arXiv subjects

Gaurava K. Jaisawal

Publications and source records attributed to Gaurava K. Jaisawal.

At least 73 records · Page 4Linked to original sources

arXiv2019

Neutron Star Interior Composition Explorer X-ray Timing of the Radio and Gamma-ray Quiet Pulsars PSR J1412+7922 AND PSR J1849-0001

We present new timing and spectral analyses of PSR J1412+7922 (Calvera) and PSR J1849-0001, which are only seen as pulsars in X-rays, based on observations conducted with the Neutron Star Interior Composition Explorer (NICER). We obtain updated and substantially improved pulse ephemerides compared to previous X-ray studies, as well as spectra that can be well-fit by simple blackbodies and/or a power law. Our refined timing measurements enable deeper searches for pulsations at other wavelengths and sensitive targeted searches by LIGO/Virgo for continuous gravitational waves from these neutron stars. Using the sensitivity of LIGO's first observing run, we estimate constraints that a gravitational wave search of these pulsars would obtain on the size of their mass deformation and r-mode fluid oscillation.

astro-ph.HE↗

arXiv2018

NICER Discovers mHz Oscillations in the "Clocked" Burster GS 1826-238

We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of mHz X-ray brightness oscillations from the "clocked burster." NICER observed the source in the periods 2017 June 20 - 29, July 11 - 13, and September 9 - 15, for a total useful exposure of 34 ks. Two consecutive dwells obtained on 2017 September 9 revealed highly significant oscillations at a frequency of 8 mHz. The fractional, sinusoidal modulation amplitude increases from 0.7 % at 1 keV to approximately 2 % at 6 keV. Similar oscillations were also detected at lower significance in three additional dwells. The oscillation frequency and amplitude are consistent with those of mHz QPOs reported in other accreting neutron star systems. A thermonuclear X-ray burst was also observed on 2017 June 22. The burst properties and X-ray colors are both consistent with the source being in a soft spectral state during these observations, findings that are confirmed by ongoing monitoring with MAXI and SWIFT-BAT. Assuming that the mHz oscillations are associated with black body emission from the neutron star surface, modeling of the phase-resolved spectra shows that the oscillation is consistent with being produced by modulation of the temperature component of this emission. In this interpretation, the black body normalization, proportional to the emitting surface area, is consistent with being constant through the oscillation cycle. We place the observations in the context of the current theory of marginally stable burning and briefly discuss the potential for constraining neutron star properties using mHz oscillations.

astro-ph.HE↗

arXiv2018

NICER Discovers the Ultracompact Orbit of the Accreting Millisecond Pulsar IGR J17062-6143

We present results of recent Neutron Star Interior Composition Explorer observations of the accreting millisecond X-ray pulsar IGR J17062-6143 that show that it resides in a circular, ultracompact binary with a 38 minute orbital period. NICER observed the source for approximately 26 ksec over a 5.3 day span in 2017 August, and again for 14 and 11 ksec in 2017 October and November, respectively. A power spectral analysis of the August exposure confirms the previous detection of pulsations at 163.656 Hz in Rossi X-ray Timing Explorer data, and reveals phase modulation due to orbital motion of the neutron star. A coherent search for the orbital solution using the Z^2 method finds a best-fitting circular orbit with a period of 2278.21 s (37.97 min), a projected semi-major axis of 0.00390 lt-sec, and a barycentric pulsar frequency of 163.6561105 Hz. This is currently the shortest known orbital period for an AMXP. The mass function is 9.12 e-8} solar masses, presently the smallest known for a stellar binary. The minimum donor mass ranges from about 0.005 - 0.007 solar masses, for a neutron star mass from 1.2 - 2 solar masses. Assuming mass transfer is driven by gravitational radiation, we find donor mass and binary inclination bounds of 0.0175 - 0.0155 solar masses and 19 deg < i < 27.5 deg, where the lower and upper bounds correspond to 1.4 and 2 solar mass neutron stars, respectively. Folding the data accounting for the orbital modulation reveals a sinusoidal profile with fractional amplitude 2.04 +- 0.11 % (0.3 - 3.2 keV).

