The first giant outburst of A0535+26 caught by INTEGRAL
A0535+26 is an archetypical Be/X-ray binary transient fueled by
the activity of its donor star. A long quiescent period ended in
2005 with a giant outburst, followed by further normal and giant
outbursts in the following years. But, as if to taunt the
interested observers, the giant outbursts of 2005 (~5.2 Crab, 15-50 keV range) and 2009 (~5.6 Crab) were not observable by INTEGRAL - and various
other satellites - because of Sun viewing constraints. Only
several smaller outbursts could be observed, see e.g.,
INTEGRAL POM October 2005
Finally, during a recent giant outburst in February 2011, which reached a
flux of ~3.8 Crab, INTEGRAL could be re-pointed to monitor the outburst's evolution. TOO observations were performed every three days starting at the peak and followed the outburst through its slow decay, as shown in the image above.
A0535+26 is a highly magnetized accreting neutron star, with a magnetic field of ~4x1012 G. The magnetic field is derived from the cyclotron lines present in the X-ray spectrum of the source at ~45 and ~100 keV. Cyclotron lines are very powerful tools to directly derive a neutron star's magnetic field. Moreover,
the dependence of the cyclotron line energy on the X-ray luminosity can be used as a test to probe the accretion theory.
Contrary to other sources, for A0535+26 so far no significant variation of the
cyclotron line with the X-ray luminosity has been observed, and this behavior
will be studied with the new INTEGRAL observations in a much wider luminosity
range.
The top panel of the image shows the Swift BAT light curve during the outburst, with red ticks indicating the times of the INTEGRAL observations. The vertical dotted line indicates the time of periastron. The insert shows the long term Swift BAT light curve, where the past 2005 and 2009 giant outbursts are seen.
Weaker normal outbursts in 2005, 2009 and 2010, also observed by INTEGRAL, are also visible.
The bottom panel shows the INTEGRAL broad-band photon spectrum of the source during one of the observations of the outburst, at a flux of ~0.47 Crab (15-50 keV range), in which the fundamental cyclotron line is observed at E=45.7 +/- 0.7 keV.
