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

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:

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. 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:

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 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.

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