48 research outputs found
Cyclotron lines: from magnetic field strength estimators to geometry tracers in neutron stars
With exactly forty years since the discovery of the first cyclotron line in
Her X-1, there have been remarkable advancements in the field related to study
of the physics of accreting neutron stars -- cyclotron lines have been a major
torchbearer in this regard, from being the only direct estimator of the
magnetic field strength, a tracer of accretion geometry and an indicator of the
emission beam in these systems. The main flurry of activities have centred
around studying the harmonic separations, luminosity dependence, pulse phase
dependence and more recently the shapes of the line and the trend for long term
evolution in the line energy. This article visits the important results related
to cyclotron lines since its discovery and reviews their significance. An
emphasis is laid on pulse phase resolved spectroscopy and the important clues a
joint timing and spectral study in this context can provide, to build a
complete picture for the physics of accretion and hence X-ray emission in
accreting neutron stars.Comment: Has appeared in Journal of Astrophysics and Astronomy special issue
on 'Physics of Neutron Stars and related objects', celebrating the 75th
birth-year of G. Srinivasa
Prospect of polarisation measurements from black hole binaries in their thermal state with a scattering polarimeter
X-ray polarisation measurement is a unique tool which may provide crucial
information regarding the emission mechanism and the geometry of various
astrophysical sources, like neutron stars, accreting black holes, pulsar wind
nebulae, AGNs, Supernova Remnants etc. and can help us to probe matter under
extreme magnetic fields and extreme gravitational fields. Although the three
other domains of X-ray Astronomy i.e. timing, spectral and imaging are well
developed, there has been very little progress in X-ray Polarimetry with only
one definitive polarisation measurement and a few upper limits available so
far. Radiation from accreting black holes in their thermal dominated (High
Soft) state is expected to be polarised due to scattering in the plane parallel
atmosphere of the disk. Also, special and general relativistic effects in the
innermost parts of the disk predicts energy dependent rotation in the plane of
polarisation and some distinct signatures which can be used as a probe for
measuring the parameters of the black hole like its spin, emissivity profile
and the angle of inclination of the system. We present the results from an
analysis of expected minimum detectable polarisation from some of the galactic
black hole binaries GRO J1655-40, GX339-4, H1743-322, Cyg X-1 and XTE J1817-330
in their thermal dominated state with a proposed Thomson X-ray Polarimeter. A
proposal for a scattering polarimeter has been submitted to the Indian Space
Research Organization (ISRO) for a dedicated small satellite mission and a
laboratory unit has been built. Along with the measurement of the degree of
polarisation, the polarisation angle measurement is also important, hence the
error in the polarisation angle measurement for a range of detection
significance is also obtained.Comment: 12 pages, 13 figures, 2 tables, accepted for publication by MNRA
Detection of a cyclotron line in SXP 15.3 during its 2017 outburst
We report the results of AstroSat and NuSTAR observations of the Be/X-ray
binary pulsar SXP 15.3 in the Small Magellanic Cloud during its outburst in
late 2017, when the source reached a luminosity level of ~ 10^{38} erg s^{-1},
close to the Eddington limit. The unprecedented broadband coverage of the
source allowed us to perform timing and spectral analysis between 3 and 80 keV.
The pulse profile exhibits a significant energy dependence, and morphs from a
double peaked profile to a single broad pulse at energies >15 keV. This can be
explained by a spectral hardening during an intensity dip seen between the two
peaks of the pulse profile. We detect a Cyclotron Resonance Scattering Feature
(CRSF) at ~5 keV in the X-ray spectrum, independent of the choice of the
continuum model. This indicates a magnetic field strength of 6x10^{11} G for
the neutron star.Comment: 5 pages, 6 figures, accepted for publication in MNRAS Letter
Temperature Measurement during Thermonuclear X-ray Bursts with BeppoSAX
We have carried out a study of temperature evolution during thermonuclear
bursts in LMXBs using broad band data from two instruments onboard BeppoSAX,
the MECS and the PDS. However, instead of applying the standard technique of
time resolved spectroscopy, we have determined the temperature in small time
intervals using the ratio of count rates in the two instruments assuming a
blackbody nature of burst emission and different interstellar absorption for
different sources. Data from a total of twelve observations of six sources were
analysed during which 22 bursts were detected. We have obtained temperatures as
high as ~3.0 keV, even when there is no evidence of photospheric radius
expansion. These high temperatures were observed in the sources within
different broadband spectral states (soft and hard).Comment: To appear in New Astronom
NGC 300 ULX1: A test case for accretion torque theory
NGC 300 ULX1 is a newly identified ultra-luminous X-ray pulsar. The system is
associated with the supernova impostor SN 2010da that was later classified as a
possible supergiant Be X-ray binary. In this work we report on the spin period
evolution of the neutron star based on all the currently available X-ray
observations of the system. We argue that the X-ray luminosity of the system
has remained almost constant since 2010, at a level above ten times the
Eddington limit. Moreover, we find evidence that the spin period of the neutron
star evolved from ~126 s down to ~18 s within a period of about 4 years. We
explain this unprecedented spin evolution in terms of the standard accretion
torque theory. An intriguing consequence for NGC 300 ULX1 is that a neutron
star spin reversal should have occurred a few years after the SN 2010da event.Comment: 10 pages with appendix, 9 figures, accepted to A&A Letter
Probing the Cyclotron line characteristics of 4U 1538-522 using AstroSat-LAXPC
We report the first report on cyclotron line studies with the LAXPC
instrument onboard AstroSat of the High mass X-ray Binary pulsar 4U 1538-52.
