In 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:
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.
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.
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. 2005, ADS/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. 2008, ADS/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:
- 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).
- 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).
- 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 2010, ADS/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.
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
Google 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):
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.):
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.):
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.):
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 ï¬ux. 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?
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., BÃ¶ttcher, M., Buemi, C., Calcidese, P., Carosati, D., Casas, R., Chen, W., Coloma, J., Diltz, C., Di Paola, A., Dolci, M., Efimova, N., FornÃ©, E., GÃ³mez, J., Gurwell, M., Hakola, A., Hovatta, T., Hsiao, H., Jordan, B., Jorstad, S., Koptelova, E., Kurtanidze, S., LÃ¤hteenmÃ¤ki, A., Larionova, E., Leto, P., Lindfors, E., Ligustri, R., Marscher, A., Morozova, D., Nikolashvili, M., Nilsson, K., Ros, J., Roustazadeh, P., Sadun, A., SillanpÃ¤Ã¤, 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., DâArcangelo, F., Smith, P., Williams, G., Larionov, V., Oh, H., Olmstead, A., Aller, M., Aller, H., McHardy, I., LÃ¤hteenmÃ¤ki, 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., BenÃtez, E., Berdyugin, A., BÃ¶ttcher, M., Buemi, C., Buttiglione, S., Carosati, D., Charlot, P., Chen, W., Dultzin, D., FornÃ©, E., Fuhrmann, L., GÃ³mez, J., Gupta, A., Heidt, J., Hiriart, D., Hsiao, W., JelÃnek, M., Jorstad, S., Kimeridze, G., Konstantinova, T., Kopatskaya, E., Kostov, A., Kurtanidze, O., LÃ¤hteenmÃ¤ki, 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 2007â2008 Astronomy and Astrophysics, 507 (2), 769-779 DOI: 10.1051/0004-6361/200912953
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 â 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.
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).
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).
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.
Looking up at the sky on any clear, dark, night, you can’t help but get a deep and often meaningful impression of the infinite voids of space. But in reality this is merely an illusion, as the universe is actually very, very crowded. I’m not talking about all the man-made space junk that we’ve launched into orbit in the five decades since Sputnik, although that is an increasing problem, but rather the sheer amount of astrophysical phenomena visible to us across the whole dome of the sky, in all the various parts of the electromagnetic spectrum. This is not a problem limited to our galaxy either, as even a cursory glance at the famous Hubble Deep Field will testify. Space is, to use the vernacular, absolutely chock-a-block with all sorts of interesting objects, and unfortunately, from our point of view, this means that often, objects overlap in the most messy of fashions. And for astronomers, it is a major problem.
New research (Chen et al. 2010) (the title of the paper is “Study of the gamma-ray source 1AGL J2022+4032 in the Cygnus Region”) by a collaboration of mainly Italian astronomers from the Istituto Nazionale Di Astrofiscia (National Institute for Astrophysics) led by Dr. Andrew Chen to be published in the forthcoming January issue of Astronomy and Astrophysics illustrates this dilemma spectacularly. It concerns a previously known supernova remnant (SNR) called G78.2+2.1, which is located in the northern Milky Way constellation of Cygnus at a distance of approximately 5000 light-years. Buried deep in the Milky Way, the region around this object is euphemistically described as “complex” in the literature. The SNR itself had previously been detected in radio and X-rays as well as in the optical, but a decade ago the orbiting Energetic Gamma Ray Experiment Telescope (EGRET) additionally detected the emission of gamma-rays (this emission was later labelled with the catchy title of 1FGL J2021.5+4026)
SNRs have historically been well-known as emitters of gamma-rays, but it was soon confirmed that the gamma-ray emission in G78.2+2.1 originated from a point source, and hence the emission did not originate from the SNR. It wasn’t until last year that researchers led by A.A. Abdo of the US Naval Research Laboratory using the NASA Fermi/LAT orbiting observatory were seemingly able to identify the source of this emission – a gamma-ray pulsar called LAT PSR J2021+4026 lying at a similar distance from Earth as G78.2+2.1 (and hence possibly physically associated with the SNR, perhaps sharing a common origin). From the introduction to Dr’s Chen paper:
“Identiﬁcation of gamma-ray-emitting Galactic sources is a long-standing problem in astrophysics. One such source, 1AGL J2022+4032, coincident with the interior of the radio shell of the supernova remnant Gamma Cygni (SNR G78.2+2.1) in the Cygnus Region, has recently been identiﬁed by Fermi as a gamma-ray pulsar, LAT PSR J2021+4026.”
