5 research outputs found
Spectral and timing studies of 2S 1417-624 during a giant outburst
We present the results obtained from timing and spectral studies of the
accretion powered X-ray pulsar 2S~1417-624 during a giant outburst in 2009 by
using {\it Rossi X-ray Timing Explorer (RXTE)} observations. X-ray pulsations
were detected in the light curves obtained from all epochs of observations. The
pulsar was found to be spinning-up during the outburst. The pulse profiles were
observed to be strongly dependent on photon energy and luminosity. A double
peaked profile at lower luminosity evolved into a triple peaked profile at the
peak of the outburst which is further reverted back to a double peaked
structure during the decay of the outburst. An anti-correlation was also
observed between the pulse fraction and the source flux. The 3-70 keV energy
spectrum of pulsar was well described with a power law modified with high
energy cutoff model along with an iron fluorescence line at 6.4 keV. Based on
the evolution of pulse profile, pulse fraction and spectral parameters across
observed luminosity, we interpret our results in terms of changes in the pulsar
beam configuration from sub-critical to super-critical regimes.Comment: 9 pages,7 figures; Accepted for publication in Monthly Notices of the
Royal Astronomical Society journal on July 2, 201
AstroSat observations of eclipsing high mass X-ray binary pulsar OAO 1657-415
We present the results obtained from analysis of two AstroSat observations of
the high mass X-ray binary pulsar OAO 1657-415. The observations covered
0.681-0.818 and 0.808-0.968 phases of the 10.4 day orbital period of the
system, in March and July 2019, respectively. Despite being outside the
eclipsing 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 from
0.808--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.Comment: 11 pages, 8 figures and 2 tables; Accepted for publication in the
Journal of Astrophysics & Astronom
Detection of X-ray pulsations at the lowest observed luminosity of Be/X-ray binary pulsar EXO 2030+375 with AstroSat
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 2.510 erg s in
0.5--30 keV energy range, 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 an extremely 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 80 keV when the source was
brighter, pulsations were limited up to 25 keV during the third AstroSat
observation at lowest source luminosity. A search for quasi-periodic
oscillations in 210 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 2. The values of photon-indices at
observed low luminosities follow the known pattern in sub-critical regime of
the pulsar.Comment: 12 pages, 8 figures and 2 tables; Accepted for publication in the
Journal of Astrophysics & Astronom