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Renewed low-level X-ray and radio activity from Swift J1910.2-0546

ATel #5063; John A. Tomsick (SSL/UCB), Stephane Corbel (Univ. Paris Diderot & CEA Saclay), Jerome Rodriguez (CEA Saclay), Tasso Tzioumis (ATNF)
on 13 May 2013; 17:04 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

Subjects: Radio, Optical, Ultra-Violet, X-ray, Black Hole, Transient

Swift J1910.2-0546 (aka MAXI J1910-057) is an X-ray transient and likely black hole binary that was discovered when it had an X-ray outburst that started in 2012 May (Krimm et al., ATEL#4139 and Usui et al., ATEL#4140). Swift/BAT and MAXI detected the source until 2013 January, but then the flux level dropped below their detection limits.

We carried out radio observations with ATCA on 2013 March 9 and May 3 and Swift observations on March 9 and May 10.

In March, the source was marginally detected by ATCA at 5.5 and 9 GHz with an average flux density near 0.06 mJy (preliminary). It was not detected by Swift/XRT during a 1031 second exposure, and the 3-sigma upper limit on the 0.6-10 keV count rate and absorbed flux are <8e-3 c/s and <4.7e-13 erg/cm2/s (using the spectral parameters described below), respectively.

On May 3, the 5.5 and 9 GHz radio fluxes had increased to 0.3-0.4 mJy (preliminary), and we requested a Swift/XRT observation to check on whether the source had increased in X-rays as well. A 1658 second exposure indicated a 0.6-10 keV count rate of (7.5+/-0.7)e-2 c/s, which is a least a factor of ten higher than in March. The X-ray spectrum is consistent with an absorbed power-law with a column density of 3.5e21 cm-2 (fixed to a value found in a brighter observation using Wilms et al. 2000 abundances) and a photon index of 1.8+/-0.3 (90% confidence errors), which is consistent with the source being in the hard state. The absorbed 0.6-10 keV flux is 4.7e-12 erg/cm2/s.

The Swift/UVOT measurements also show that the outburst from Swift J1910.2-0546 is continuing. Even in March, the source was at B = 16.13 (+/-0.4 statistical) (+/-0.2 systematic), which is nearly as bright as its outburst level (see Rau et al., ATEL#4144). In May, the UVOT observation was done with a the UVM2 filter (at an effective wavelength of 231 nm), and the source was detected at 20.4+/-0.2 magnitudes.

Thus, currently, multi-wavelength observations of Swift J1910.2-0546 are useful for studies of the hard state (e.g., compact jet). With its low extinction, optical observations in quiescence may yield a compact object mass measurement, but the source is still much too bright for such observations.

We thank Neil Gehrels for approving the Swift TOO requests, and the Swift duty scientists and planners for carrying out the observations.