The image below shows observations at different energies of the region superimposed from each other and is from Dr. Chen’s paper:
As mentioned in the except above from Dr. Chen’s paper, calculations published earlier this year in the Monthly Notes of the Royal Astronomical Society by a separate group of astronomers (Trepl et al. 2010) confirmed that the spectra and flux of the 1FGL J2021.5+4026 source were both consistent with that of a gamma-ray pulsar and that LAT PSR J2021+4026 was indeed probably physically associated with SNR G78.2+2.1. They concluded that it was highly likely that this was a case of a SNR containing a gamma-ray emitting pulsar, a noteworthy astrophysical occurrence and one definitely worthy of further study.
But as Dr. Chen notes, there’s a problem with this scenario. Emissions from gamma-ray pulsars are known to be (and not taking into account changes in flux due to phase) steady over the short-term (i.e. on a time period ranging from weeks to months). But the gamma-ray emission from 1FGL J2021.5+4026 isn’t steady – since 2008 it has flared on several occasions.
So the Italian reseachers have analysed almost three years worth of archival observations of 1FGL J2021.5+4026 in gamma-rays with the AGILE satellite (in accordance with standard procedure, the AGILE-observed source was renamed 1AGL J2022+4032 and I’ll be using that name from now on), and in their paper they report on their results.
They find that the variability of the gamma-ray flux from this source is sufficiently statistically significant enough to dispute the gamma-ray pulsar scenario. As a precautionary check they also applied the same analysis to the variability of the nearby (and previously confirmed steady) gamma-ray source 1AGL J2022+4032 over the same period; they found that this latter source did not, as expected, fluctuate to a statistically significant degree.
And thus they conclude that the association of this gamma-ray source with the gamma-ray pulsar LAT PSR J2021+4026 may not be so concrete after all.
But if 1FGL J2021.5+4026/1AGL J2022+4032 doesn’t correspond to a gamma-ray pulsar, where do the observed gamma-rays come from? And what type of object is responsible for their emission? In their paper, the researchers led by Dr. Chen discuss and evaluate a number of possible alternatives.
As can be seen from the above image, only one source, called [WSC2006] S21 is consistent with the position of all detections of 1AGL J2022+4032. But this X-ray source has previously identified by Trepl et al. (2010) as corresponding to the gamma-ray pulsar LAT PSR J2021+4026. A second possible X-ray source, called [WSC2006] S25 seems to be variable in X-rays over the long-term, but they report that it appears to be a normal star.
The next alternative they consider is the possibility that the gamma-rays come from a background blazar and have nothing to do with our galaxy:
There are no known blazars nearby (although it would be extremely difficult to detect them in this region due to interference and extinction from galactic sources), but on geometric grounds they calculate the possibility of finding a corresponding (but previously undetected) blazar as only ~0.02 (or 2%).
The third alternative they then investigate is an X-ray quiet microquasar. This is a peculiar type of microquasar deficient in X-rays proposed in 2009 (Romero & Vila 2009) by the Argentine astronomers Dr. Gustavo Romero and Gabriela Soledad Vila to account for situations such as this, where a high-energy source detected by AGILE for example, has no counterpart at lower energies. This type of microquasar would have a relativistic jet dominated by protons:
“…we propose a complete lepto/hadronic jet model to explain the unidentiﬁed variable AGILE sources. This model assumes a strong component of relativistic primary protons and takes into account all radiative processes that might occur at the base of the jets. The predicted SEDs are in accordance with what we know about of these sources. The jet model is independent of the nature of the donor star, so it could explain both low- and high-latitude galactic sources.”
However disappointingly for the Russian Doll scenario referred to in the title of this post, Dr Chen and his researchers calculate that such a microquasar would have to lie at a distance of only ~1000 years from Earth, i.e. much closer than both SNR G78.2+2.1 and LAT PSR J2021+4026.