During the observation of source which spanned about one day with a net
exposure of 50 ks,the source X-ray flux remained constant. Pulse profile is
double peaked in low energy range and has a single peak in high energy range,
the transition taking place around the cyclotron line energy of the source.
Cyclotron Scattering Feature (CRSF) is detected at 22 keV with a very
high significance in phase averaged spectrum. It is one of the highest signal
to noise ratio detection of CRSF for this source. We performed detailed pulse
phase resolved spectral analysis with 10 independent phase bins. We report the
results of pulse phase resolved spectroscopy of the continuum and CRSF
parameters. The cyclotron line parameters show pulse phase dependence over the
entire phase with a CRSF energy variation of 13% which is in agreement
with previous studies. We also confirm the increase in the centroid energy of
the CRSF observed between the 1996-2004 (RXTE) and the 2012 (Suzaku)
observations, reinforcing that the increase was a long-term change.Comment: 6 pages, 7 figures, Accepted for publication in MNRAS Lette
Evolution of timing and spectral characteristics of 4U 1901+03 during its 2019 outburst using the Swift and NuSTAR observatories
We report the results from a detailed timing and spectral study of transient
X-ray pulsars, 4U 1901+03 during its 2019 outburst. We performed broadband
spectroscopy in the 1-70 keV energy band using four observations made with
Swift and NuSTAR at different intensity levels. Our timing results reveal the
presence of highly variable pulse profiles dependent on both luminosity and
energy. Our spectroscopy results showed the presence of a cyclotron resonance
scattering feature (CRSF) at ~30 keV. This feature at 30 keV is highly
luminosity and pulse-phase dependent. Phase-averaged spectra during the last
two observations, made close to the declining phase of the outburst showed the
presence of this feature at around 30 keV. The existence of CRSF at 30 keV
during these observations is well supported by an abrupt change in the shape of
pulse profiles found close to this energy. We also found that 30 keV feature
was significantly detected in the pulse-phase resolved spectra of observations
made at relatively high luminosities. Moreover, all spectral fit parameters
showed a strong pulse phase dependence. In line with the previous findings, an
absorption feature at around 10 keV is significantly observed in the
phase-averaged X-ray spectra of all observations and also showed a strong pulse
phase dependence.Comment: 18 pages, accepted for publication in MNRA
IKT 16: the first X-ray confirmed composite SNR in the SMC
Aims: IKT 16 is an X-ray and radio-faint supernova remnant (SNR) in the Small
Magellanic Cloud (SMC). A detailed X-ray study of this SNR with XMM-Newton
confirmed the presence of a hard X-ray source near its centre, indicating the
detection of the first composite SNR in the SMC. With a dedicated Chandra
observation we aim to resolve the point source and confirm its nature. We also
acquire new ATCA observations of the source at 2.1 GHz with improved flux
density estimates and resolution.
Methods: We perform detailed spatial and spectral analysis of the source.
With the highest resolution X-ray and radio image of the centre of the SNR
available today, we resolve the source and confirm its pulsar wind nebula (PWN)
nature. Further, we constrain the geometrical parameters of the PWN and perform
spectral analysis for the point source and the PWN separately. We also test for
the radial variations of the PWN spectrum and its possible east west asymmetry.
Results: The X-ray source at the centre of IKT 16 can be resolved into a
symmetrical elongated feature centering a point source, the putative pulsar.
Spatial modeling indicates an extent of 5.2 arcsec of the feature with its axis
inclined at 82 degree east from north, aligned with a larger radio feature
consisting of two lobes almost symmetrical about the X-ray source. The picture
is consistent with a PWN which has not yet collided with the reverse shock. The
point source is about three times brighter than the PWN and has a hard spectrum
of spectral index 1.1 compared to a value 2.2 for the PWN. This points to the
presence of a pulsar dominated by non-thermal emission. The expected E_{dot} is
~ 10^37 erg s^-1 and spin period < 100 ms. However, the presence of a compact
nebula unresolved by Chandra at the distance of the SMC cannot completely be
ruled out.Comment: 9 pages, 6 figures, 2 tables, Accepted for publication in Astronomy &
Astrophysic