nebula, or Messier 1 (M1), the remains of a supernova that exploded in 1054 CE. Driving the nebula (by injecting energetic electrons into the surrounding environment) is a central pulsar, one of the first of these types of objects discovered, and an extremely important class of astrophysical object.

In the specific context of gamma-ray astronomy, the Crab Nebula is even more important: for decades since its discovery, its output at gamma-ray energies has always been, absent a previous flaring event in 2009 (Abdo et al. 2011, ADS/ArXiv), remarkably constant, to the point where, as I previously noted, it has been used as a calibration unit for gamma-ray fluxes from other objects in the sky.

However, as a new letter to the Editor of the Astronomy & Astrophysics journal (from a group of Swiss/German astronomers led by Matteo Balbo of the University of Geneva) (A&A 527, L4 (2011)ADS/arXiv) to be published in the March edition notes, on the 18th September last year, the previously stable Crab Nebula erupted in three massive gamma-ray flares, before returning to normal over the next few weeks:

Observations of the light curve of the Crab Nebula in the range 0.1 to 300 GeV showing the three observed flares. Each horizontal unit represents a 12h time bin. The red lines represent the average flux before and during the flares (image: Balbo et al. 2011).

Observing with a number of instruments, including Fermi/LAT, Balbo et al. report that each flare involved a sudden increase in the flux from the Crab Nebula by a factor of ~3.3 followed by a decay back to previous levels of emission over the course of approximately a day. During this time, they also report that the pulsar flux was constant; i.e. no change in the emission from the pulsar was detected. This discontinuity between the flaring and the pulsar luminosity has important consequences, and I’ll come back to these later in the post.

Don’t panic, its only a flare!

These flares caused quite a kerfuffle in the high-energy astrophysical community, and it was initially thought that perhaps a previously unknown Active Galactic nuclei positionally coincident with the Crab Nebula (i.e. that happened to lie in the same area of the sky) might have been the cause of the increased emission, and not the Crab Nebula itself. However, no changes in the X-ray flux from the area of the sky including the Crab Nebula was observed, ruling out this scenario (AGN are typically very bright in X-rays).

So, on the reasonable assumption that these flares came from the Crab Nebula, Balbo et al. consider the two obvious questions that present themselves: what are the physical mechanisms responsible for the flares, and given the comparatively large size (~10 light years across) of the Crab Nebula as a whole, where exactly did they originate from?

It turns out that Balbo et al. find that these two questions can be answered without having to introduce new physics or making too many changes to what we know about the Crab Nebula. Since we know that whatever their cause, the pulsar itself wasn’t directly responsible (due to the pulsar flux remaining constant), we can use the observed properties of the flares themselves to constrain the size and location of the emitting region using standard astrophysical techniques:

  • The length of the flares themselves limit the size of the region responsible for the emission (this is a standard geometric argument often used in AGN research: the size of an emitting region equals the distance light can travel during the shortest variation of its brightness, for example: a one-day variation means a size of one light-day), and Balbo et al. hence report that the size of the flare-emitting region is approximately < 1015 cm (which is approximately 0.001 ly).
  • The flare luminosity can be used to compute upper limits on the location of the emitting region, by comparing its luminosity to a calculated property known as the pulsar spin-down luminosity (which is derived from, amongst other things, the pulsar inertia & spin period) and using another calculated property known as the apparent efficiency for VHE gamma-ray production (i.e. a measure of how efficient the pulsar is at converting spin-down energy into observed gamma-rays) for this object. Balbo et al. thus constrain the position of the emitting region to very close (< 6 × 1016 cm) to the pulsar at the centre of the nebula.

This limitation on the position of the emitting region is important: as at the centre of the nebula, surrounding the pulsar, is a jet and torus of high-energy X-ray emitting particles approximately 0.1 ly across (and hence many times the size of both the emitting region and its distance from the central pulsar):

The Crab Nebula Pulsar Jet and Torus as imaged by CHANDRA (Image: NASA)

The emitting region is small enough that an increase in the energy of the particles in a particular part of the torus could be responsible for the emission, or interactions between the jet and the torus, or even changes in the jet itself: that the torus was somehow involved is given additional weight by subsequent observations (on the 2nd October), of the torus having increased in luminosity.

However, Balbo et al. discount this latter scenario (i.e. of the flares arising from the jet itself) as unlikely as since the jet in the Crab Nebula is likely to be relativistic, the emission would be highly-beamed and given the inclination of the jet, it would make the flares very difficult to detect.

Next, they consider the physical mechanisms responsible for the flares. As noted by Abdo et al. (2011), there are two main physical processes that are considered responsible for the flux from the Crab Nebula: synchrotron emission (where charged particles accelerate in a magnetic field) and Inverse Comptonisation (IC) (which I’ve talked about in detail in this post, but in summary involves collisions of low energy photons with relativistic electrons). However, if the flare was due to the IC component, then the duration of the flare would be much longer. As Abdo et al. note, in relation to the 2009 flare that lasted approximately 4 days:

The extrapolation of the the LAT spectrum of low-energy component to lower frequencies suggest that it represents synchrotron emission… The brevity of the gamma-ray flares strengthens this scenario: If the flare were instead produced by IC radiation or Bremsstrahlung, the cooling time of the emitting electrons would greatly exceed the flare duration. The cooling via Bremsstrahlung in particle densities <10 cm−3 happens over ∼106 years. Similarly, electrons cooling via IC emission of 100 MeV gamma rays on the photons of the synchrotron component of the Crab Nebula have cooling times ~ 107 years. The average magnetic field inside the Crab Nebula is estimated to be ~200 G as deduced from modeling of the broad-band SED, and might be enhanced locally by up to an order of magnitude in the inner nebula. These fields imply synchrotron cooling times <15 days, comparable to the flare duration, leaving synchrotron radiation as the only plausible process responsible for the gamma-ray emission during the flares.