But they do suggest the chances of such a microquasar being responsible for the gamma-ray emission from 1AGL J2022+4032 are considerably higher than those of an extragalactic blazar being responsible (X-ray binaries tend to be concentrated in regions of the galaxy such as the Cygnus region) :
“We consider the possibility of a nearby X-ray quiet microquasar contributing to the ﬂux of 1AGL J2022+4032 to be more likely than the hypotheses of a background blazar or intrinsic gamma-ray variabilty of LAT PSR J2021+4026.”
Thus it is likely that as well as the supernova remnant SNR G78.2+2.1, the gamma-ray pulsar LAT PSR J2021+4026, there is an additional microquasar that lies coincidentally within our line of sight and is responsible for the gamma-ray emission detected as 1AGL J2022+4032. They finally suggest followup observations in an attempt to ascertain the true nature of 1AGL J2022+4032 concentrating especially on its variability:
“Future observations of 1AGL J2022+4032 by both AGILE and Fermi will reveal whether this fascinating source continues to show evidence [of variability] over the long-term.”
Of course, this all raises new questions. Very little research has been done on Romero & Vila’s proposed X-ray quiet microquasars (not including the current paper under discussion, its only been cited four times in almost two years). If 1AGL J2022+4032 is indeed such a microquasar, then there could be many others out there, and the population estimates referred to in my last post on the subject could be a massive underestimate.
From a personal point of view, the main lesson to take away from this is that the Universe is a fiendishly complicated and crowded place, and we’re lucky to have the technology and the science that allows us to at least nibble at the surface of true understanding of objects such as pulsars, supernova remnants and microquasars. That such a tiny portion of sky offers such a rich variety of objects to analyse and research confirms my bias that astrophysics is the most fascinating of all branches of science! (In any case, we have the things that make the biggest boom!)
P.S. This is my first post for Research Blogging. If you have any thoughts regarding the content and the especially the format/style of this post, please leave a comment or email me direct (my email address is available on the about page)
Chen, A., Piano, G., Tavani, M., Trois, A., Dubner, G., Giacani, E., Argan, A., Barbiellini, G., Bulgarelli, A., Caraveo, P., Cattaneo, P., Costa, E., D’Ammando, F., De Paris, G., Del Monte, E., Di Cocco, G., Donnarumma, I., Evangelista, Y., Feroci, M., Ferrari, A., Fiorini, M., Fuschino, F., Galli, M., Gianotti, F., Giuliani, A., Giusti, M., Labanti, C., Lazzarotto, F., Lipari, P., Longo, F., Marisaldi, M., Mereghetti, S., Moretti, E., Morselli, A., Pacciani, L., Pellizzoni, A., Perotti, F., Picozza, P., Pilia, M., Prest, M., Pucella, G., Rapisarda, M., Rappoldi, A., Sabatini, S., Scalise, E., Soffitta, P., Striani, E., Trifoglio, M., Vallazza, E., Vercellone, S., Vittorini, V., Zambra, A., Zanello, D., Pittori, C., Giommi, P., Verrecchia, F., Lucarelli, F., Santolamazza, P., Colafrancesco, S., Antonelli, L., & Salotti, L. (2010). Study of the gamma-ray source 1AGL J2022+4032 in the Cygnus region
Astronomy and Astrophysics, 525 DOI: 10.1051/0004-6361/201015279
