Variations in the pulsation and spectral characteristics of OAO 1657−415

Abstract

We present broad-band pulsation and spectral characteristics of the accreting X-ray pulsar OAO 1657-415 with a 2.2 d long Suzaku observation carried out covering its orbital phase range ∼ 0.12-0.34, with respect to the mid-eclipse. During the last third of the observation, the X-ray count rate in both the X-ray Imaging Spectrometer (XIS) and the HXD-PIN instruments increased by a factor of more than 10. During this observation, the hardness ratio also changed by a factor of more than 5, uncorrelated with the intensity variations. In two segments of the observation, lasting for ∼ 30-50 ks, the hardness ratio is very high. In these segments, the spectrum shows a large absorption column density and correspondingly large equivalent widths of the iron fluorescence lines. We found no conclusive evidence for the presence of a cyclotron line in the broad-band X-ray spectrum with Suzaku. The pulse profile, especially in the XIS energy band shows evolution with time but not so with energy. We discuss the nature of the intensity variations, and variations of the absorption column density and emission lines during the duration of the observation as would be expected due to a clumpy stellar wind of the supergiant companion star. These results indicate that OAO 1657-415 has characteristics intermediate to the normal supergiant systems and the systems that show fast X-ray transient phenomena.

Fast flaring observed from XMMU J053108.3−690923 by eROSITA: a supergiant fast X-ray transient in the Large Magellanic Cloud

“…This implies a characteristic volume of the clump of V c ≃ R c 3 ≈ 10 35 cm 3 , which would then give the clump responsible for the bright flare a mass of M c ∼ 10 21 g. The estimated values of the mass and radius of the clump are compatible with estimates of the clump mass and radius obtained for other supergiant HMXBs (Bozzo et al 2011;Martínez-Núñez et al 2014;Pradhan et al 2014). The derived value is also in qualitative agreement with the predictions of the clumpy wind model of HMXBs (Ducci et al 2009).…”

New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-Ray Binaries from Chandra High-energy Transmission Grating X-Ray Spectroscopy

“…In the last few years, a number of papers have presented analyses for some of these objects using broadband X-ray data from missions such as NuSTAR or Suzaku. Thus Jaisawal & Naik (2015) analyze Suzaku data for 4U1700−37; Sasano et al (2014) analyze Suzaku data for 4U1822−371; Naik et al (2011) analyze Suzaku, andFarinelli et al (2016) Suzaku and NuSTAR data for Cen X−3; Tomsick et al (2014) and Parker et al (2015) analyze Suzaku and NuSTAR data for Cyg X−1, while Walton et al (2016) NuSTAR-only data for the same object; Suchy et al (2012) analyze Suzaku data for GX 301−2; Yoshida et al (2017) analyze Suzaku data for GX 1+4; Fürst et al (2013) and Wolff et al (2016) analyze NuSTAR, andFarinelli et al (2016) Suzaku and NuS-TAR data for Her X−1; Shtykovsky et al (2017) analyze NuSTAR, and Hung et al (2010) Suzaku data for LMCX−4; Jaisawal & Naik (2014) and Pradhan et al (2014) analyze Suzaku data for OAO 1657−415;and Maitra & Paul (2013) analyze Suzaku data for Vela X−1.…”

“…Note that an additional Gaussian component had to be included for both the BN11 and MYTORUS models due to excess line-like emission at ∼ 6.68 keV, likely due to Fe XXV(r). Pradhan et al (2014) interpret the strong spectral and Xray intensity variations of their phase-resolved Suzaku observations as evidence of a highly inhomogeneous, clumpy stellar wind from the supergiant Ofpe/WN9-type companion, which belongs to a class characterized by exceptionally intense stellar winds with low terminal velocities and high mass loss rates (Martins et al 2007;Mason et al 2012). They estimate clump masses of the order of ∼ 3 × 10 24 g, and further suggest that this object could belong to a distinct class between Supergiant Fast X-ray Transients (SFXTs), which are known to show irregular outbursts lasting from minutes to hours, and "normal" HMXBs.…”

AstroSat observations of eclipsing high mass X-ray binary pulsar OAO 1657-415

“…Being located at a low galactic latitude, OAO 1657-415 is highly absorbed with a column density of 10 23 cm À2 (Polidan et al 1978;Kamata et al 1990). The energy spectrum of the pulsar can be described by a power law continuum with an exponential high energy cutoff along with a soft excess and prominent emission lines at 6.4, 6.7 and 7.1 keV (Kamata et al 1990;Audley et al 2006;Barnstedt et al 2008;Pradhan et al 2014Pradhan et al , 2019Jaisawal & Naik 2014).…”