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INTEGRAL detection of the onset of renewed X-ray activity from 1A 0535+26

ATel #5942; V. Sguera (INAF-IASF Bologna, Italy), M. Fiocchi (INAF-IAPS Roma, Italy), L. Sidoli (INAF-IASF Milano, Italy), P. Kretschmar (ESA/ESAC, Spain), E. Kuulkers (ESA/ESAC, Spain)
on 3 Mar 2014; 17:19 UT
Credential Certification: Eleonora Torresi (torresi@iasfbo.inaf.it)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 6043, 6259

An increase of X-ray flux from the Be high mass X-ray binary 1A 0535+26 was detected by INTEGRAL during a public observation of the Crab from 2014 Feb 21 05:55 (UTC) to 2014 Feb 23 21:00 (UTC).

1A 0535+26 was in the IBIS/ISGRI field of view for a total of 199 ks (on source), it was detected at a 29-sigma level in the 18-60 keV energy range, with an average flux of 12.5+/-0.4 mCrab. The 22-60 keV spectrum can be fitted with a power law (reduced chi-squared 1.8 for 12 dof) with photon index 3.3+/-0.3 and average 18-60 keV flux of 3.2 x 10-10 erg cm-2 s-1. The source was also in the JEM-X1 smaller field of view for a total of 38 ks (on source), it was detected at a 16-sigma level (3-10 keV) with average flux of 8.1+/-0.5 mCrab (3-10 keV).

An X-ray re-brightening of 1A0535+26 was reported on 2014 Feb 27 from MAXI/GSC observations with a flux level of about 20 mCrab (4-10 keV; ATel #5931); it was also mentioned a flux increase in the Swift/BAT light curve. The MAXI/GSC measured flux is a factor of about 2.5 larger than that measured by INTEGRAL/JEM-X reported above, about a week earlier. We also note that optical activity from 1A 0535+26 was reported in ATel #5941 from optical observations performed between 2014 16 Feb and 2014 26 Feb.