Trepl, L., Hui, C., Cheng, K., Takata, J., Wang, Y., Liu, Z., & Wang, N. (2010). Multiwavelength properties of a new Geminga-like pulsar: PSR J2021+4026 Monthly Notices of the Royal Astronomical Society DOI: 10.1111/j.1365-2966.2010.16555.x
Romero, G., & Vila, G. (2009). On the nature of the AGILE galactic transient sources Astronomy and Astrophysics, 494 (3) DOI: 10.1051/0004-6361:200811283
Abdo, A., Ackermann, M., Ajello, M., Anderson, B., Atwood, W., Axelsson, M., Baldini, L., Ballet, J., Barbiellini, G., Baring, M., Bastieri, D., Baughman, B., Bechtol, K., Bellazzini, R., Berenji, B., Bignami, G., Blandford, R., Bloom, E., Bonamente, E., Borgland, A., Bregeon, J., Brez, A., Brigida, M., Bruel, P., Burnett, T., Caliandro, G., Cameron, R., Caraveo, P., Casandjian, J., Cecchi, C., Celik, O., Chekhtman, A., Cheung, C., Chiang, J., Ciprini, S., Claus, R., Cohen-Tanugi, J., Conrad, J., Cutini, S., Dermer, C., de Angelis, A., de Luca, A., de Palma, F., Digel, S., Dormody, M., do Couto e Silva, E., Drell, P., Dubois, R., Dumora, D., Farnier, C., Favuzzi, C., Fegan, S., Fukazawa, Y., Funk, S., Fusco, P., Gargano, F., Gasparrini, D., Gehrels, N., Germani, S., Giebels, B., Giglietto, N., Giommi, P., Giordano, F., Glanzman, T., Godfrey, G., Grenier, I., Grondin, M., Grove, J., Guillemot, L., Guiriec, S., Gwon, C., Hanabata, Y., Harding, A., Hayashida, M., Hays, E., Hughes, R., Johannesson, G., Johnson, R., Johnson, T., Johnson, W., Kamae, T., Katagiri, H., Kataoka, J., Kawai, N., Kerr, M., Knodlseder, J., Kocian, M., Kuss, M., Lande, J., Latronico, L., Lemoine-Goumard, M., Longo, F., Loparco, F., Lott, B., Lovellette, M., Lubrano, P., Madejski, G., Makeev, A., Marelli, M., Mazziotta, M., McConville, W., McEnery, J., Meurer, C., Michelson, P., Mitthumsiri, W., Mizuno, T., Monte, C., Monzani, M., Morselli, A., Moskalenko, I., Murgia, S., Nolan, P., Norris, J., Nuss, E., Ohsugi, T., Omodei, N., Orlando, E., Ormes, J., Paneque, D., Parent, D., Pelassa, V., Pepe, M., Pesce-Rollins, M., Pierbattista, M., Piron, F., Porter, T., Primack, J., Raino, S., Rando, R., Ray, P., Razzano, M., Rea, N., Reimer, A., Reimer, O., Reposeur, T., Ritz, S., Rochester, L., Rodriguez, A., Romani, R., Ryde, F., Sadrozinski, H., Sanchez, D., Sander, A., Parkinson, P., Scargle, J., Sgro, C., Siskind, E., Smith, D., Smith, P., Spandre, G., Spinelli, P., Starck, J., Strickman, M., Suson, D., Tajima, H., Takahashi, H., Takahashi, T., Tanaka, T., Thayer, J., Thompson, D., Tibaldo, L., Tibolla, O., Torres, D., Tosti, G., Tramacere, A., Uchiyama, Y., Usher, T., Van Etten, A., Vasileiou, V., Vilchez, N., Vitale, V., Waite, A., Wang, P., Watters, K., Winer, B., Wolff, M., Wood, K., Ylinen, T., & Ziegler, M. (2009). Detection of 16 Gamma-Ray Pulsars Through Blind Frequency Searches Using the Fermi LAT Science, 325 (5942), 840-844 DOI: 10.1126/science.1175558
(This is the first in a continuing series of posts on various aspects of galactic high-energy astrophysics. I’ll also be covering some of the recent papers published in this area each week or so for Research Blogging)
Sometimes its hard to grasp just how profound the rate of scientific progress is in some fields, especially in my own area of study, galactic high-energy astrophysics. Until a few decades ago, for example, we didn’t know that X-ray binaries (XRBs) in our own galaxy could emit relativistic jets. Jets were thought to be the domain of farflung and exotic extragalactic objects such as Active Galactic Nuclei or Quasars, indeed the first such jet had been discovered as far back as 1918!