This conclusion is also reached by Balbo et al., who also note that in the case of the September 2010 flares, this mechanism requires electrons to be accelerated to extreme energies in the PeV (Peta-Electron Volt) range (1015 eV), i.e. orders of magnitude above the energies reached in blazars and microquasars.

As a measure of how much energy this actually is, and in terms of units of energy we’re more familiar with in our everyday lives, 1 PeV is equal to ~1.6× 10-4 Joules. Now imagine a golf ball with a mass of ~46 grams.  Using standard formula for kinetic energy, a golf ball moving at a speed of 0.08 m/s (3 inches/second), or ~300 m/hr, has the same (kinetic) energy as the electron energies that the Crab Nebula is accelerating to. Doesn’t seem scary, except a golf ball is 1028 times as massive as an electron!

If this truely is the case, then this makes the Crab Nebula a natural Pevatron, and an astrophysical wonder.

References:

Balbo, M., Walter, R., Ferrigno, C., & Bordas, P. (2011). Twelve-hour spikes from the Crab Pevatron Astronomy & Astrophysics, 527 DOI: 10.1051/0004-6361/201015980

Abdo, A., Ackermann, M., Ajello, M., Allafort, A., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Bechtol, K., Bellazzini, R., Berenji, B., Blandford, R., Bloom, E., Bonamente, E., Borgland, A., Bouvier, A., Brandt, T., Bregeon, J., Brez, A., Brigida, M., Bruel, P., Buehler, R., Buson, S., Caliandro, G., Cameron, R., Cannon, A., Caraveo, P., Casandjian, J., Celik, O., Charles, E., Chekhtman, A., Cheung, C., Chiang, J., Ciprini, S., Claus, R., Cohen-Tanugi, J., Costamante, L., Cutini, S., D’Ammando, F., Dermer, C., de Angelis, A., de Luca, A., de Palma, F., Digel, S., do Couto e Silva, E., Drell, P., Drlica-Wagner, A., Dubois, R., Dumora, D., Favuzzi, C., Fegan, S., Ferrara, E., Focke, W., Fortin, P., Frailis, M., Fukazawa, Y., Funk, S., Fusco, P., Gargano, F., Gasparrini, D., Gehrels, N., Germani, S., Giglietto, N., Giordano, F., Giroletti, M., Glanzman, T., Godfrey, G., Grenier, I., Grondin, M., Grove, J., Guiriec, S., Hadasch, D., Hanabata, Y., Harding, A., Hayashi, K., Hayashida, M., Hays, E., Horan, D., Itoh, R., Johannesson, G., Johnson, A., Johnson, T., Khangulyan, D., Kamae, T., Katagiri, H., Kataoka, J., Kerr, M., Knodlseder, J., Kuss, M., Lande, J., Latronico, L., Lee, S., Lemoine-Goumard, M., Longo, F., Loparco, F., Lubrano, P., Madejski, G., Makeev, A., Marelli, M., Mazziotta, M., McEnery, J., Michelson, P., Mitthumsiri, W., Mizuno, T., Moiseev, A., Monte, C., Monzani, M., Morselli, A., Moskalenko, I., Murgia, S., Nakamori, T., Naumann-Godo, M., Nolan, P., Norris, J., Nuss, E., Ohsugi, T., Okumura, A., Omodei, N., Ormes, J., Ozaki, M., Paneque, D., Parent, D., Pelassa, V., Pepe, M., Pesce-Rollins, M., Pierbattista, M., Piron, F., Porter, T., Raino, S., Rando, R., Ray, P., Razzano, M., Reimer, A., Reimer, O., Reposeur, T., Ritz, S., Romani, R., Sadrozinski, H., Sanchez, D., Parkinson, P., Scargle, J., Schalk, T., Sgro, C., Siskind, E., Smith, P., Spandre, G., Spinelli, P., Strickman, M., Suson, D., Takahashi, H., Takahashi, T., Tanaka, T., Thayer, J., Thompson, D., Tibaldo, L., Torres, D., Tosti, G., Tramacere, A., Troja, E., Uchiyama, Y., Vandenbroucke, J., Vasileiou, V., Vianello, G., Vitale, V., Wang, P., Wood, K., Yang, Z., & Ziegler, M. (2011). Gamma-Ray Flares from the Crab Nebula Science, 331 (6018), 739-742 DOI: 10.1126/science.1199705

Why you should be frightened of Jupiter

February 16, 2011
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(from http://www.bradgoodspeed.com).

Never trust an unstable Asymptotic Giant Branch Star…

January 15, 2011
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From the ever reliable xkcd:

Eta Carinae: Nature’s own Large Hadron Collider

January 11, 2011

ResearchBlogging.orgTo say that Eta Carinae is one of the most remarkable and marvellous stars in the sky is probably an understatement of hyperbolic proportions. It is one of the most fascinating objects in the universe. Not only is it one of the most massive stars in the Universe (weighing approximately 100 solar masses), it is also amongst the most luminous known (with a luminosity some five million times that of Sol), and is extremely unstable and dynamic, undergoing periodic nova-events (including the famous outburst of 1843 where it became briefly the second-brightest star in the sky after Sirius).

