S.E. Motta (Oxford), A. Sanna (UNICA), D. Altamirano (Southampton), A. Rushton (Oxford, Southampton), R. Fender (Oxford)

ATel #7591; S. E. Motta (Oxford), A. Sanna (UNICA), D. Altamirano (Southampton), A. Rushton (Oxford, Southampton), R. Fender (Oxford)
on 5 Jun 2015; 09:08 UT
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Credential Certification: Sara Elisa Motta (smotta@sciops.esa.int)

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Following the announcement of the re-brightening of the black hole candidate XTE J1856+053 (ATel #7579), we requested Swift/XRT X-ray and Arcminute Microkelvin Imager Large Array (AMI-LA) radio observations to study the source evolution during a possible outburst. The Swift/XRT observation taken on 2015-06-03 (Obs Id: 00030906011) was carried out in "auto" mode (75s PC-mode data plus 407s WT-mode data). The source was clearly visible in both the PC-mode and WT-mode data (net source count rate 5 and 18 counts/s respectively). The energy spectrum from the WT data is well-fitted by an absorbed (Nh = 2.90x10e22) multi-color disk blackbody, with and inner-disk temperature of 0.75keV, resulting in an unabsorbed flux of 8.1e-10 erg cm-2 s-1 in the 2-10 keV energy band. The energy spectrum could not be fitted with an absorbed powerlaw. We also extracted a power density spectrum from the WT data that appears to be consistent with weak red noise (rms ~ 1.5%). We also checked the Swift/BAT light curve of XTE J1856+053 and we did not find any significant brightening of the source in the hard X-rays. We triggered the AMI-LA (2 cm) telescope in Cambridge, UK, as part of the 4 PI SKY project. Observations were taken on 2015-06-01 and we found no evidence of radio emission from XTE J1856+053 above ~0.5 mJy; the only radio flux detected came from the large extended structure previously identified as a confusing background source (see Atel #7338). This emission limits deeper radio observations at this resolution. Based on the soft, disk-dominated spectrum, and on the low variability from the source, we argue that XTE J1856+053 might be going through a soft outburst of the kind often shown by other black hole candidates, such as H1743-322. This statement is supported by the lack of hard emission in the BAT data and of radio emission in the AMI data. We will continue monitoring the source with Swift during the upcoming days. Multi-Wavelengths observations are encouraged. We thank the Swift and AMI staff for scheduling the requested observations and the ERC for funding 4 PI SKY (4pisky.org).