It wasn’t until 1979 that the presence of relativistic jets in our galaxy was finally confirmed. The unusual elongated radio source SS433 turned out to be a relativistic jet being emitted from a galactic binary system, and thus was what we now know to be the first galactic microquasar (Image source here):
It was long thought to be a unique object, but with the discoveries of other galactic microquasars in the early nineties by, amongst others, Felix Mirabel and Luis Felipe Rodriguez (see here an early but comprehensive review by Mirabel) it was soon realised that a good sized-proportion of the 700 or so XRBs in our galaxy possessed jets and were local analogues of quasars (the current best estimate for the total size of the galactic microquasar population, according to a recent review by Josep Paredes and Victor Zabalza of the University of Barcelona is ~65.
These jets usually emit most of their energy in the radio. but a small number of them have recently been discovered to emit also at more energetic frequences, in gamma-rays, much like their larger cousins do, via the mechanism of Inverse Compton Scattering where a low energy photon (such as one emitted from a star) bounces off a lepton (e.g. an electron) travelling at relativistic speeds (such as one in a relativistic jet) and energy is transferred from the lepton to the photon causing the photon to ‘upscatter’ and reach extremely high energy levels (original image here):
Up until now, only a few of these so-called “gamma-ray binaries” have been identified (yes, astronomers love to categorise and pigeonhole things to an extent that would make even Carl Linneus blush). But just to complicate matters, it turns out that some of them don’t even have relativistic jets, but instead are an entirely different type of binary called a Pulsar Wind Binary. The diagram (source: F. Mirabel) below summarises the differences between the two types of gamma-ray binary:
Unlike how they are generated in a microquasar, the gamma-rays in a young windy pulsar are emitted due to interactions between the fast (i.e. relativistic) stellar wind from the pulsar and a gaseous circumstellar disk that orbits the main star in the binary. But the end emission is to all intents and purposes identical in both scenarios, and that gives astrophysicists a problem: how can you figure out what type of object a particular gamma-ray binary is?
I’ll be covering the methods used in some upcoming posts.
W. Bednarek & R. Protheroe (1997) Interactions of Stars with AGN Jets: Gamma Ray Production in Relativistic Jets in AGNs, Proceedings of the International Conference, p.318-323 (ADS)
G.Dubus (2006) A&A 456 801 (ADS)
F. Mirabel & L. Rodriguez (1998) Nature 392 673 (ADS)
F. Mirabel (2010) Microquasars: Summary and Outlook in The Jet Paradigm, Lecture Notes in Physics, Volume 794. Springer-Verlag Berlin Heidelberg, 2010 (ADS)
J. M. Paredes & V. Zabalza, V (2010) Microquasars in the GeV-TeV era Invited review at the “7th Agile Meeting and The Bright Gamma-Ray Sky”, held in Frascati, Italy, 29 September to 1 October 2009 (ADS)
R. Spencer (1979) Nature 282 483 (ADS)
There’s been much excitement recently over the discovery by Fermi/LAT of two giant gas bubbles approximately 25,000 light years across in our galaxy. They span more than half the gamma-ray sky corresponding to a location in the visible sky stretching fron Virgo to Grus.
As with most of these discoveries, there is much rampant (and probably premature) speculation about their origin:
“The bubble emissions are much more energetic than the gamma-ray fog seen elsewhere in the Milky Way, and appear to have defined edges, suggesting it formed as a result of a large and rapid energy release. One possible culprit includes a particle jet from the supermassive black hole in the Galaxy’s centre, a phenomena observed in other galaxies, too. But while there is no evidence for such a jet being active today, the bubble could represent an ancient jet. An alternative theory is that the bubbles were blown out from gas outflow during a burst of star formation, another process also seen in other galaxies.”
If they are the result of a relativistic jet from the black hole at the centre of our galaxy, then this is yet another example of a correspondance between galactic-scale astrophysical proceses and similar local-scale processes.