Eta Carinae (Image: NASA)

For a comparatively long time now we’ve known that Eta Carinae sheds vast quantities of material via its extremely strong stellar winds -the 1843 event is calculated to have involved the loss of 10 solar masses of material (Smith et al. 2003, ADS), and its ongoing mass loss is perhaps as much as 10-3 solar masses annually (van Boekel et al. 2003, ADS/arXiv) – by contrast, Sol (thankfully!) loses only ~1/1014 of its mass annually via its own stellar winds. The mass loss from the 1843 event created the nebula around Eta Carinae now known as the Homunculus:

The Homunculus Nebula & Eta Carinae (Image: NASA)

As this material streams away from Eta Carine it (de)accelerates, and hence produces synchrotron radiation (De Becker 2007, ADS/arXiv), which has been detected at radio wavelengths. But as we’ve already seen, in other exotic systems such as Pulsar Wind Binaries, even higher-energy emission is possible where particles in the stellar wind collide with ambient particles in the local Interstellar Medium producing energies up to gamma-ray range (> ~100 GeV), and such emissions have indeed been detected recently from Eta Carinae (Leyder et al. 2010, ADS/arXiv), which suggests that Eta Carinae could also be binary system. The binarity of Eta Carinae is very important, as we will see later in this post.

But this marvellous star is the site of something even more stranger & energetic, according to a new report (A&A 526, A57 (2011)ADS/arXiv) to be published in the February issue of Astronomy & Astrophysics and authored by a small group of Swiss & Belgian Astronomers led by Christian Farnier of the Data Centre for Astrophysics at the University of Geneva.

They report, using long-term observations of Eta Carinae with our old friend Fermi/LAT, evidence of two separate mechanisms for the production of the observed gamma-rays.

Spectral energy distribution of Eta Carinae as measured and modelled by Farnier et al. (2010). From low to high energies are shown the synchrotron, stellar emission, inverse Compton and neutral pion  decay spectral components (Image: Farnier et al. 2010)

As well as the ‘standard’ mechanism for production of the gamma-rays by inverse Compton scattering of stellar photons by extremely relativistic electrons, the authors found evidence for an additional and different mechanism for production of gamma-rays; a process called neutral pion decay. Pions are subatomic particles with one quark and one antiquark and are the lightest type of mesons. They come in two forms: charged (π+, π) and neutral (π0). Neutral pions are typically produced by interactions between accelerated protons (hadrons). They then decay extremely quickly (they have a lifetime typically of only 9 X 10-17 seconds) into gamma-rays, producing a small amount of electrons and positrons, and possibly neutrinos (although unfortunately our current instrumentation is not yet sensitive enough to detect these neutrinos at the Earth’s surface).

Photodisintegration (Image: Swinburne Astronomy Online)

So what in the Eta Carinae system could be responsible for the production and acceleration of such protons? We know that the stellar winds of massive stars such as Eta Carinae contain atomic nuclei ranging from helium up to oxygen. These hadrons are then photodisintegrated into neutrons and protons (Bednarek 2005, ADS/arXiv). Farnier et al. suggest that a process called diffusive shock acceleration, or Fermi acceleration could be responsible for the acceleration of these protons to speeds where collisions could produce neutral pions. This process would occur where the stellar wind collided with a previous disturbance (or “shock”).

Is this a realistic scenario? There are a number of relevant factors that the authors evaluate which could impinge upon the probability of gamma-rays being produced in the above way:

  1. Proton Density: The observed gamma-ray flux requires a certain density of protons as a prerequisite . Unfortunately, the most obvious candidate region for this, the interface between the Homunculus nebula and the stellar wind from the primary star in the system does not meet this threshold. However, the authors calculate that a region in the centre of the system where the stellar winds from both stars collide with each other producing a hydro-dynamical shock would contain enough protons for diffusive shock acceleration to theoretically occur.
  2. Magnetic Field Strength: A known constraint upon Fermi acceleration is the necessity of a strong enough magnetic field. As with the proton density criteria, the interface region between Homunculus and the primary stellar wind does not appear to contain a field of sufficient strength. However, regions of space closer to the primary star, such as the interface region could be sufficiently magnetised for Fermi acceleration to be a feasible source of proton acceleration.
  3. Available Energy: To drive the acceleration of protons, there needs to be sufficient energy available that can be transferred to the protons. The authors calculate the amount of mechanical energy in Eta Carinae’s stellar wind and find that it is many times the amount necessary – they suggest provisionally that maybe only ~1% of the total stellar wind energy is transferred in this way.

So this model for the gamma-ray production looks pretty solid. But it rests or founders on the binarity of Eta Carinae. Fortunately, the evidence for this is now reasonably conclusive (see for example Abraham & Falceta-Goncalves (2010), ADS/arXiv).

Artists Impression of a Colliding Wind Binary such as Eta Carinae (Image: ESA)

So the next time you look up into the night sky (although Eta Carinae is only visible from the tropics and the Southern Hemisphere), or watch a documentary on the Large Hadron Collider, remember the universe’s own particle acceleration laboratory, Eta Carinae.

The Large Hadron Collider (LHC) (Image: CERN)
References:

Farnier, C., Walter, R., & Leyder, J. (2010). Eta Carinae: a very large hadron collider
Astronomy and Astrophysics, 526 DOI: 10.1051/0004-6361/201015590

Abraham, Z., & Falceta-Gonsalves, D. (2010). Precession and nutation in the Eta Carinae binary system: evidence from the X-ray light curve Monthly Notices of the Royal Astronomical Society, 401 (1), 687-694 DOI: 10.1111/j.1365-2966.2009.15692.x

Bednarek, W. (2005). GeV gamma-rays and TeV neutrinos from very massive compact binary systems: the case of WR 20a Monthly Notices of the Royal Astronomical Society: Letters, 363 (1) DOI: 10.1111/j.1745-3933.2005.00081.x

Becker, M. (2007). Non-thermal emission processes in massive binaries The Astronomy and Astrophysics Review, 14 (3-4), 171-216 DOI: 10.1007/s00159-007-0005-2

Leyder, J., Walter, R., & Rauw, G. (2010). Hard X-ray identification of Carinae and steadiness close to periastron Astronomy and Astrophysics, 524 DOI: 10.1051/0004-6361/201014316

