Suzaku observation of Be / X-ray binary pulsar EXO 2030+375

We study the timing and spectral properties of Be/X-ray binary pulsar EXO 2030＋375 using a Suzaku observation taken on 2012 May 23, during a less intense Type I outburst. Pulsations were clearly detected in the X-ray light curves at a barycentric period of 41.2852 s, which suggest that the pulsar is spinning-up. The pulse profiles were found to be peculiar, e.g. unlike those obtained from the earlier Suzaku observation acquired on 2007 May 14. A single-peaked narrow profile at soft X-rays （0.5-10 keV range） changed to a double-peaked broad profile in the 12-55 keV energy range and again reverted back to a smooth single-peaked profile at hard X-rays （55-70 keV range）. The 1.0-100.0 keV broadband spectrum of the pulsar was found to be well described by three continuum models described as （i） a partial covering high energy cut-off power-law model, （ii） a partially absorbed power-law with high- energy exponential rolloff and （iii） a partial covering Negative and Positive power law with EXponential （NPEX） continuum model. Unlike the earlier Suzaku observation during which several low energy emission lines were detected, a weak and narrow Iron Ka emission line at 6.4 keV was only present in the pulsar spectrum during the 2012 May outburst. Non-detection of any absorption like feature in the 1-100 keV energy range supports the claim of the absence of the cyclotron resonance scattering feature in EXO 2030＋375 from the earlier Suzaku observation. Pulse-phase resolved spectroscopy revealed the presence of additional dense matter causing the absence of a second peak from the soft X-ray pulse profiles. The details of the results are described in the paper.

Accretion properties of MAXI J1813-095 during its failed outburst in 2018

We present the results obtained from detailed timing and spectral studies of a black hole candidate MAXI J1813-095 using Swift,NICER,and NuSTAR observations during its 2018 outburst.The timing behavior of the source is mainly studied by examining NICER light curves in the 0.5−10 keV range.We did not find any signature of quasi-periodic oscillations in the power density spectra of the source.We carry out spectral analysis with a combined disk blackbody&power law model,and physical two-component advective flow(TCAF)model.From the combined disk blackbody&power-law model,we extracted thermal and non-thermal fluxes,photon index and inner disk temperature.We also find evidence for weak reflection in the spectra.We have tested the physical TCAF model on a broadband spectrum from NuSTAR and Swift/XRT.The parameters like mass accretion rates,the size of Compton clouds and the shock strength are extracted.Our result affirms that the source remained in the hard state during the entire outburst which indicates a‘failed’outburst.We estimate the mass of the black hole as 7.4±1.5M⊙from the spectral study with the TCAF model.We apply the LAOR model for the Fe K line emission.From this,the spin parameter of the black hole is ascertained as a^(∗)>0.76.The inclination angle of the system is estimated to be in the range of 28°−45°from the reflection model.We find the source distance to be∼6 kpc.

Broad-band spectroscopy of the transient X-ray binary pulsar KS 1947+300 during 2013 giant outburst: Detection of pulsating soft X-ray excess component

We present the results obtained from detailed timing and spectral studies of the Be/X-ray binary pulsar KS 1947＋300 during its 2013 giant outburst. We used data from Suzaku observations of the pulsar at two epochs, i.e. on 2013 October 22 （close to the peak of the outburst） and 2013 November 22. X- ray pulsations at - 18.81 s were clearly detected in the light curves obtained from both observations. Pulse periods estimated during the outburst showed that the pulsar was spinning up. The pulse profile was found to be single-peaked up to -10 keV beyond which a sharp peak followed by a dip-like feature appeared at hard X-rays. The dip-like feature has been observed up to -70 keV. The 1-110 keV broad-band spectroscopy of both observations revealed that the best-fit model was comprised of a partially absorbed Negative and Positive power law with EXponential cutoff （NPEX） continuum model along with a blackbody component for the soft X-ray excess and two Gaussian functions at 6.4 and 6.7 keV for emission lines. Both the lines were identified as emission from neutral and He-like iron atoms. To fit the spectra, we included the previously reported cyclotron absorption line at 12.2 keV. From the spin-up rate, the magnetic field of the pulsar was estimated to be -1.2 x 10^12 G and found to be comparable to that obtained from the detection of the cyclotron absorption feature. Pulse-phase resolved spectroscopy revealed the pulsating nature of the soft X-ray excess component in phase with the continuum flux. This confirms that the accretion column and/or accretion stream are the most probable regions of the soft X-ray excess emission in KS1947＋300. The presence of the pulsating soft X-ray excess in phase with continuum emission may be the possible reason for not observing the dip at soft X-rays.