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
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).
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 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.
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
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.
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:
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:
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.
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 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.
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
From the ever reliable xkcd:
To 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).
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:
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.
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).
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:
- 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.
- 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.
- 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).
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.
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
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