Smith, N., Gehrz, R., Hinz, P., Hoffmann, W., Hora, J., Mamajek, E., & Meyer, M. (2003). Mass and Kinetic Energy of the Homunculus Nebula around η Carinae The Astronomical Journal, 125 (3), 1458-1466 DOI: 10.1086/346278

van Boekel, R., Kervella, P., Scholler, M., Herbst, T., Brandner, W., de Koter, A., Waters, L., Hillier, D., Paresce, F., Lenzen, R., & Lagrange, A. (2003). Direct measurement of the size and shape of the present-day stellar wind of Eta Carinae Astronomy and Astrophysics, 410 (3) DOI: 10.1051/0004-6361:20031500

In the journals…

December 13, 2010
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Here are some recent and soon-to-be published articles from the journals that
have caught my eye this month. Watch out for more detailed articles on some of these soon for Research Blogging:

Romero, G., Reynoso, M., & Christiansen, H. (2010). Gravitational radiation from precessing accretion disks in gamma-ray bursts Astronomy and Astrophysics, 524 DOI: 10.1051/0004-6361/201014882

Abdo, A., Ackermann, M., Ajello, M., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Bellazzini, R., Blandford, R., Bloom, E., Bonamente, E., Borgland, A., Bouvier, A., Brandt, T., Bregeon, J., Brigida, M., Bruel, P., Buehler, R., Buson, S., Caliandro, G., Cameron, R., Caraveo, P., Carrigan, S., Casandjian, J., Charles, E., Chaty, S., Chekhtman, A., Cheung, C., Chiang, J., Ciprini, S., Claus, R., Cohen-Tanugi, J., Conrad, J., DeCesar, M., Dermer, C., de Palma, F., Digel, S., do Couto e Silva, E., Drell, P., Dubois, R., Dumora, D., Favuzzi, C., Fortin, P., Frailis, M., Fukazawa, Y., Fusco, P., Gargano, F., Gasparrini, D., Gehrels, N., Germani, S., Giglietto, N., Giordano, F., Glanzman, T., Godfrey, G., Grenier, I., Grondin, M., Grove, J., Guillemot, L., Guiriec, S., Hadasch, D., Harding, A., Hays, E., Jean, P., Jóhannesson, G., Johnson, T., Johnson, W., Kamae, T., Katagiri, H., Kataoka, J., Kerr, M., Knödlseder, J., Kuss, M., Lande, J., Latronico, L., Lee, S., Lemoine-Goumard, M., Llena Garde, M., Longo, F., Loparco, F., Lovellette, M., Lubrano, P., Makeev, A., Mazziotta, M., Michelson, P., Mitthumsiri, W., Mizuno, T., Monte, C., Monzani, M., Morselli, A., Moskalenko, I., Murgia, S., Naumann-Godo, M., Nolan, P., Norris, J., Nuss, E., Ohsugi, T., Omodei, N., Orlando, E., Ormes, J., Pancrazi, B., Parent, D., Pepe, M., Pesce-Rollins, M., Piron, F., Porter, T., Rainò, S., Rando, R., Reimer, A., Reimer, O., Reposeur, T., Ripken, J., Romani, R., Roth, M., Sadrozinski, H., Saz Parkinson, P., Sgrò, C., Siskind, E., Smith, D., Spinelli, P., Strickman, M., Suson, D., Takahashi, H., Takahashi, T., Tanaka, T., Thayer, J., Thayer, J., Tibaldo, L., Torres, D., Tosti, G., Tramacere, A., Uchiyama, Y., Usher, T., Vasileiou, V., Venter, C., Vilchez, N., Vitale, V., Waite, A., Wang, P., Webb, N., Winer, B., Yang, Z., Ylinen, T., & Ziegler, M. (2010). A population of gamma-ray emitting globular clusters seen with the Large Area Telescope
Astronomy and Astrophysics, 524 DOI: 10.1051/0004-6361/201014458

Klimek, M., Points, S., Smith, R., Shelton, R., & Williams, R. (2010). An X-ray Investigation of Three Supernova Remnants in the Large Magellanic Cloud The Astrophysical Journal, 725 (2), 2281-2289 DOI: 10.1088/0004-637X/725/2/2281

Böttcher, M., Hivick, B., Dashti, J., Fultz, K., Gupta, S., Gusbar, C., Joshi, M., Lamerato, A., Peery, T., Principe, D., Rajasingam, A., Roustazadeh, P., & Shields, J. (2010). Optical Spectral Variability of the Very High Energy Gamma-ray Blazar 1ES 1011+496 The Astrophysical Journal, 725 (2), 2344-2348 DOI: 10.1088/0004-637X/725/2/2344

Marengo, M., Evans, N., Barmby, P., Matthews, L., Bono, G., Welch, D., Romaniello, M., Huelsman, D., Su, K., & Fazio, G. (2010). An Infrared Nebula Associated with δ Cephei: Evidence of Mass Loss? The Astrophysical Journal, 725 (2), 2392-2400 DOI: 10.1088/0004-637X/725/2/2392

Is an unusual new galactic gamma-ray source a possible “dark accelerator”?

December 8, 2010

ResearchBlogging.orgIn my last post on gamma-ray binaries, I mentioned that only a few of these exotic X-ray binaries (XRB) have been observed, and that they appear to fall into two distinct categories: microquasars, where the gamma-ray emission is caused by leptonic or hadroni particle interactions in the relativistic jet (Inverse Compton Scattering and Neutral Pion Decay respectively) and pulsar wind binaries. where the gamma-rays are generated leptonic interactions between the pulsar wind and a circumstellar disk of gas surrounding the star:

Microquasars and Windy Binary Pulsars (image: Mirabel 2010)

They are, of corse, not the only type of object in the universe to emit gamma-rays, I have recently blogged about gamma-ray-pulsars and blazars; but supernovae/supernova remnants, gamma-ray bursters (GRBs) and active galaxies (AGN) are also well-known as cosmic sources of gamma-rays.

