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INTEGRAL observes enhanced activity of A0535+26 outside of outbursts

ATel #2496; I. Caballero (CEA Saclay), F. Lebrun (APC (CEA/IRFU)), J. Rodriguez, S. Soldi (CEA Saclay), F. Mattana (APC), A. Santangelo (IAAT), J. Wilms, I. Kreykenbohm (Dr Karl Remeis Obs./ECAP), P. Kretschmar (ISOC/ESAC), C. Ferrigno (ISDC), K. Pottschmidt (CRESST/GSFC/UMBC), R. Rothschild (CASS/UCSD)
on 20 Mar 2010; 20:35 UT
Distributed as an Instant Email Notice Transients
Credential Certification: I. Caballero (isabel.caballero@cea.fr)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 2541

During recent Crab calibration observations performed with INTEGRAL, the Be/X-ray binary A 0535+26 has been detected at ~ 32.4 mCrab in the 20-100 keV range. The source showed a giant outburst in December 2009 (ATel#2337) that lasted about 45 days associated with the periastron. The A 0535+26 observations took place between March 3-9 2010 (MJD 55258-55264), about 3 weeks before the next periastron passage of the source. It is known from optical observations of the system that a disk of circumstellar matter was present around the Be star during and after the giant outburst (Camero et al. 2010, in: 11th HEAD Meeting, BAAS 42 #2, 43.19).

From the inspection of NRT data, we significantly detect a fundamental cyclotron line at E=47.8+1.7/-1.4 keV with ISGRI (90% confidence). The effective exposure of the observation is ~220 ks. The 20-100 keV flux is F=5.236e-10 erg cm-2 s-1, giving a luminosity (assuming d=2kpc) of L=2.5e35 erg s-1. This is to our knowledge the first detection of a cyclotron line for this source outside of outbursts.

The cyclotron line was measured at E = 46.7+0.6/-0.7 keV with RXTE and E = 45.9 ± 0.3 keV with INTEGRAL during a normal outburst in 2005 (Caballero et al. 2007A&A, 465, L21). From recent INTEGRAL, RXTE, and Suzaku and observations of the source during a peculiar double-peaked normal outburst (ATel #2161) the cyclotron line was measured at E=46.1 ± 0.3 keV (1sigma errors).