• PRAHLAD R. EPILI

      Articles written in Journal of Astrophysics and Astronomy

    • Detection of X-ray pulsations at the lowest observed luminosity of Be/X-ray binary pulsar EXO 2030+375 with AstroSat

      GAURAVA K. JAISAWAL SACHINDRA NAIK SHIVANGI GUPTA P. C. AGRAWAL ARGHAJIT JANA BIRENDRA CHHOTARAY PRAHLAD R. EPILI

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      We present the results obtained from timing and spectral studies of Be/X-ray binary pulsar EXO 2030$+$375 using observations with the Large Area Xenon Proportional Counters and Soft X-ray Telescope of AstroSat, at various phases of its Type-I outbursts in 2016, 2018, and 2020. The pulsar was faint during these observations as compared to earlier observations with other observatories. At the lowest luminosity of $2.5\times10^{35}$ erg s$^{-1}$ in 0.5–30 keV energy range, $\approx$41.3 s pulsations were clearly detected in the X-ray light curves. This finding establishes the first firm detection of pulsations in EXO 2030$+$375 at anextremely low mass accretion rate to date. The shape of the pulse profiles is complex due to the presence of several narrow dips. Though pulsations were detected up to $\sim$80 keV when the source was brighter, pulsations were limited up to $\sim$25 keV during the third AstroSat observation at lowest source luminosity. A search for quasi-periodic oscillations in $2\times 10^4$ Hz to 10 Hz yielded a negative result. Spectral analysis of the AstroSat data showed that the spectrum of the pulsar was steep with a power-law index of $\sim$2. Thevalues of photon-indices at observed low luminosities follow the known pattern in sub-critical regime of the pulsar.

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

      GAURAVA K. JAISAWAL SACHINDRA NAIK PRAHLAD R. EPILI BIRENDRA CHHOTARAY ARGHAJIT JANA P. C. AGRAWAL

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      We present the results obtained from analysis of two AstroSat observations of the high mass Xray binary pulsar OAO 1657-415. The observations covered 0.681–0.818 and 0.808–0.968 phases of the $\sim$10.4 day orbital period of the system, in March and July 2019, respectively. Despite being outside theeclipsing regime, the power density spectrum from the first observation lacks any signature of pulsation or quasi-periodic oscillations. However, during July observation, X-ray pulsations at a period of 37.0375 s were clearly detected in the light curves. The pulse profiles from the second observation consist of a broad single peak with a dip-like structure in the middle across the observed energy range. We explored evolution of the pulse profile in narrow time and energy segments. We detected pulsations in the light curves obtained from0.808–0.92 orbital phase range, which is absent in the remaining part of the observation. The spectrum of OAO 1657-415 can be described by an absorbed power-law model along with an iron fluorescent emission line and a blackbody component for out-of-eclipse phase of the observation. Our findings are discussed in the frame of stellar wind accretion and accretion wake at late orbital phases of the binary.

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