astro-ph.HE↗

arXiv2018

On the 2018 outburst of the accreting millisecond X-ray pulsar Swift J1756.9-2508 as seen with NICER

We report on the coherent timing analysis of the 182 Hz accreting millisecond X-ray pulsar Swift J1756.9$-$2508 during its 2018 outburst as observed with the Neutron Star Interior Composition Explorer (NICER). Combining our NICER observations with Rossi X-ray Timing Explorer observations of the 2007 and 2009 outbursts, we also studied the long-term spin and orbital evolution of this source. We find that the binary system is well described by a constant orbital period model, with an upper limit on the orbital period derivative of $|\dot P_b| < 7.4\times 10^{-13}$ s s$^{-1}$. Additionally, we improve upon the source coordinates through astrometric analysis of the pulse arrival times, finding R.A. = $17^{\rm h}56^{\rm m}57.18^{\rm s}\pm0.08^{\rm s}$ and Decl. = $-25^{\circ}06'27.8''\pm3.5''$, while simultaneously measuring the long-term spin frequency derivative as $\dotν= -7.3\times 10^{-16}$ Hz s$^{-1}$. We briefly discuss the implications of these measurements in the context of the wider population of accreting millisecond pulsars.

astro-ph.HE↗

arXiv2018

NICER Detects a Soft X-ray Kilohertz Quasi-periodic Oscillation in 4U 0614+09

We report on the detection of a kilohertz quasi-periodic oscillation (QPO) with the Neutron Star Interior Composition Explorer (NICER). Analyzing approximately 165 ks of NICER exposure on the X-ray burster 4U 0614+09, we detect multiple instances of a single-peak upper kHz QPO, with centroid frequencies that range from 400 Hz to 750 Hz. We resolve the kHz QPO as a function of energy, and measure, for the first time, the QPO amplitude below 2 keV. We find the fractional amplitude at 1 keV is on the order of 2% rms, and discuss the implications for the QPO emission process in the context of Comptonization models.

astro-ph.HE↗

arXiv2018

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.

astro-ph.HE↗

arXiv2018

A NICER look at the Aql X-1 hard state

We report on a spectral-timing analysis of the neutron star low-mass X-ray binary Aql~X-1 with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station. Aql~X-1 was observed with NICER during a dim outburst in 2017 July, collecting approximately $50$ ks of good exposure. The spectral and timing properties of the source correspond to that of an (hard) extreme island state in the atoll classification. We find that the fractional amplitude of the low frequency ($<0.3$ Hz) band-limited noise shows a dramatic turnover as a function of energy: it peaks at 0.5 keV with nearly 5\% rms, drops to $12\%$ rms at 2 keV, and rises to $15\%$ rms at 10 keV. Through the analysis of covariance spectra, we demonstrate that band-limited noise exists in both the soft thermal emission and the power-law emission. Additionally, we measure hard time lags, indicating the thermal emission at $0.5$ keV leads the power-law emission at 10 keV on a timescale of $\sim100$ ms at $0.3$ Hz to $\sim10$ ms at $3$ Hz. Our results demonstrate that the thermal emission in the hard state is intrinsically variable, and driving the modulation of the higher energy power-law. Interpreting the thermal spectrum as disk emission, we find our results are consistent with the disk propagation model proposed for accretion onto black holes.

astro-ph.HE↗

arXiv2018

A curious case of the accretion-powered X-ray pulsar GX 1+4

We present detailed spectral and timing studies using a NuSTAR observation of GX 1+4 in October 2015 during an intermediate intensity state. The measured spin period of 176.778 s is found to be one of the highest values since its discovery. In contrast to a broad sinusoidal-like pulse profile, a peculiar sharp peak is observed in profiles below ~25 keV. The profiles at higher energies are found to be significantly phase-shifted compared to the soft X-ray profiles. Broadband energy spectra of GX 1+4, obtained from NuSTAR and Swift observations, are described with various continuum models. Among these, a two component model consisting of a bremsstrahlung and a blackbody component is found to best-fit the phase-averaged and phase-resolved spectra. Physical models are also used to investigate the emission mechanism in the pulsar, which allows us to estimate the magnetic field strength to be in $\sim$(5-10)$\times$10$^{12}$ G range. Phase-resolved spectroscopy of NuSTAR observation shows a strong blackbody emission component in a narrow pulse phase range. This component is interpreted as the origin of the peculiar peak in the pulse profiles below $\le$25 keV. The size of emitting region is calculated to be $\sim$400 m. The bremsstrahlung component is found to dominate in hard X-rays and explains the nature of simple profiles at high energies.

