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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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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 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 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.
We carried out optical high-dispersion spectroscopic monitoring of the Be disk in a Be/X-ray binary A 0535+262/V725 Tau from 2009 to 2012, covering two giant outbursts and several normal outbursts. This monitoring was performed in order to investigate variabilities of the Be disk due to the interaction with the neutron star in recent X-ray active phase from 2008 to 2011. Such variabilities give a clue to uncleared detailed mechanism for very bright X-ray outbursts, which are unique to some Be/X-ray binaries with relatively wide and eccentric orbit. In the previous letter (Moritani et al. 2011), a brief overview of line profile variabilities around the 2009 giant outburst was given and the possibility of a warped Be disk was discussed. In this paper, a full analysis of the Halpha line profiles as well as other line profiles is carried out. A bright blue component, or blue shoulder, showing up after periastron indicates the presence of a dense gas stream toward the neutron star, which is associated with observed outbursts. We re-analyze the Halpha line profiles before 2009 (down to 2005) in order to investigate the variability of the the disk structure in the innermost region, which seems to have detached from the Be star surface by 2008. A redshifted enhanced component is remarkable in all emission lines observed around the 2009 giant outburst, occasionally forming a triple peak. These features indicate that the Be disk was warped in X-ray active phase. We estimate the position of the warped region from fitting the radial velocity of the redshifted enhanced component of Halpha, and find that it was very close to the periastron when two giant outbursts in 2009 and 2011 and a bright normal outburst in 2010 March occurred. These facts strongly suggest that the warped Be disk triggered these giant outbursts.
A normal outburst of the Be/X-ray binary system A0535+26 has taken place in August 2009. It is the fourth in a series of normal outbursts that have occured around the periastron passage of the source, but is unusual by starting at an earlier orbital phase and by presenting a peculiar double-peaked light curve. A first flare (lasting about 9 days from MJD 55043 on) reached a flux of 440 mCrab. The flux then decreased to less than 220 mCrab, and increased again reaching 440 mCrab around the periastron at MJD 55057. Target of Opportunity observations have been performed with INTEGRAL, RXTE and Suzaku. First results of these observations are presented, with special emphasis on the cyclotron lines present in the X-ray spectrum of the source, as well as in the pulse period and energy dependent pulse profiles of the source.
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