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Integral observes possible cyclotron line at 47 keV for 1A 0535+262

ATel #601; P. Kretschmar (ESA/ESAC, Spain); I. Kreykenbohm (IAA Tuebingen/ISDC); K. Pottschmidt (CASS/UC San Diego); J. Wilms (Warwick); W. Coburn, S. Boggs (SSL/UC Berkeley); R. Staubert, A. Santangelo, E. Kendziorra (IAA Tuebingen); A. Segreto (IASF/CNR, Palermo), M. Orlandini (IASF/CNR, Roma), Lars Bildsten (KITP/UC Santa Barbara), R. Araya-Gochez (Caltech)
on 3 Sep 2005; 16:18 UT
Credential Certification: Peter Kretschmar (Peter.Kretschmar@obs.unige.ch)

Subjects: X-ray, Gamma Ray, Neutron Star, Transient, Pulsar

Referred to by ATel #: 605, 613

The Be/X-ray binary 1A 0535+262 is known as the accreting X-ray pulsar with the highest observed magnetic field. Usually the field strength is quoted as approx. 1O^13 G, based on the strong cyclotron line feature at ~110 keV observed by CGRO/OSSE during a giant outburst in 1994 (Grove et al., 1995, ApJ 438, L25). Observations by HEXE and TTM on Mir/Kvant of another giant outburst in 1989 had indicated cyclotron line features at ~50 and ~100 keV but the lower energy feature was of low significance (Kendziorra et al., 1994, A&A 291, L31). Since the outburst in 1994 the source had remained in quiescence.

Earlier this year, the source underwent another giant outburst, but too close to the Sun to be observed by most satellites (ATel #504, #557). Another outburst was recently announced by Finger (ATel #595, #597). We have analyzed near real time data from Integral observations on Aug 31 and Sep 1, 2005. The average 5-100 keV flux during these observations is ~300 mCrab.

The 5-150 keV spectrum shows a strong, broad line feature (12 sigma at ~50 keV in ISGRI). Fitting the line with a profile with Gaussian optical depth (Kreykenbohm et al., 2004, A&A 427, 975), the SPI line parameters are: centroid 47+-2 keV, width 10+4-1 keV and depth 0.5+-0.1. The parameters for ISGRI are consistent within calibration uncertainties.