astro-ph.HE↗

arXiv2017

Understanding the spectral and timing behavior of a newly discovered transient X-ray pulsar Swift J0243.6+6124

We present the results obtained from timing and spectral studies of the newly discovered accreting X-ray binary pulsar Swift J0243.6+6124 using a NuSTAR observation in 2017 October at a flux level of ~280 mCrab. Pulsations at 9.85423(5) s were detected in the X-ray light curves of the pulsar. Pulse profiles of the pulsar were found to be strongly energy dependent. A broad profile at lower energies was found to evolve into a double peaked profile in $\ge$30keV. The 3-79 keV continuum spectrum of the pulsar was well described with a negative and positive exponential cutoff or high energy cutoff power law models modified with a hot blackbody at $\sim$3 keV. An iron emission line was also detected at 6.4 keV in the source spectrum. We did not find any signature of cyclotron absorption line in our study. Results obtained from phase-resolved and time-resolved spectroscopy are discussed in the paper.

astro-ph.HE↗

arXiv2017

A Suzaku View of Accretion Powered X-ray Pulsar GX 1+4

We present results obtained from a Suzaku observation of the accretion powered X-ray pulsar GX 1+4. Broad-band continuum spectrum of the pulsar was found to be better described by a simple model consisting of a blackbody component and an exponential cutoff power-law than the previously used compTT continuum model. Though the pulse profile had a sharp dip in soft X-rays ($<$10 keV), phase-resolved spectroscopy confirmed that the dimming was not due to increase in photoelectric absorption. Phase-sliced spectral analysis showed the presence of a significant spectral modulation beyond 10 keV except for the dip phase. A search for the presence of cyclotron resonance scattering feature in the Suzaku spectra yielded a negative result. Iron K-shell (K$_α$ and K$_β$) emission lines from nearly neutral iron ions ($<$Fe III) were clearly detected in the source spectrum. A significant K$_α$ emission line from almost neutral Ni atoms was detected for the first time in this source. We estimated the iron abundance of $\sim$80 % of the solar value and Ni/Fe abundance ratio of about two times of the solar value. We searched for a iron Ly$_α$ emission line and found a significant improvement in the spectral fitting by inclusion of this line.

astro-ph.HE↗

arXiv2017

Decade long RXTE monitoring observations of Be/X-ray binary pulsar EXO 2030+375

We present a comprehensive timing and spectral studies of Be/X-ray binary pulsar EXO 2030+375 using extensive Rossi X-ray Timing Explorer observations from 1995 till 2011, covering numerous Type I and 2006 Type II outbursts. Pulse profiles of the pulsar were found to be strongly luminosity dependent. At low luminosity, the pulse profile consisted of a main peak and a minor peak that evolved into a broad structure at high luminosity with a significant phase shift. A narrow and sharp absorption dip, also dependent on energy and luminosity, was detected in the pulse profile. Comparison of pulse profiles showed that the features at a particular luminosity are independent of type of X-ray outbursts. This indicates that the emission geometry is solely a function of mass accretion rate. The broadband energy spectrum was described with a partial covering high energy cutoff model as well as a physical model based on thermal and bulk Comptonization in accretion column. We did not find any signature of cyclotron resonance scattering feature in the spectra obtained from all the observations. A detailed analysis of spectral parameters showed that, depending on source luminosity, the power-law photon index was distributed in three distinct regions. It suggests the phases of spectral transition from sub-critical to super-critical regimes in the pulsar as proposed theoretically. A region with constant photon index was also observed in ~(2-4) x 10^37 erg/s range, indicating critical luminosity regime in EXO 2030+375.