Artists Impression of an Active Galactic Nuclei (image: NASA)

A paper (A&A 525, A45 (2011), ADS/arXiv)) to be published in the January issue of Astronomy & Astrophysics reports on the discovery of a new source of very high energy (VHE) gamma rays which could be yet another type of cosmic gamma-ray emitter.

The paper, entitled “Discovery and follow-up studies of the extended, off-plane, VHE gamma-ray source HESS J1507-622” and authored by the H.E.S.S.Collaboration reports on the discovery of an unusual new source of very-high-energy (VHE) gamma-rays called HESS J1507-622.

Using the Namibian-based High-Energy Stereoscopic System (HESS) they employed a technique called atmospheric Cherenkov imaging to observe gamma-rays with energies ranging from 100 GeV up to 100 TeV from a source called HESS J1507-622.

Gamma-ray flux of HESS J1507-622. The contours displayed are confidence levels (image: H.E.S.S. Collaboration 2010)

The authors report that HESS J1507-622 is unusual for several reasons:

  • Unlike other gamma-ray sources discovered by HESS, which cluster within a degree of the galactic equator, HESS J1507-622 lies some ~3.5 degrees south of the galactic equator.
  • It appears to lack any plausible counterpart, i.e. a corresponding source at the same place in the sky detected in an alternative part of the electromagnetic spectrum such as in X-rays or radio (searches in X-rays were carried out using the the CHANDRA orbiting X-ray observatory without success).
  • There is no indication as to its distance from us – it could either be a nearby local object (in astrophysical terms this means within a few kiloparsecs!) or could be located right out in the galactic halo.

The lack of detection of any other object at the same location in the sky is unusual. If there truly is no X-ray/radio emission, then they suggest that HESS J1507-622 could be an example of a mysterious type of object known as a “dark accelerator“.

Dark accelerators were first proposed in 2005 (Aharonian et al. 2005ADS/arXiv) by the astronomer Dr Felix Aharonian (who also co-authored on this paper) to explain a small number of HESS-observed galactic VHE gamma-ray sources that have no apparent X-ray counterpart. Very little is known about them, and their existence is highly disputed (for a sceptical counter-view see Butt et al. 2008ADS/arXiv). But as they emit gamma-rays they must be a site of non-thermal particle acceleration involving either either high-energy electrons or photons.

But, like any good scientists, the authors outline a number of alternative explanations for the observed gamma-ray emission. These include:

The plerion at the centre of the Crab Nebula as imaged by CHANDRA (image: NASA)
  • HESS J1507-622 is a Pulsar Wind Nebula (PWN, or Plerion), surrounding an ancient (perhaps up to a million years old) pulsar; these are known to emit gamma-rays (via leptonic Inverse Compton Scattering) whilst having only very faint lower-energy counterparts (which in the case of HESS J1507-622 would mean that the X-ray flux is at a level low enough to be unobservable with CHANDRA).

 

The Supernova Remnant Cassiopeia A as imaged by CHANDRA (Image: NASA)

  • Alternatively HESS J1507-622 might be a young (and thus still very small) Supernova Remnant (SNR). These are thought to use hadronic processes to accelerate particles to gamma-ray energies. Perhaps the most famous SNR in the sky is the Crab Nebula (which is incidentally also a plerion), which is the canonical cosmic emitter of gamma-rays (so much so that gamma-ray flux from cosmic emitters is usually quoted in units equivalent to the flux from the Crab Nebula (‘crab’); the emission from HESS J1507-622 is ~0.08 crab).

Artist;s impression of a collision between two neutron stars (image: NASA)

  • The third alternative (and most exotic, but also the most unlikely, in their opinion) hypothesis that the authors put forward is that HESS J1507-622 is a remnant of a merger of two compact objects (such as black holes or neutron stars). These are thought to be one of the causes of Short Gamma-Ray-Bursts (SGRBs).(see Rosswog 2010ADS/arXiv for a recent review).

Although the tone of the paper is (correctly) tentative, with objects like HESS J1507-622, unless all the right ‘boxes’ are ticked, or the initial observations are done at a fortuitous time, it is usually extremely difficult to draw any firm conclusions without years of follow-up observations. As the authors state:

Upcoming deeper X-ray observations (XMM-Newton and Suzaku) will undoubtedly offer deeper insight in this VHE source.

But whatever it is, it is clear that HESS J1507-622 represents an extremely unusual astrophysical object, and one worthy of much study in years to come. Expect many papers to be published on this exotic object in the near future.

References:

