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Register a new userStudy of Timing and Spectral Properties of the X-ray Pulsar A 0535+262 During the Giant Outburst in Nov-Dec 2020
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We made a multi-wavelength study of the timing and spectral properties of the X-ray pulsar A 0535+262 during a recent giant outburst in November and December 2020. The flux of the pulsar reached a record value of $sim$12.5 Crab on 19th November 2020 (MJD 59172). We have used the NuSTAR, Swift, and NICER data for our study. We have studied the evolution of pulse frequency, pulse profile, and different spectral parameters during the giant outburst. The variation of pulse fraction in different energy ranges has been studied. We have detected a textit q like feature for the X-ray pulsar during the outburst from the hardness intensity diagram. We investigated the evolution of the pulse period and found the spin period of the neutron star to be $P = 103.58pm 0.01$ s based on NuSTAR data during the rising phase of the outburst. It was found that the spin period decreased with time at a rate of $dot P= -1.50pm 0.05times10^{-7}$ ss$^{-1}$ during the outburst. The timing results revealed the presence of highly variable pulse profiles. The pulse profile evolved from a double peak feature to a single peak in a higher energy range and prominent energy dependence of the pulse profile was established. The variation of pulse fraction with energy is studied during the different days of the observations. The cyclotron resonant scattering feature (CRSF) from the spectrum have been detected at $sim$44 keV and the corresponding magnetic field is B $sim$4.9$times10^{12}$ G. We have studied the broadband spectrum of the source which can be described by a composite model with two continuum components -- a blackbody emission and a cut-off power law. An emission line of iron ($K_{alpha}$) near 6.4 keV has been detected from the energy spectrum.
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Giant X-ray outbursts, with luminosities of about $ 10^{37}$ erg s$^{-1}$, are observed roughly every 5 years from the nearby Be/pulsar binary 1A 0535+262. In this article, we present observations of the source with VERITAS at very-high energies (VHE; E$>$100 GeV) triggered by the X-ray outburst in December 2009. The observations started shortly after the onset of the outburst, and they provided comprehensive coverage of the episode, as well as the 111-day binary orbit. No VHE emission is evident at any time. We also examined data from the contemporaneous observations of 1A 0535+262 with the Fermi/LAT at high energy photons (HE; E$>$0.1 GeV) and failed to detect the source at GeV energies. The X-ray continua measured with the Swift/XRT and the RXTE/PCA can be well described by the combination of blackbody and Comptonized emission from thermal electrons. Therefore, the gamma-ray and X-ray observations suggest the absence of a significant population of non-thermal particles in the system. This distinguishes 1A~0535+262 from those Be X-ray binaries (such as PSR B1259--63 and LS I +61$^{circ}$303) that have been detected at GeV--TeV energies. We discuss the implications of the results on theoretical models.
1A 0535+262 is a Be X-ray binary pulsar and one of the only galactic pulsar systems to show radio jet emission. Characterizing the very high energy emission (VHE, >100 GeV) in these extreme microquasars is critical to understanding their contribution to the origin of galactic cosmic rays. The 2020 giant outburst of this system, where X-ray fluxes exceeded 12 Crab, marked a rare opportunity to investigate the gamma-ray and rapid optical variability of these transient systems while in such an extreme state. This month of activity marked the brightest flare measured in this system. VERITASs developing optical capabilities in tandem with the ability to measure TeV gamma rays allowed for a unique campaign to be undertaken. VERITASs observations of this system during the outburst will be presented in the context of observations at lower energies and previous observations of this system by imaging atmospheric Cherenkov telescopes.
We report on a detailed spectral analysis of the transient X-ray pulsar 1A~0535+262, which underwent the brightest giant outburst ever recorded for this source from November to December 2020 with a peak luminosity of $1.2$ $times10^{38} rm erg s^{-1}$. Thanks to the unprecedented energy coverage and high cadence observations provided by Insight-HXMT, we were able to find for the first time evidence for a transition of the accretion regime. At high luminosity, above the critical luminosity $6.7times10^{37}$ erg s$^{-1}$, the cyclotron absorption line energy anti-correlates with luminosity. Below the critical luminosity, a positive correlation is observed. The 1A~0535+262 becomes, therefore, the second source after V~0332+53, which clearly shows an anti-correlation above and transition between correlation and anti-correlation around the critical luminosity. The evolution of both the observed CRSF line energy and broadband X-ray continuum spectrum throughout the outburst exhibits significant differences during the rising and fading phases: that is, for a similar luminosity the spectral parameters take different values which results in hysteresis patterns for several spectral parameters including the cyclotron line energy. We argue that, similarly to V~0332+53, these changes might be related to different geometry of the emission region in rising and declining parts of the outburst, probably due to changes in the accretion disk structure and its interaction with the magnetosphere of the neutron star.
We present results from a study of broadband timing and spectral properties of EXO 2030+375 using a Suzaku observation. Pulsations with a period of 41.41 s and strong energy dependent pulse profiles were clearly detected up to ~100 keV. Narrow dips are seen in the profiles up to ~70 keV. Presence of prominent dips at several phases in the profiles up to such high energy ranges were not seen before. At higher energies, these dips gradually disappeared and the profile appeared single-peaked. The 1.0-200.0 keV broad-band spectrum is found to be well described by a partial covering high energy cut-off power-law model. Several low energy emission lines are also detected in the pulsar spectrum. We fitted the spectrum using neutral as well as partially ionized absorbers along with above continuum model yielding similar parameter values. The partial covering with partially ionized absorber resulted into marginally better fit. The spectral fitting did not require any cyclotron feature in the best fit model. To investigate the changes in spectral parameters at dips, we carried out pulse-phase-resolved spectroscopy. During the dips, the value of additional column density was estimated to be high compared to other pulse phases. While using partially ionized absorber, the value of ionization parameter is also higher at the dips. This may be the reason for the presence of dips up to higher energies. No other spectral parameters show any systematic variation with pulse phases of the pulsar.
We have studied the evolution of different timing and spectral properties of the X-ray pulsar 2S 1417--624 during the recent outburst in January 2021 based on the Neutron Star Interior Composition Explorer (NICER) observations. The spin period during the outburst is $P sim$17.3622 s based on the NICER data and the period decreases slowly with time. The evolution of the spin period and pulsed flux is studied with Fermi/GBM during the outburst and the spin-up rate was found to be varied between $simeq$(0.8--1.8)$times$10$^{-11}$ Hz s$^{-1}$. The pulse profile shows strong energy dependence and variability. The pulse profile shows multiple peaks and dips which evolve significantly with energy. The pulsed fraction shows a positive correlation with energy. The evolution of the spectral state is also studied. The NICER energy spectrum is well described with a composite model of -- power-law and a blackbody emission along with a photo-electric absorption component. An iron emission line is detected near 6.4 keV in the NICER spectrum with an equivalent width of about 0.05 keV. During the recent outburst, the flux was relatively low compared to the 2018 outburst and the mass accretion rate was also low. The mass accretion rate is estimated to be $simeq$1.3 $times$ 10$^{17}$ g s$^{-1}$ near the peak of the outburst. We have found a positive correlation between the pulse frequency derivatives and luminosity. The GL model was applied to estimate the magnetic field in low mass accretion rate and different spin-up rates, which is compared to the earlier estimated magnetic field at a relatively high mass accretion rate. The magnetic field is estimated to be $simeq$10$^{14}$ G from the torque-luminosity model, which is comparatively higher than most of the other Be/XBPs.
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