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Register a new userLuminosity dependence of the cyclotron line energy in 1A 0535+262 observed by Insight-HXMT during 2020 giant outburst
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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.
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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.
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.
Cyclotron line scattering features are detected in a few tens of X-ray pulsars (XRPs) and used as direct indicators of a strong magnetic field at the surface of accreting neutron stars (NSs). In a few cases, cyclotron lines are known to be variable with accretion luminosity of XRPs. It is accepted that the observed variations of cyclotron line scattering features are related to variations of geometry and dynamics of accretion flow above the magnetic poles of a NS. A positive correlation between the line centroid energy and luminosity is typical for sub-critical XRPs, where the accretion results in hot spots at the magnetic poles. The negative correlation was proposed to be a specific feature of bright super-critical XRPs, where radiation pressure supports accretion columns above the stellar surface. Cyclotron line in spectra of Be-transient X-ray pulsar GRO J1008-57 is detected at energies from $sim 75 -90$ keV, the highest observed energy of cyclotron line feature in XRPs. We report the peculiar relation of cyclotron line centroid energies with luminosity in GRO J1008-57 during the Type II outburst in August 2017 observed by Insight-HXMT. The cyclotron line energy was detected to be negatively correlated with the luminosity at $3.2times 10^{37},ergs<L<4.2times 10^{37},ergs$, and positively correlated at $Lgtrsim 5times 10^{37},ergs$. We speculate that the observed peculiar behavior of a cyclotron line would be due to variations of accretion channel geometry.
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.
The Be/X-ray binary 3A 0535+262 has the highest magnetic field determined by cyclotron line studies of all accreting X-ray pulsars, despite an open debate if the fundamental line was rather at ~50 or above 100 keV as observed by different instruments in past outbursts. The source went into quiescence for more than ten years since its last outbursts in 1994. Observing during a `normal outburst August/September 2005 with Integral and RXTE we find a strong cyclotron line feature at ~45 keV and have for the first time since 1975 determined the low energy pulse profile.
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