Optical Timing of X-Ray Transient MAXI J1727-203 with ULTRACAM/NTT

ATel #11701; J. A. Paice, P. Gandhi (Univ. Southampton), V. S. Dhillon (Sheffield), T. R. Marsh (Warwick), D. A.H. Buckley, M. M. Kotze (SAAO), D. Altamirano, P. A. Charles (Southampton)
on 8 Jun 2018; 16:16 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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MAXI J1727-203 is a new X-ray transient discovered on 5th June 2018 14:29 UT by MAXI (ATel #11683), with an optical counterpart detected by GROND (Atel #11690) and confirmed by the Hiltner telescope (ATel #11691), MASTER (ATel #11692) and Swift/UVOT (ATel #11697). The nature of the source remains unknown, though it has been suggested to be a Galactic black hole X-ray binary (ATel #11696).

We report on optical observations by ULTRACAM on the ESO/NTT (Dhillon et al. 2007, MNRAS, 378, 825) over two nights:

June 6th, 09:21 UT:
Length of Observation: 3600 seconds
Bands: u' g' r'
Frame Rate: 1.5 Hz (u'), 15 Hz (g', r')
Signal/Noise: 5-7 (u', g', r')

June 7th, 08:01 UT:
Length of Observation: 900 seconds
Bands: u' g' i'
Frame Rate: 1.5 Hz (u'), 5 Hz (g', i')
Signal/Noise: 7 (u'), 16-17 (u', g')

There was variable transparency, especially during during night 1, and the target counts were corrected with a simultaneously observed bright comparison star. The light curves show a lack of significant optical flaring intrinsic to the target, and no correlation between the faster r' and g' bands (See link at bottom of page). The fractional r.m.s (on a lightcurve using 1-second bins) was limited to <3-5% on the first night in r' and g', and <2.5% on the second in i' and g'. We found no evidence of any optical periodicity.

This lack of flaring differs from that seen in other black hole transient outburst rises, which typically occur in the hard state (e.g. ATel #10820, #11437). The optical fractional r.m.s during hard states is typically ~10% or higher on similar timescales in g' and r' (e.g. Gandhi et al. 2010 MNRAS 407 2166). Instead, MAXI J1727-203 currently shows a very soft X-ray spectrum and a significant disc component (ATel #11689, #11697), plausibly explaining the lack of fast optical variations. The ULTRACAM observations also occurred close in time to the hard-to-soft state transition reported by MAXI (ATel #11696).

We note, however, that NICER also reports the presence of significant X-ray variability on timescales of seconds. The X-ray and optical light curves thus appear to be decoupled on these rapid timescales.

We also checked Gaia Data Release 2 (GDR2; Gaia Collaboration 2018 A&A in press, arXiv:1804.09365) for a possible optical counterpart to this source. Within a search radius of 2 arcsec, there is a single GDR2 source with designation 4121064315074589696, G mag of 19.46 (+/-0.02 stat.), and no parallax measurement. This is located 1.9 arcsec to the south-west of the GROND position, and has counterparts in PANSTARRS as well as 2MASS; it is thus not associated with MAXI J1727-203.

Follow up observations to confirm this lack of variability, and to further constrain the nature of this source, are highly encouraged. We gratefully acknowledge C. Clark, T. Cunningham and G. Voisin as the observers for this run and R. Breton for help with the observations schedule.

ULTRACAM light curve example