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):