Optical timing observations of Swift J1753.5-0127 during its Feb 2017 rejuvenation

ATel #10118; P. Gandhi (Southampton), V. S. Dhillon (Sheffield), T. R. Marsh, M. Green, R. P. Ashley (Warwick), D. Altamirano, A. Beri (Southampton), A. W. Shaw (Alberta), J. A. Tomsick (Berkeley)
on 23 Feb 2017; 23:45 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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The black hole transient Swift J1753.5-0127 (hereafter, J1753) is currently showing rejuvenated accretion activity. X-ray, optical, and radio observations find that the source is returning to near pre-quiescent flux levels following a remarkably brief lull (ATel #10075, #10081, #10097, #10110, #10114). The source is currently in the hard state (ATel #10114).

Here we report that J1753 is also showing optical timing behaviour qualitatively similar to that seen in prior active hard states. We observed the source with the ULTRASPEC high-speed imaging photometer (Dhillon et al. 2014 MNRAS 444 4009) on the 2.4 m Thai National Telescope on Feb 20, 21 and 22 UT at a time resolution of ~0.50 s (exposure of ~0.49 s with dead time of ~0.02 s). ULTRASPEC utilises frame transfer CCDs for rapid, near-continuous imaging. Conditions were photometric throughout, with variable seeing of ~1.5-3 arcsec (FWHM). Observations were carried out at the end of each night (local time) at airmasses of ~1.7-1.3, for durations of ~1.1-1.9 hours limited by the onset of morning twilight. A wide KG5 filter sensitive between ~3200-6700 A was employed. A nearby bright comparison star was observed simultaneously, and divided through the target light curves to compensate for atmospheric variations.

The source shows clear excess variability noise on timescales of minutes, with no strong variations on the longer ~hour-long durations of each light curve. The measured fractional r.m.s. levels (see Vaughan et al. 2003 MNRAS 345 1271) range over 0.08-0.10 between the nights, significantly higher than the comparison star fractional r.m.s. of 0.01-0.04 which is related to atmospheric variations.

The power spectra are flat at low frequencies, breaking around 0.04 Hz, and show a power-law-like decline at high frequencies. The maxima lie in the ~0.02-0.04 Hz range, where the most prominent feature is a narrow peak at a Fourier frequency of ~0.035 Hz (~28 s) present on all three nights. This feature has a quality factor significantly greater than 2, and we classify it as a low frequency quasi-periodic oscillation (QPO). It is not present in the comparison star power spectrum, arguing that it is not an artefact related to the observing conditions or setup. We cannot rule out the presence of other weaker features at higher frequencies, e.g. around 0.07 Hz.

This timing behaviour is qualitatively similar to that seen during the 2007 hard state, when a flat power spectrum at low frequencies and a red noise power-law were reported during the hard state (Durant et al. 2009 MNRAS 392 309, Gandhi 2009 ApJ 697 L167). A QPO close to 0.08 Hz was also reported in 2007 (Durant et al. 2009, Veledina et al. 2015 MNRAS 454 2855).

The key differences between the present observation and the 2007 one are that the source is currently still X-ray faint (Durant et al. reported F[2-20 keV]~1.6e-9 erg/s/cm^2 in the 2007 hard state, as compared to a recent corresponding ~20x fainter flux of F[2-20]~9e-11 erg/s/cm^2 extrapolated from ATel #10114). No Swift/BAT or MAXI detection has been reported so far during the rejuvenation. In addition, the optical power spectrum break is shifted to lower frequencies (Durant et al. found a break closer to 0.07 Hz). These facts may point to a recessed disc that is still building up and currently truncated at a large radius, filled with a larger hot inner flow than seen during the 2007 observation. In this scenario, the QPO would be expected to evolve to higher frequencies as source activity rises and the disc moves inwards.

The night time visibility of J1753 is now improving, and continued multiwavelength timing observations are encouraged. At least one more ULTRASPEC observation is scheduled on the night of Feb 24, around UT 22:00. It is unclear how long the current outburst will continue, though the source flux is still rising (ATel #10114).

ULTRASPEC data from 2017 Feb 21