Bubbles of gas created by the action of relativistic jets from X-ray binaries (XRBs) upon the local interstellar medium (ISM) are actually quite common, for example, the bubble around the microquasar Cygnus X-1, as described by the astronomer Elena Gallo in her 2006 paper (Gallo 2006):
“More recently, a low surface brightness arc of radio emission has been discovered around Cygnus X-1…and interpreted in terms of a shocked compressed hollow sphere of free-free emitting gas driven by an under-luminous synchrotron lobe inﬂated by the jet of Cygnus X-1”
Unlike the famous ring around SN1987A, this material definitely isn’t the result of a supernova explosion. From the discovery paper by Gallo et al (2005):
“A ring of radio emission – with a diameter of ∼1 million AU – appears northeast of Cygnus X-1…and seems to draw an edge between the tail of the nearby HII nebula Sh2-101 (whose distance is consistent with that to Cygnus X-1) and the direction of the radio jet powered by Cygnus X-1. Since Cygnus X-1 moves in the sky along a trajectory which is roughly perpendicular to the jet and thus can not possibly be traced back to the ring centre, this rules out that the ring might be the low-luminosity remnant of the natal supernova of the black hole. In analogy with extragalactic jet sources, the ring of Cygnus X-1 could be the result of a strong shock that develops at the location where the collimated jet impacts on the ambient interstellar medium. The jet particles inﬂate a radio lobe which is over-pressured with respect to the surroundings, thus the lobe expands sideways forming a spherical bubble of shock-compressed ISM, which we observe as a ring because of limb brightening effects.”
Even though Cygnus X-1 is a relatively well-known galactic object, probably the most exotic example of a relativistic-jet driven nebula is that around the Ultraluminous X-Ray source S26 in the galaxy NGC7793, which is located 12 million light-years distant in the constellation of Sculptor:
“A black hole only slightly heavier than our Sun is emitting the most powerful jets of energy ever seen, rivaling that of quasars a million times larger, and creating a bubble of hot gas and fast-moving particles 1000 light-years across…The bubble has a diameter of 1000 light-years and is expanding at about a million kilometers per hour. The black hole, located 12 million light-years away in the outer spiral of galaxy NGC 7793, has been blowing the bubble for about 200,000 years.”
“We have studied the structure and energetics of the powerful microquasar/shock-ionized nebula S26 in NGC7793, with particular focus on its radio and X-ray properties. Using the Australia Telescope Compact Array, we have resolved for the first time the radio lobe structure and mapped the spectral index of the radio cocoon. The steep spectral index of the radio lobes is consistent with optically-thin synchrotron emission; outside the lobes, the spectral index is flatter, suggesting an additional contribution from free-free emission, and perhaps ongoing ejections near the core. The radio core is not detected, while the X-ray core has a 0.3-8 keV luminosity ~6 × 1036 erg s-1. The size of the radio cocoon matches that seen in the optical emission lines and diffuse soft X-ray emission. The total 5.5-GHz flux of cocoon and lobes is ~2.1 mJy, which at the assumed distance of 3.9 Mpc corresponds to about three times the luminosity of Cas A. The total 9.0-GHz flux is ~1.6 mJy. The X-ray hotspots (combined 0.3-8 keV luminosity ~2 × 1037 erg s-1) are located ~20 pc outwards of the radio hotspots (i.e. downstream along the jet direction), consistent with a different physical origin of X-ray and radio emission (thermal-plasma and synchrotron, respectively). The total particle energy in the bubble is ~1053 erg: from the observed radio flux, we estimate that only approximately a few times 1050 erg is stored in the relativistic electrons; the rest is stored in protons, nuclei and non-relativistic electrons. The X-ray-emitting component of the gas in the hotspots contains ~1051 erg, and ~1052 erg over the whole cocoon. We suggest that S26 provides a clue to understand how the ambient medium is heated by the mechanical power of a black hole near its Eddington accretion rate”.
ULX-driven nebula are an important area of research because their projenitors represent a possible “missing link” between galactic-scale quasars and their smaller stellar cousins, microquasars:
“Recent X-ray observations of galaxies have uncovered a populations of sources that have high X-ray luminosities but are not coincident with the nucleus of the galaxy. They have luminosities which are greater than that possible for a normal black hole to be powering them.
There is a maximum possible luminosity of a black hole as photons carry momentum and so can exert a pressure. This means that if there are enough of them they will blow away in-falling material, which sets this maximum luminosity, and it depends on the mass of the central object. Hence astronomers know that the luminosities are too high for a black hole which has a mass a few times that of the Sun to be powering these sources. These sources are long-lived, and so they cannot be special types of supernovae for example.”