H.E.S.S. Collaboration,, Acero, F., Aharonian, F., Akhperjanian, A., Anton, G., Barres de Almeida, U., Bazer-Bachi, A., Becherini, Y., Behera, B., Bernlöhr, K., Bochow, A., Boisson, C., Bolmont, J., Borrel, V., Brucker, J., Brun, F., Brun, P., Bühler, R., Bulik, T., Büsching, I., Boutelier, T., Chadwick, P., Charbonnier, A., Chaves, R., Cheesebrough, A., Chounet, L., Clapson, A., Coignet, G., Dalton, M., Daniel, M., Davids, I., Degrange, B., Deil, C., Dickinson, H., Djannati-Ataï, A., Domainko, W., O’C. Drury, L., Dubois, F., Dubus, G., Dyks, J., Dyrda, M., Egberts, K., Emmanoulopoulos, D., Espigat, P., Farnier, C., Feinstein, F., Fiasson, A., Förster, A., Fontaine, G., Füßling, M., Gabici, S., Gallant, Y., Gérard, L., Gerbig, D., Giebels, B., Glicenstein, J., Glück, B., Goret, P., Göring, D., Hauser, D., Hauser, M., Heinz, S., Heinzelmann, G., Henri, G., Hermann, G., Hinton, J., Hoffmann, A., Hofmann, W., Holleran, M., Hoppe, S., Horns, D., Jacholkowska, A., de Jager, O., Jahn, C., Jung, I., Katarzyński, K., Katz, U., Kaufmann, S., Kerschhaggl, M., Khangulyan, D., Khélifi, B., Keogh, D., Klochkov, D., Kluźniak, W., Kneiske, T., Komin, N., Kosack, K., Kossakowski, R., Lamanna, G., Lenain, J., Lohse, T., Marandon, V., Martineau-Huynh, O., Marcowith, A., Masbou, J., Maurin, D., McComb, T., Medina, M., Méhault, J., Moderski, R., Moulin, E., Naumann-Godo, M., de Naurois, M., Nedbal, D., Nekrassov, D., Nicholas, B., Niemiec, J., Nolan, S., Ohm, S., Olive, J., de Oña Wilhelmi, E., Orford, K., Ostrowski, M., Panter, M., Paz Arribas, M., Pedaletti, G., Pelletier, G., Petrucci, P., Pita, S., Pühlhofer, G., Punch, M., Quirrenbach, A., Raubenheimer, B., Raue, M., Rayner, S., Reimer, O., Renaud, M., Rieger, F., Ripken, J., Rob, L., Rosier-Lees, S., Rowell, G., Rudak, B., Rulten, C., Ruppel, J1, Sahakian, V., Santangelo, A., Schlickeiser, R., Schöck, F., Schwanke, U., Schwarzburg, S., Schwemmer, S., Shalchi, A., Sikora, M., Skilton, J., Sol, H., Stawarz, �., Steenkamp, R., Stegmann, C., Stinzing, F., Superina, G., Szostek, A., Tam, P., Tavernet, J., Terrier, R., Tibolla, O., Tluczykont, M., van Eldik, C., Vasileiadis, G., Venter, C., Venter, L., Vialle, J., Vincent, P., Vivier, M., Völk, H., Volpe, F., Wagner, S., Ward, M., Zdziarski, A., & Zech, A. (2010). Discovery and follow-up studies of the extended, off-plane, VHE gamma-ray source HESS J1507-622 Astronomy and Astrophysics, 525 DOI: 10.1051/0004-6361/201015187

Aharonian, F. (2005). A New Population of Very High Energy Gamma-Ray Sources in the Milky Way Science, 307 (5717), 1938-1942 DOI: 10.1126/science.1108643

Butt, Y., Combi, J., Drake, J., Finley, J., Konopelko, A., Lister, M., Rodriguez, J., & Shepherd, D. (2008). TeV J2032+4130: a not-so-dark accelerator? Monthly Notices of the Royal Astronomical Society, 385 (4), 1764-1770 DOI: 10.1111/j.1365-2966.2008.12959.x

Mirabel, I. F. (2010). Microquasars: Summary and Outlook The Jet Paradigm, Lecture Notes in Physics, Springer-Verlag Berlin Heidelberg, 2010, Volume 794 DOI: 10.1007/978-3-540-76937-8_1

S. Rosswog (2010). Compact binary mergers: an astrophysical perspective Invited review at “Nuclei in the Cosmos” (NIC XI) arXiv: 1012.0912v1

Off the straight and narrow: The kinky jets of Blazars

December 4, 2010

ResearchBlogging.orgGoogle any of the myriad artists impressions of blazars and relativistic jets available online and they all share one common feature: the jet streams out helically from the central accretion disc/black hole region in a long thin straight line that would make any Roman road builder green with envy. For example (image: University of Michigan):

Artists Impression of a Blazar

Yet this is actually a misrepresentation, as a new paper published in this month’s Astronomy and Astrophysics discusses. The paper, entitled Another look at the BL Lacertae flux and spectral variability and authored by a multinational group of astronomers led by Claudia Raiteri of the Osservatorio Astronomico di Torino suggests that the relativistic jet of the famous blazar BL Lacertae is anything but straight – in addition to the previously known spiral rotation, it contains a bend or ‘kink’.

The authors analysed the flux from the canonical blazar BL Lacertae (also known as BL Lac) over a period of a year covering most of 2008 in a wide variety of wavelengths (radio, near-IR and optical) using a wide variety of telescopes from the Whole Earth Blazar Telescope (WEBT) network including both ground and space-borne instruments (including the AGILE and Fermi (GLAST) satellites). The flux from BL Lac is well-known to be variable, but a major flare was observed near the start of the monitoring period (diagram: Raiteri et al.):

R-band composite light curve of BL Lacertae by the collaborating observatories from February 2008 to February2009

From these observations, they built a broad-band Spectral Energy Distribution (SED) of BL Lac which they then used to develop an enhanced model of the jet that includes for the first time a ‘kink’ to explain the observed emissions (diagram: Raiteri et al.):

Broad-band SEDs (dashed/dotted lines) of BL Lacertae in August 2008 (blue) and July 1997 (red). Solid lines represent model fits from Raiteri et al.

Using their model, the authors show that the observed SED and flux of the jet are explained in both a flared and non-flared state by a jet has has a bent section or ‘kink’ (Image: Raiteri et al.):

Sketch of the Raiteri et al. helical jet model during both the 2008 faint state and the 1997 outburst state. The different colours represent  different regions & emission: purple-blue – inner region, emitting the high-energy synchrotron plus self inverse-Compton component;  green-yellow-red – the outer zone, where the low-energy emission component is produced

In addition to the ‘kink’, as mentioned in the introduction to this post, the whole jet is rotating helically like a corkscrew, and is also constantly changing its alignment with respect to our line of sight (the angles in the above diagram have been exaggerated for clarity).