astro-ph.HE↗

arXiv2017

Cyclotron absorption lines in the era of Suzaku and NuSTAR

Cyclotron resonance scattering features or cyclotron absorption lines are unique features observed in the hard X-ray spectra of accretion powered X-ray pulsars with magnetic field of the order of 10$^{12}$ G. Detection of these features enables us for the direct estimation of strength of the magnetic field close to the neutron star surface. Corresponding to magnetic field of $\sim$10$^{12}$ G, the fundamental cyclotron lines are expected in 10-100 keV energy range with harmonics expected at multiples of fundamental line energy. However, we detected first harmonic of cyclotron line at less than twice of the fundamental line energy ($\sim$1.7 times the fundamental line energy) in Be/X-ray binary pulsar Cep X-4. With the broadband spectral capability of $Suzaku$ and $NuSTAR$ observatories, we have investigated cyclotron resonance scattering features in several X-ray pulsars to understand the shape of the lines, width, magnetic field mapping, anharmonicity in the line energies and luminosity-dependent properties of cyclotron lines. The results obtained from these works and new detection of cyclotron line in unknown/poorly studied sources are presented in this paper.

astro-ph.HE↗

arXiv2017

Detection of Cyclotron Resonance Scattering Feature in High Mass X-ray Binary Pulsar SMC X-2

We report broadband spectral properties of the high mass X-ray binary pulsar SMC X-2 by using three simultaneous $NuSTAR$ and $Swift$/XRT observations during its 2015 outburst. The pulsar was significantly bright, reaching a luminosity up to as high as $\sim$5.5$\times$10$^{38}$~ergs~s$^{-1}$ in 1-70 keV range. Spin period of the pulsar was estimated to be 2.37 s. Pulse profiles were found to be strongly luminosity dependent. The 1-70 keV energy spectrum of the pulsar was well described with three different continuum models such as (i) negative and positive power-law with exponential cutoff, (ii) Fermi-Dirac cutoff power-law and (iii) cutoff power-law models. Apart from the presence of an iron line at $\sim$6.4 keV, a model independent absorption like feature at $\sim$27 keV was detected in the pulsar spectrum. This feature was identified as a cyclotron absorption line and detected for the first time in this pulsar. Corresponding magnetic field of the neutron star was estimated to be $\sim$2.3$\times$10$^{12}$ G. The cyclotron line energy showed a marginal negative dependence on the luminosity. The cyclotron line parameters were found to be variable with pulse phase and interpreted as due to the effect of emission geometry or complicated structure of the pulsar magnetic field.

astro-ph.HE↗

arXiv2016

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 $\sim$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 $\sim$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 $\sim$70 keV. The 1-110 keV broad-band spectroscopy of both observations revealed that the best-fit model 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 atom. 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 $\sim$1.2$\times$10$^{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 pulsar's soft X-ray excess emission. 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.

astro-ph.SR↗

arXiv2016

Investigation of iron emission lines in the eclipsing high mass X-ray binary pulsar OAO 1657-415

We present the results obtained from timing and spectral studies of high mass X-ray binary pulsar OAO 1657-415 using a Suzaku observations in 2011 September. X-ray pulsations were detected in the light curves up to $\sim$70 keV. The continuum spectra during the high- and low-flux regions in light curves were well described by high energy cutoff power-law model along with a blackbody component and iron fluorescent lines at 6.4 keV and 7.06 keV. Time resolved spectroscopy was carried out by dividing the entire observations into 18 narrow segments. Presence of additional dense matter at various orbital phases was confirmed as the cause of low-flux regions in the observations. Presence of additional matter at several orbital phases of the pulsar was interpreted as due to the inhomogeneously distributed clumps of matter around the neutron star. Using clumpy wind hypothesis, the physical parameters of the clumps causing the high- and low-flux episodes in the pulsar light curve were estimated. The equivalent width of iron emission lines was found to be significantly large at certain orbital phases of low-flux segments. We investigated the iron line emitting regions and suggest the existence of neutral and ionized iron atoms in emission sites that are located within the accretion radius.

