Increasing X-ray/UV flux from Swift J1753.5-0127

ATel #10114; John A. Tomsick (SSL/UCB), Aarran Shaw (U. Alberta), Poshak Gandhi (U. Southampton), Farid Rahoui (Harvard Univ.), and Tolga Dincer (Yale Univ.)
on 23 Feb 2017; 01:21 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

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Swift J1753.5-0127 is a black hole transient that was recently reported to be back in outburst in X-rays, UV, optical, and radio (ATELs #10075, #10081, #10097, #10110). As part of a larger multi-wavelength program, a Swift observation (ObsID 00030090118) occurred on 2017 Feb. 22 from 9.4 to 12.8 h UT. The XRT exposure time was 4.4 ks, and measurements were made with all six UVOT filters.

The XRT was in Photon Counting mode and the count rate was 0.97+/-0.01 c/s. As this rate is high enough to cause photon pile-up, we extracted a source spectrum from an annular region, cutting out the core of the PSF. The 0.3-10 keV spectrum is well-described (reduced-chi2 = 1.1 for 121 dof) by an absorbed power-law with nH = (2.8+/-0.4)e21 cm-2 (using Wilms et al. 2000 abundances), Gamma = 1.51+/-0.07, and a 0.3-10 keV unabsorbed flux of (7.4+/-0.3)e-11 erg/cm2/s. The uncertainties are 90% confidence. The UVOT magnitudes (in the Vega system) are V = 17.03+/-0.07+/-0.01, B = 17.53+/-0.05+/-0.02, U = 16.74+/-0.04+/-0.02, UW1 = 17.07+/-0.05+/-0.03, UM2 = 17.67+/-0.07+/-0.03, and UW2 = 17.50+/-0.04+/-0.03, where the first and second error values are statistical and systematic 1-sigma uncertainties.

The previous two Swift observations occurred on Feb. 16 and 19, and 0.3-10 keV unabsorbed X-ray fluxes of 6e-11 and (5.8+/-0.5)e-11 erg/cm2/s were reported (ATELs #10081 and #10097). Thus, between these observations and the Feb. 22 observation, the X-ray flux increased by about 25%. On Feb. 16, U = 16.85+/-0.05+/-0.02 (ATEL #10081), and on Feb. 19, UW1 = 17.17+/-0.05+/-0.03 (ATEL #10097). While the error regions overlap with those of the Feb. 22 measurements, the magnitudes suggest a small brightening in the UV between Feb. 16/19 and Feb. 22.

These measurements suggest that the flux is slowly increasing, but it is still unclear whether the current activity will develop into a larger outburst and how long this outburst will last. In the radio/X-ray correlation plot, Swift J1753.5-0127 was on the outlier track during its previous outburst (e.g., Corbel et al. 2013, MNRAS, 428, 2500; Rushton et al. 2016, MNRAS, 463, 628), but it currently appears to be close to the standard track (see radio fluxes reported in ATEL #10110). Further X-ray and radio monitoring would help to understand the current behavior.

While we were fortunate to be able to thank Neil Gehrels many times in the past for approving Swift observations, we thank Brad Cenko for approving the Swift TOO request and the Swift duty scientists for carrying out the observations.