Another advantage of this new model is that it also confirms previous calculations by the same authors published last year (Raiteri et al. 2009) that reduced the number of emitting regions (“components”) in the jet that were required to explain the observed emission:

“Photons coming from the disc or broad line region could then enter the jet, and be inverse-Compton scattered, giving rise to other high-energy emission components that are sometimes invoked to account for the SED properties of blazars. In particular, the 1997 outburst state has previously been interpreted by Madejski et al. (1999) in terms of three emission components: synchrotron, synchrotron self-Compton, and Comptonisation of the broad emission line flux. Similar results were obtained by Bottcher & Bloom (2000) and by Ravasio et al. (2002).”

“Our ‘geometrical’€interpretation does not require these external-Compton emission components, which are not expected to contribute if the jet emission regions are parsecs away from the central black hole.”

This reappraisal of the physical structure of the jet is important, because, as the authors state:

“…the whole range of BL Lacertae multi-wavelength variability can be interpreted in terms of orientation effects. Although the rotating helical jet model we have adopted in the previous section is not a physically complete model, but more a phenomenological approach, it has the advantage of taking into account variations of the orientation of the emitting regions with respect to the line of sight, with consequent changes of the Doppler beaming factor. This is an aspect that is usually neglected by theoretical models of blazar emission,which explain flux and spectral changes uniquely in terms of energetic processes inside the jet.”

So what could cause the intricate structure of the jet? We know that there are three possible factors that are involved – curved magnetic fields in the region of the jet, the rotation of the parent black hole/accretion disc, and interaction of the particles in the jet with the surrounding medium.

The idea that relativistic jets (and the jet of BL Lac in particular) can possess helical structure caused by curved magnetic fields is not new. In 2008, a team led by Prof. Alan Marscher (who also contributed to this new paper) observed material in the jet of the same blazar, BL Lac, and observed that material close to the base of the jet appeared to follow a corkscrew-shaped path caused by twisting magnetic fields (image: Boston University).

Marscher et al. also noted that the radiation emitted by the moving material brightened when its rotating path was aimed almost directly toward Earth (due to doppler boosting), an effect Raiteri et al. have confirmed. From the original Marscher press release:

“[Theorists predicted] that material moving outward in this close-in acceleration region would follow a corkscrew-shaped path inside the bundle of twisted magnetic fields. They also predicted that light and other radiation emitted by the moving material would brighten when its rotating path was aimed most directly toward Earth. Marscher and his colleagues anticipated that there might also be a flare later when the material hits a stationary shock wave called the “core” some time after it has emerged from the acceleration region. “That behaviour is exactly what we saw,” Marscher said, when his team followed an outburst of radiation from BL Lac. In late 2005 and early 2006, the astronomers watched BL Lac with an international collection of ground- and space-based telescopes as a bright knot of condensed material was ejected outward through the jet. As the material sped out from the neighbourhood of the black hole, the VLBA could pinpoint its location, while other telescopes measured the properties of the radiation emitted from the knot.

Bright bursts of light, X rays, and gamma rays came when the knot was precisely at locations where the theories said such bursts would be seen. In addition, the property of the radio and light waves called polarization rotated as the knot wound its corkscrew path inside the tight throat of twisted magnetic fields.”

But why are the magnetic fields twisted? The answer is that the rotation of the black hole is distorting space itself, an effect known as frame-dragging. As space itself twists, any magnetic fields present also twist (image: NASA):

The big question raised by this paper is whither this ‘kink’ is unique to the jet of BL Lacertae or if its a common feature of other relativistic jets. And if it turns out the jets of AGN can be bent, what about the jets of their smaller galactic cousins, microquasars?

References:

Raiteri, C., Villata, M., Bruschini, L., Capetti, A., Kurtanidze, O., Larionov, V., Romano, P., Vercellone, S., Agudo, I., Aller, H., Aller, M., Arkharov, A., Bach, U., Berdyugin, A., Blinov, D., Böttcher, M., Buemi, C., Calcidese, P., Carosati, D., Casas, R., Chen, W., Coloma, J., Diltz, C., Di Paola, A., Dolci, M., Efimova, N., Forné, E., Gómez, J., Gurwell, M., Hakola, A., Hovatta, T., Hsiao, H., Jordan, B., Jorstad, S., Koptelova, E., Kurtanidze, S., Lähteenmäki, A., Larionova, E., Leto, P., Lindfors, E., Ligustri, R., Marscher, A., Morozova, D., Nikolashvili, M., Nilsson, K., Ros, J., Roustazadeh, P., Sadun, A., Sillanpää, A., Sainio, J., Takalo, L., Tornikoski, M., Trigilio, C., Troitsky, I., & Umana, G. (2010). Another look at the BL Lacertae flux and spectral variability Astronomy and Astrophysics, 524 DOI: 10.1051/0004-6361/201015191

Marscher, A., Jorstad, S., D’Arcangelo, F., Smith, P., Williams, G., Larionov, V., Oh, H., Olmstead, A., Aller, M., Aller, H., McHardy, I., Lähteenmäki, A., Tornikoski, M., Valtaoja, E., Hagen-Thorn, V., Kopatskaya, E., Gear, W., Tosti, G., Kurtanidze, O., Nikolashvili, M., Sigua, L., Miller, H., & Ryle, W. (2008). The inner jet of an active galactic nucleus as revealed by a radio-to-gamma-ray outburst Nature, 452 (7190), 966-969 DOI: 10.1038/nature06895