The key debate over the true nature of ULXes currently is whither they are a new type of object – so-called Intermediate Mass Black Holes (IMBH), or simply standard XRBs that appear more luminous due to relativistic beaming (effectively making them, by a similar analogy as quasars and microquasars, microblazars):
“These sources are too luminous for a normal black hole – how about a more massive one? The masses required are in the region of 100 Solar Masses or more. The problem with this explanation is that there is no clear way to create black holes of this mass. Either stars could merge at the centre of clusters, faster than they evolve, so forming a very massive star, which when it dies forms a massive black hole; or black holes could merge, also at the centres of clusters. Both of these scenarios have their disadvantages, merging black holes tend to get catapulted out of the centre of the clusters and heavy stars have a very strong wind, see High Mass X-ray Binaries, and so the final black hole mass would be less than the mass of the merged stars. Surveys of some clusters show that there is some mass present that we do not see, and this could be as a result of these IMBHs.
However, what could occur is that the emission from these objects is not isotropic, but concentrated into beams, see jets. This reduces the necessity for the most massive black holes, and means that ULXs could just be special X-ray Binaries. As to why some X-ray Binaries have emission concentrated into particular directions and others do not is another matter. The other explanation is that the maximum luminosity limit is temporarily over-stepped for a few years or decades.”
The Soria et al paper is important in this context because it demonstrates that whatever its rrue nature, S26 demonstrates properties in common with both its smaller and larger cousins. They observed similar components in the nebula that also occur in certain powerful radio galaxies (called Fanaroff-Riley Type II (FRII) galaxies):
“We showed that its structure is a scaled-down version of powerful FRII radio galaxies, with a core, radio lobes, X-ray hot spots and cocoon. It is the ﬁrst time that all these elements have been found in a non-nuclear BH”.
Yet this object also shares many characteristics with standard microquasars such as the aforementioned Cygnus X-1, yet on a much larger scale:
“Hα images (PSM10) may suggest an even larger size, ~ 340×170 pc. Thus, the volume-averaged shell radius Rs ≈ 100 pc. Its characteristic size is an order of magnitude larger than the jet driven bubble around Cyg X-1, which has an estimated jet power ~ 1037 erg s−1“
However they report an unusual spacial anticorrelation between X-ray and radio emitting regions, which is the reverse of the standard configuration found in microquasars:
“We showed that the radio and X-ray hot spots are not spatially coincident: the X-ray hot spots are ≈ 20 pc further out than the peak of the radio intensity in the lobes.”
They then suggest that this means that X-ray and radio emission come from different populations of radiating particles, with the X-ray emission thermal in origin (like that from accretion disks), and the radio emission originating, like microquasars, via the Sychrotron process.
As mentioned above, the calculated jet-power for objects such as S26 is actually greater than is seemingly possible using standard BH models, and on the assumption that relativistic beaming isn’t involved, they propose, like another recent paper on this object (Pakull et al. 2010), that the extreme luminosity shown in this microquasar is due primarily to thermal emission from non-relativistic protons and nuclei, outstripping any contribution from non-thermal relativistic particles that are normally found in standard microquasars:
“The total particle energy in the bubble is ~ 1053 erg. Based on the measured radio ﬂux and size of the bubble, and using standard equipartition relations for microquasar lobes, we estimated that the energy carried by the synchrotron-emitting relativistic electrons is a few 100 times less than the energy stored in protons, nuclei and non-relativistic electrons; non-relativistic particles provide most of the pressure to inﬂate the bubble.”
Whatever its true nature though, S26 will continue to be an object of great importance to those scientists engaged in unravelling the astrophysical processes behind relativistic jets in the Universe.
E. Gallo (2006) Radio emission and jets from Galactic microquasars Proceedings of the VI Microquasar Workshop: Microquasars and Beyond. September 18-22, 2006, Como, Italy., p.9.1 (ADS)
E. Gallo et al (2005) Nature 436 819 (ADS)
M. Pakull et al (2010) Nature 466 209 (ADS)
R. Soria et al (2010) MNRAS 409 541 (ADS)