INTEGRAL detection of the on-going activity from the Be HMXB GX 304-1

ATel #9067; V. Sguera (INAF-IASF Bologna, Italy), L. Sidoli (INAF-IASF Milano, Italy)
on 20 May 2016; 07:37 UT
Credential Certification: Eleonora Torresi (torresi@iasfbo.inaf.it)

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The HMXB GX 304-1 is currently undergoing weak X-ray activity as detected by MAXI/GSC starting on May 17th (Atel#9064), the onset phase of the present brightening has changed with respect to the trend observed from the previous brightening events.

INTEGRAL has recently performed public observations of the Musca region, in the direction of GX 304-1, during revolution 1678 (starting from 14 May 00:25 UTC to 14 May 17:22 UTC) as well as 1679 (starting from 16 May 14:17 UTC to 17 May 10:13 UTC).

During revolution 1678, GX 304-1 was in the field of view of IBIS/ISGRI with an effective exposure time of 3.3 ks. It was detected at about 10.7 sigma level (22-60 kev) with an average flux of 32.1+/-3.0 mCrab. No significant emission was detected in the band 60-100 keV. The source was never in the field of view of the JEM-X monitor during this observation.

During the subsequent revolution 1679, GX 304-1 was also in the field of view of IBIS/ISGRI with an effective exposure time of 10.5 ks. It was detected at about 17.5 sigma level (22-60 kev) with an average flux of 27.9+/-1.6 mCrab, compatible within the errors with the previous measurement. No significant emission was detected in the band 60-100 keV. During this observation the source was briefly in the field of view of the JEM-X monitor. It was barely detected at about 3 sigma level (3-10 keV) in the combined JEM-X1+JEM-X2 mosaic (total effective exposure of about 2.6 ks) with a flux of 3.1+/-1.0 mCrab.

Previous observations with INTEGRAL were reported by Manousakis et al. 2008 (ATel #1613), Klochkov et al. (2012) and Malacaria et al. (2015), especially devoted to the investigation of the correlation of the 50 keV cyclotron line energy with the source flux.