2011ATel.3204....1C

Two INTEGRAL target of opportunity observations were performed from 2011-02-23 21:40 to 2011-02-25 02:20 UT and from 2011-02-27 11:25 to 2011-02-28 01:44

(total exposure time ∼150 ks) to study the on-going giant outburst of the Be X-ray binary 1A 0535+262 (ATels #3166, #3173, and #3176 ).

We investigated the average broad band spectral emission in the 5-200 keV range by using the combined JEM-X (5-25 keV) + IBIS/ISGRI (20-200 keV) + SPI (25-200 keV) spectrum. The continuum emission could be reasonably well described by using a power-law model with an exponential roll-over at high energy (photon index=0.19 +0.07/-0.08, cut-off energy=13.5 ± 0.4 keV). One broad absorption feature was clearly visible from the residuals of the fit, interpreted as a cyclotron line. We accounted for this feature by adding to the spectral model one multiplicative Gaussian line and obtained the following preliminary results:

centroid energy: 43.3 +1.3/-1.6 keV

Gaussian width: 14.6 +2.8/-2.1 keV

optical depth: 0.33 +0.11/-0.06

(all uncertainties are at 90% c.l.).

We obtain a chi2(min)/dof=1.8 for 136 dof adding a systematic uncertainty of 5% to the JEMX-1, JEMX-2 data points, 2% to ISGRI and 1% to SPI. The estimated 5-200 keV X-ray flux of the source was (7.8 ± 0.2 )1e-8 erg s^-1 cm^-2 (i.e., ∼3.6 Crab), roughly constant throughout the observations. We note that INTEGRAL never observed 1A 0535+262 at such a high luminosity.

Further INTEGRAL and RXTE target of opportunity observations of the outburst are on-going.