Long-term RXTE/ASM (1.5-12 keV) light curve of 1A 0535+262, from 1996 January 5 to 2010 September 9. Note the presence of outbursts of diff erent types.

Reference: V. A. Acciari et al. (The VERITAS Collaboration), Astrophysical Journal 733: 96, 2011

Full text version

ArXiv: ArXiV:1103.3250

Contacts: Wei Cui, Gernot Maier, Angelo Varlotta

Triggered by the December 2009 major X-ray outburst of the Be/X-ray binary system 1A0535+262, we have conducted multiwavelength observations of this system with VERITAS at very-high energies (VHE; E>100 GeV) and with Fermi/LAT at high energies (HE; E>0.1 GeV). The VERITAS observations were carried out from 2009/12/06 to 2010/02/20, while the contemporaneous Fermi/LAT observations were conducted between 2009/11/30 and 2010/02/22. Neither observation resulted in a detection. X-ray observations were also conducted by Swift/XRT (from 2009/12/07 to 2010/12/27) and by RXTE/PCA (from 2009/12/04 to 2010/01/07).

High-mass X-ray binaries are binary systems in which the mass of the secondary star exceeds 10 solar masses. The majority are Be/X-ray binaries (BeXRBs), consisting of a Be or O star and a neutron star, and some show up observationally as X-ray pulsars. Be or Oe stars are known to be rapid rotators, forming a circumstellar disk. BeXRBs exhibit flux variability over a wide range of wavelengths, which can be regular or in sudden and strong outbursts.

The data for 1A 0535+262 were arranged in four periods: rising X-ray, falling X-ray, apastron and approaching periastron period. No VHE or HE emission has been detected over the whole orbital period, either during or after the X-ray flare. We model the Swift/XRT and RXTE/PCA data jointly, understanding that the source could have changed between the two. The thermal Comptonization model (comptt) with untied black-body temperature and normalization parameters gave the best fit, which seems to indicate the thermal origin of the X-rays.

Results from X-ray spectral modeling show that the observed X-ray emission is likely of thermal origin. No significant contribution from non-thermal particles is seen. This is in line with the lack of gamma-ray production in the source. Cheng & Ruderman (1989, 1991, ApJ) proposed a mechanism in which protons are accelerated in the electrostatic potential gap of a pulsar and collide with protons in the accretion disk producing neutral pions, which then decay to produce gamma rays. Applying it to 1A0535+262, Orellana et al. (2007, ApJ) predicted the fluxes of gamma-ray emission from the source. The VERITAS and Fermi upper limits are comparable to or below the predictions, so our results begin to severely constrain this hadronic model. On the other hand, PSR B1259-63, which is also a BeXRB, has been detected both at GeV and TeV energies. We are beginning to see diverse behaviors among BeXRBs in terms of gamma ray production.

Figures from paper (click to get full size image):

Figure 1: Long-term RXTE/ASM (1.5-12 keV) light curve of 1A 0535+262, from 1996 January 5 to 2010 September 9. Note the presence of outbursts of di fferent types.

FIgure 2a: Swift/BAT counting rate vs time in the 15-50 keV energy range. The horizontal line at 0.1 cts/cm^2/s indicates the trigger threshold for observations with VERITAS.

Figure 2b: Relative orbit of the neutron star around the Be star. The primary star lies in the focus of the ellipse (0,0) and the axis units are multiples of the semi-major axis of the orbit. Note that the inclination of the system is unknown. Indicated in colors is the Swift/BAT counting rate in the 15-50 keV energy range for the orbit starting in November 2009. The dashed lines indicate nights with VERITAS observations, covering the are, apastron, and periods close to periastron. Orbital parameters after Coe et al. (2006).

FIgure 3: VERITAS signi cance map of the region around 1A 0535+262 in equatorial coordinates for the period MJD 57166-55250. The location of 1A 0535+262 is indicated by a black cross at the center. Also shown are regions excluded from the background calculation due to bright stars. The numbers beside the excluded regions indicate the B magnitude of these stars. The circle at the bottom right indicates the angular resolution of the VERITAS observations.

Figure 4a: Joint Swift/XRT and RXTE/PCA spectral modeling of 1A 0535+262 for the rising portion of the X-ray outburst. The upper panel shows the Swift/XRT data (lower curve) and the RXTE/PCA data (upper curve), and the lower panel shows the residuals of the fit. The fit to the data is a model that consists of blackbody radiation and thermal Comptonization, with all physical parameters tied for the XRT and PCA data sets.

FIgure 4b: Joint Swift/XRT and RXTE/PCA spectral modeling of 1A 0535+262 for the rising portion of the X-ray outburst. The upper panel shows the Swift/XRT data (lower curve) and the RXTE/PCA data (upper curve), and the lower panel shows the residuals of the fi t. A fit to the data with the same model as in Figure 4a but with the blackbody temperature and normalization untied between the two data sets.