1. Spectral evolution of LMXBs (and MSPs)

Using multi-wavelength observations (e.g. Swift/XRT and Chandra in X-rays, and Gemini in Infrared and Optical) I try to study accretion and properties of the compact object in these systems. In our collaboration's most recent paper in this field, we have focused on X-ray spectral variations of quiescent LMXBs over a decade (Bahramian et al. 2015).

Terzan 5 as seen in X-rays by Chandra; Credit: NASA/CXO, Bahramian et al.

I also work on monitoring spectral evolution of transient LMXBs during outbursts. In one of our previous works, we have discovered and monitored a newly transient LMXB in globular cluster Terzan 5 (Bahramian et al. 2014A).

Optical/infrared spectroscopy of LMXBs' counterpart is another important source of information that I'm interested in. In these frequencies we can learn about the companion star and what the accretion disk might contain. Using Near-Infrared spectroscopy, we have been able to identify and study a new Symbiotic LMXB (Bahramian et al. 2014B).

I'm also interested in the evolutionary link between LMXBs and MSPs; It's been shown that LMXBs are progenitors of MSPs and recently, some LMXBs have been identified to show a transitional state. These transitional millisecond pulsars (tMSPs) seem to switch between accretion power and rotation power. I've been part of a collaboration which studied one of these tMSPs in globular cluster M 28 (Linares et al. 2014).

Very faint X-ray binaries are one of less well-studied subclasses of LMXBs. These systems show faint outbursts (\(<10^{36}\) erg s \(^{-1}\)) and thus their accretion is hard to explain using standard disk models. In a recent paper, we found similarities between some of these very faint X-ray binaries and tMSPs which might suggest a link between the two groups (Heinke et al. 2015).

Moving light echoes around V404 Cyg; Credit: NASA/Swift, Bahramian et al.

I've also been part of a collaboration that studied the 2015 outburst of V404 Cyg black hole X-ray binary. V404 Cyg is a relatively close (~2 kpc) black hole which went through a massive outburst in June and July 2015, and it slowly declined back to quiescence over a few weeks (e.g. Astronomers Telegrams #7763 and #7788). One of many amazing aspects of this outburst was the clear detection of light echoes after a strong X-ray burst and their outward motion.

Over the last few years, there have been some other transient X-ray sources that I've been involved in studying them; These include GRS 1747-312 in Terzan 6 (ATel #4915), 1A 1744-361 (ATel #5301) and IGR J17451-3022 (ATel #6459).

2. Formation of X-ray binaries and MSPs in Globular Clusters

Population density of X-ray binaries (XRBs) have been observed to be orders of magnitude higher in globular clusters (GCs) than the rest of the Galaxy. This is thought to be due to high concentration of stars (and thus higher chances of encounter between cluster members). This overpopulation also seem to vary from one GC to another. It has been shown that population of XRBs in GCs correlates with various properties of GCs (e.g. Mass, density, velocity dispersion). One of the main quantities that correlates with XRB population is stellar encounter rate (denoted as \(\Gamma \)). Gamma can be written as: $$ \Gamma \propto \int \frac{\rho^2}{\sigma} dV$$ Where \(\rho \) is GC stellar density, \(\sigma \) is velocity dispersion and the integral is over the volume of the GC.

To investigate correlation between properties of GCs and XRB population, it is important to calculate \(\Gamma \) accurately and take into account observational uncertainties. In Bahramian et al. 2013, we calculated \(\Gamma \) for 124 Galactic GCs and used Monte-Carlo simulations to incorporate observational uncertainties into final results.

Now that we have calculated stellar encounter rate for a large sample of GCs, I'm focusing on parameterizing the correlations between GC properties and XRB population.

Correlation between number of XRBs in GCs and stellar encounter rate. Credit: Bahramian et al.

3. Other topics:

Besides my main research work, I'm also interested in a few other loosely related topics:

  • Bayesian statistics, Computational methods (in Python and R mostly) and Astronomical application.

  • 3D Vizualization of astronomical systems (mostly in Blender).

A snapshot from my simple vizualization of an accretion disk made in Blender.