Raiteri, C., Villata, M., Capetti, A., Aller, M., Bach, U., Calcidese, P., Gurwell, M., Larionov, V., Ohlert, J., Nilsson, K., Strigachev, A., Agudo, I., Aller, H., Bachev, R., Benítez, E., Berdyugin, A., Böttcher, M., Buemi, C., Buttiglione, S., Carosati, D., Charlot, P., Chen, W., Dultzin, D., Forné, E., Fuhrmann, L., Gómez, J., Gupta, A., Heidt, J., Hiriart, D., Hsiao, W., Jelínek, M., Jorstad, S., Kimeridze, G., Konstantinova, T., Kopatskaya, E., Kostov, A., Kurtanidze, O., Lähteenmäki, A., Lanteri, L., Larionova, L., Leto, P., Latev, G., Le Campion, J., Lee, C., Ligustri, R., Lindfors, E., Marscher, A., Mihov, B., Nikolashvili, M., Nikolov, Y., Ovcharov, E., Principe, D., Pursimo, T., Ragozzine, B., Robb, R., Ros, J., Sadun, A., Sagar, R., Semkov, E., Sigua, L., Smart, R., Sorcia, M., Takalo, L., Tornikoski, M., Trigilio, C., Uckert, K., Umana, G., Valcheva, A., & Volvach, A. (2009). WEBT multiwavelength monitoring and XMM-Newton observations of
BL Lacertae in 2007–2008 Astronomy and Astrophysics, 507 (2), 769-779 DOI: 10.1051/0004-6361/200912953

Astrophysics 102: Bremsstrahlung

December 3, 2010

If there ever was a language that from the outside seems designed by a language enthusiast, it has to be German. Forget Esperanto or Sindarin, the language that features the compound noun rindfleischetikettierungsuberwachungsaufgabenubertragungsgesetz (it’s actually a beef-labelling law) is one worthy of serious respect. Certainly amongst crossword-compilers and scrabble-board makers! I have a serious point here though. Many aspects of the English Language have been enriched by importing terms from German. One of the most famous is zugzwang, a chess term which means:

“…a situation where one player is put at a disadvantage because he has to make a move – the player would prefer to pass and make no move (but is compelled to do so). In game theory, it specifically means that it directly changes the outcome of the game from a win to a loss.”

Astronomy also has imported some German terms into its vernacular. One of the most famous is gegenschein (“counter glow”), which refers to:

“a rarely discernible faint glow known as the gegenschein (German for “counter glow”) [that] can be seen 180 degrees around from the Sun in an extremely dark sky. The gegenschein is sunlight back-scattered off small interplanetary dust particles. These dust particles are millimeter sized splinters from asteroids and orbit in the ecliptic plane of the planets.”

However, the most oft-used and referred formerly German term in astrophysics has to be bremsstrahlung (“braking radiation”). An impressive sounding word. But what is it? The English equivalent gives us a clue:

Bremsstrahlung is electromagnetic radiation produced by the acceleration of a charged particle when it is deflected by another charged particle.

It was originally discovered by Nikola Tesla during research he conducted between 1888 and 1897 (Image: The Vernadsky National Library of Ukraine).

Nikola Tesla

Although the definition of bremsstrahlung above could be interpreted to also include synchrotron radiation (which is caused by the acceleration of charged particles through a magnetic field), the standard definition refers usually only to the specific case of the acceleration of electrons by charged atomic nuclei (ions) via deflection from their Columb fields (Image: Nondestructive Testing)

Bremsstrahlung is also referred to as free-free emission because there is no particle capturing involved – the electrons are free before and after the interaction with the charged nuclei. If the electrons possess a thermal distribution of energies (i.e. a spread of energies around a mean value relating to their temperature) then the resulting radiation has a characteristic continuous spectrum. This is because the energy of the photon produced is related to the kinetic energy of the electron that is deflected, resulting in a spectrum that becomes more intense and shifts toward higher frequencies as the energy of the electrons involved increase (Image: Hyperphysics):

In an astrophysical context, bremsstrahlung is important because the photons emitted are usually observed as X-rays (although they can also be observed in the radio, and if energetic enough, as gamma-rays).

But where is bremsstrahlung produced? There are three main scenarios in which free-free emission is generated. The first is solar and stellar flares (Image: NASA):

A Solar Flare

I don’t intend to cover bremsstrahlung production in solar and stellar flares in this post (for more information please refer to this NASA web page). But I will talk in detail about the second and third scenarios in which bremsstrahlung is emitted.

Within galaxies, the regions between stars is filled with what is known as the Interstellar Medium (ISM). The vast majority of this interstellar medium is gas, with only a tiny amount (~1%) of dust. As a whole, it is extremely dilute, with an average density of only 1 atom per cm3! Usually found in a cold neutral state, around hot young stars in an HII region or the  exposed hot stellar core in a planetary nebula the gas is heated up and ionized (both the young stars and the stellar core emit intense ultraviolet radiation). The electrons produced as a result of the ionization then interact with the ions to produce bremsstrahlung (Image: DSS).

Galactic HII Region

As well as within galaxies, galaxy clusters are also a major source of bremsstrahlung. Between the galaxies in a cluster lies what is known as the Intracluster medium (ICM). This is an extremely tenuous gas consisting of ionized hydrogen and helium, although the mass of the ICM is typically approximately 15% of the mass of the parent galaxy cluster. At the centre of a galaxy cluster, the ICM is extremely hot – with a temperature of approximately 107 to 108 K, which means that the gas emits extremely strongly in X-rays via bremsstrahlung – typically this emission is much stronger than the optical equivalent. For example, below is a comparison of images of the Coma Cluster in X-rays and in optical (Image: Swinburne Astronomy Online):

The two bright points on the left (X-ray) image correspond to the two bright elliptical galaxies at the centre of the right (optical) image. As can be seen from the comparison, the emission from the ICM in X-rays is greater than the emission from individual galaxies in both optical and X-rays.

For a long time, it wasn’t known how and why this gas is still extremely hot –  since it is losing energy by emitting massive amounts of X-rays, it should cool down and condense, and formed more galaxies. But this is not happened. We now think this is because of the presence of massive black holes at the centre of active galaxies lying at the heart of galaxy clusters. These black holes swallow up any gas coming close to them emitting enormous amounts of energy in the process. This energy drives very narrow outflows of gas in the form of relativistic jets which reheats the intra-cluster gas. Thus the black holes act like thermostats, regulating the temperature of the gas surrounding them.