Research

My main research interests involve the understanding of processes leading to particle acceleration and high-energy emission, determining the nature of the astrophysical sources associated with such processes and studying their population. In particular, my current research focuses on Galactic gamma-ray sources, high-mass X-ray binaries, pulsars, and pulsar-wind nebulae. I’m also interested in the formation and evolution of interacting binary systems (both galactic and extragalactic), and studying and classifying various X-ray sources. Here are my publications on ADS.

Galactic TeV Sources

During the past decade TeV gamma-ray observatories have revealed a large number of very- high energy (VHE) sources. We found that (Kargaltsev et al. 2013) pulsar-wind nebulae (PWNe), shell-type supernova remnants (SNRs), and microquasar-type high-mass X-ray binaries (HMXBs) appear to be firmly established sources of the leptonic cosmic rays in our Galaxy. They account for 48% of the total number (∼90) of Galactic VHE sources, with 28 PWNe, 10 SNRs and 5 HMXBs. There is also a large number of extended TeV sources positionally coincident with young energetic pulsars; in most cases they can be considered as TeV PWN candidates. In addition, there remains a sizable fraction of unidentified VHE sources (∼20). For some of these sources, multi-wavelength (MW) observations suggest a possible counterpart (such as an SNR interacting with a molecular cloud, or a star-forming region), but most of these associations are still uncertain because at least some of these sources still could be powered by offset pulsars whose PWNe are too faint in X-rays. Finally, there are “dark” VHE sources, for which neither radio nor X-ray images reveal any plausible counterparts in the reasonable vicinity of the VHE source.

MW observations of the HESS J1809-193 field reveal a complex picture. Although the majority of the bright TeV emission can be attributed to the PWN of PSR J1809-1917, several super-nova remnants, as well as another PWN, may contribute to the observed VHE emission. My recent work (Rangelov et al. 2014) involves an extension toward north-east, which could be a separate TeV source, seen in the H.E.S.S. image. We analyze the results from three Chandra and two Suzaku observations of this region, and investigate the MW properties of point sources found in the data. One of the X-ray sources in the field is the low-mass X-ray binary (LMXB) candidate XTE J1810-189. Its X-ray properties and outburst history suggest that this is a “normal” LMXB, unlikely to produce the observed TeV emission. Another candidate would be hadronic emission from the eastern edge of SNR G11.4-0.1, which is located not too far from the TeV NE extension. No γ-ray emission is seen with Fermi. There is no obvious counterpart to the TeV emission from the HESS J1809-193 NE extension, which adds this source to the list of “dark” accelerators.

Gamma-Ray Binaries

xrbThanks to recent advances in space-based X-ray, GeV, and ground-based TeV γ-ray observations, an emerging population of high-mass gamma-ray binaries (HMGBs) has become an important topic in modern high-energy astrophysics. In Kargaltsev et al. 2013 we review five VHE HMGBs firmly detected in TeV and/or GeV γ-rays, which can be observationally separated into two types: (1) binaries where a rotation-powered pulsar interacts with the strong wind of the massive stellar companion (LS 2883/B1259-63), and (2) microquasars (LS 5039 and LS I +61 303). The types of the other two HMGB, HESS J0632+057 and HESS J1018-589A/1FGL J1018.6-5856, are so far uncertain. The latter is the only HMGB coincident with the super- nova remnant (G284.3–1.8). In addition to the five VHE HMGBs there are several other massive binary systems (e.g., Cygnus X-3, η Carina) which were detected in the GeV band with Fermi-LAT but so far lack the TeV detections. In most of the VHE binaries the GeV light curves are modulated with the orbital period determined from optical spectra and/or radio measurements.

X-Ray Binaries and Star Clusters

It has been previously suggested (Kaaret et al. 2004) that XRBs may have formed in young star clusters, because they are preferentially located near star clusters, albeit with a significant displacement (~200 pc on average).

I used UBV IHα images from Hubble Space Telescope (HST; WFPC2 and ACS) to derive masses and ages for the 129 star clusters I discovered. I performed a detailed comparison between the cluster population and the 23 XRBs discovered in Chandra data. I compared the observations with synthetic X-ray source populations, and found that while the specific recipe used to create synthetic X-ray source populations can somewhat affect the results, it is clear that the observed spatial distribution of the XRBs differs from the simulated populations. This is especially true for separations of 100 pc or less, suggesting that a large fraction of HMXBs are not far from their birth-sites.

Almost all of the HMXBs are found close to but no coincident with star clusters. In my recent studies of the HMXB populations in NGC 4449 and the Antennae galaxies, I found that high fraction (∼20-25%) of the HMXBs still reside within their parent clusters (in contrast with our galaxy or the Magellanic Clouds), based on high quality data from the HST. The chance superposition between the XRBs and clusters is only 1-2% in both systems, and it is highly unlikely to be the reason for the observed coincidences. This is a strong evidence that HMXBs from in star clusters. However, many HMXBs are displaced from their birth sites likely due to dynamical interaction or asymmetry in the supernova kicks. Knowledge of the cluster coincident or related to an XRB could provide indirect, yet solid constraints on the nature of the XRBs. Similarly, the parent clusters could provide important information regarding the origin and evolution of compact object such as neutron stars (NSs) and black holes (BHs) that have left their birth-sites.

A recent new results from N-body simulations indicate that dynamics can help constrain the type of the compact object (i.e., BH or NS) in HMXBs. For example, our N-body simulations (Garofali et al. 2012) suggest that HMXBs hosted by star clusters have a BH as the compact object, regardless of the metallicity-dependent evolutionary path that led to the formation of the compact object, because dynamically these are much less likely to be expelled from their parent clusters. The main result from our simulations is that an HMXB found within its parent star cluster almost certainly has a BH and not a NS as the compact object.

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