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Russian Dolls in Space: a possible Microquasar inside a Gamma-Ray Pulsar inside a Supernova Remnant

November 30, 2010

ResearchBlogging.orgLooking 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:

SNR G78.2+2.1, 1FGL J2021.5+4026 and LAT PSR J2021+4026 (from Chen et al. 2010). The orange represents intensity in radio wavelengths as measured by the DRAO Radio telescope; the white contours represent gamma-ray intensity from 1FGL J2021.5+4026 as measured by the AGILE satellite (green contour is 95% confidence level for emissions with an energy > 100 MeV) and the black circle is the gamma-ray pulsar LAT PSR J2021+4026

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.

Gamma-Ray Light Curve for PSR J2021+4026 as observed by Fermi/LAT in the range of 0.1 GeV -300 GeV (from Trepl et al. 2010)

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.

They first look for any corresponding X-ray sources using data from the CHANDRA orbiting observatory:

Possible X-ray counterparts of 1AGL J2022+4032 (from Chen et al. 2010). The various coloured circles represent the positional error boxes from the various surveys of the gamma-ray source

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:

An artists impression of a Blazar (Image: NASA)

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)

References:

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

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