astro-ph.SR↗

arXiv2016

Suzaku view of Be/X-ray binary pulsar GX 304-1 during Type I X-ray outbursts

We report the timing and spectral properties of Be/X-ray binary pulsar GX 304-1 by using two Suzaku observations during its 2010 August and 2012 January X-ray outbursts. Pulsations at ~275 s were clearly detected in the light curves from both the observations. Pulse profiles were found to be strongly energy-dependent. During 2010 observation, prominent dips seen in soft X-ray ($\leq$10 keV) pulse profiles were found to be absent at higher energies. However, during 2012 observation, the pulse profiles were complex due to the presence of several dips. Significant changes in the shape of the pulse profiles were detected at high energies ($>$35 keV). A phase shift of $\sim$0.3 was detected while comparing the phase of main dip in pulse profiles below and above $\sim$35 keV. Broad-band energy spectrum of pulsar was well described by a partially absorbed Negative and Positive power-law with Exponential cutoff (NPEX) model with 6.4 keV iron line and a cyclotron absorption feature. Energy of cyclotron absorption line was found to be $\sim$53 and 50 keV for 2010 and 2012 observations, respectively, indicating a marginal positive dependence on source luminosity. Based on the results obtained from phase-resolved spectroscopy, the absorption dips in the pulse profiles can be interpreted as due to the presence of additional matter at same phases. Observed positive correlation between cyclotron line energy and luminosity, and significant pulse-phase variation of cyclotron parameters are discussed in the perspective of theoretical models on cyclotron absorption line in X-ray pulsars.

astro-ph.SR↗

arXiv2016

Detection of fundamental and first harmonic cyclotron line in X-ray pulsar Cep X-4

We report the broad-band spectral properties of the X-ray pulsar Cep X-4 by using a Suzaku observation in 2014 July. The 0.8-70 keV spectrum was found to be well described by three continuum models - Negative and Positive power-law with Exponential cutoff (NPEX), high energy cutoff power-law and CompTT models. Additional components such as a cyclotron line at $\sim$28 keV and two Gaussian components for iron lines at 6.4 and 6.9 keV were required in the spectral fitting. Apart from these, an additional absorption feature at $\sim$45 keV was clearly detected in residuals obtained from the spectral fitting. This additional feature at $\sim$45 keV was clearly seen in phase-resolved spectra of the pulsar. We identified this feature as the first harmonic of the fundamental cyclotron line at $\sim$28 keV. The ratio between the first harmonic and fundamental line energies (1.7) was found to be in disagreement with the conventional factor of 2, indicating that the heights of line forming regions are different or viewed at larger angles. The phase-resolved spectroscopy of the fundamental and first harmonic cyclotron lines shows significant pulse-phase variation of the line parameters. This can be interpreted as the effect of viewing angle or the role of complicated magnetic field of the pulsar.

astro-ph.SR↗

arXiv2015

Broadband spectroscopy of the eclipsing high mass X-ray binary 4U 1700-37 with Suzaku

We present the results obtained from broadband spectroscopy of the high mass X-ray binary 4U 1700-37 using data from a Suzaku observation in 2006 September 13-14 covering 0.29-0.72 orbital phase range. The light curves showed significant and rapid variation in source flux during entire observation. We did not find any signature of pulsations in the light curves. However, a quasi-periodic oscillation at ~20 mHz was detected in the power density spectrum of the source. The 1-70 keV spectrum was fitted with various continuum models. However, we found that the partially absorbed high energy cutoff power-law and Negative and Positive power-law with Exponential cutoff (NPEX) models described the source spectrum well. Iron emission lines at 6.4 keV and 7.1 keV were detected in the source spectrum. An absorption like feature at ~39 keV was detected in the residuals while fitting the data with NPEX model. Considering the feature as cyclotron absorption line, the surface magnetic field of the neutron star was estimated to be ~3.4 times 10^12 Gauss. To understand the cause of rapid variation in the source flux, time-resolved spectroscopy was carried out by dividing the observation into 20 narrow segments. The results obtained from the time-resolved spectroscopy are interpreted as the accretion of inhomogeneously distributed matter in the stellar wind of the supergiant companion star as the cause of observed flux variation in 4U 1700-37. A sharp increase in column density after ~0.63 orbital phase indicates the presence of an accretion wake that blocks the continuum and produces the eclipse like low-flux segment.

astro-ph.SR↗