The 2011 outburst of the black hole candidate IGR J17091-3624 followed the canonical track of state transitions along with the evolution of Quasi-Periodic Oscillation (QPO) frequencies before it began exhibiting various variability classes similar to GRS 1915+105. We use this canonical evolution of spectral and temporal properties to determine the mass of IGR J17091-3624, using three different methods, viz: Photon Index (Γ) -QPO frequency (ν) correlation, QPO frequency (ν) -Time (day) evolution and broadband spectral modelling based on Two Component Advective Flow. We provide a combined mass estimate for the source using a Naive Bayes based joint likelihood approach. This gives a probable mass range of 11.8 M ⊙ − 13.7 M ⊙ . Considering each individual estimate and taking the lowermost and uppermost bounds among all the three methods, we get a mass range of 8.7 M ⊙ − 15.6 M ⊙ with 90% confidence. We discuss the possible implications of our findings in the context of two component accretion flow.

No abstract

We study the variations in the Cyclotron Resonant Scattering Feature (CRSF) during 2011 outburst of the high mass X-ray binary 4U 0115+63 using observations performed with Suzaku, RXTE, Swift and INTEGRAL satellites. The wide-band spectral data with low energy coverage allowed us to characterize the broadband continuum and detect the CRSFs. We find that the broadband continuum is adequately described by a combination of a low temperature (kT ∼ 0.8 keV) blackbody and a power-law with high energy cutoff (E cut ∼ 5.4 keV) without the need for a broad Gaussian at ∼ 10 keV as used in some earlier studies. Though winds from the companion can affect the emission from the neutron star at low energies (< 3 keV), the blackbody component shows a significant presence in our continuum model. We report evidence for the possible presence of two independent sets of CRSFs with fundamentals at ∼ 11 keV and ∼ 15 keV. These two sets of CRSFs could arise from spatially distinct emitting regions. We also find evidence for variations in the line equivalent widths, with the 11 keV CRSF weakening and the 15 keV line strengthening with decreasing luminosity. Finally, we propose that the reason for the earlier observed anti-correlation of line energy with luminosity could be due to modelling of these two independent line sets (∼ 11 keV and ∼ 15 keV) as a single CRSF.