Red sub-second optical flaring in MAXI J1820+070 observed by ULTRACAM/NTT

ATel #11437; Poshak Gandhi, John A. Paice (U. Southampton), Stuart P. Littlefair, Vik S. Dhillon (U. Sheffield), Paul Chote, Tom R. Marsh (U. Warwick)
on 17 Mar 2018; 00:32 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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The new X-ray transient MAXI J1820+070/ASASSN-18ey (ATel #11399, #11400) may be a bright and rapidly-rising hard state black hole binary in outburst (ATel #11403, #11404, #11406, #11418, #11420, #11421, #11423, #11424, #11425, #11426, #11427, #11432).

We observed the source with the fast triple-beam camera ULTRACAM (Dhillon et al. 2007 378 825) at NTT/La Silla from UT 08:45 to UT 10:10 on 2018 March 16 in the SDSS u'g'i' filters. We used ULTRACAM's drift mode and 50 pixel windows to image at a frame rate of 72.5 Hz, with a mean deadtime of 1.2 ms. The sky was photometric, with seeing ~1-2 arcseconds throughout. The g' and i' lightcurves described here were extracted using optimal photometry without bias or flat field corrections.

Dereddened median source mags are g'=12.03 and i'=12.24, calibrated against a field comparison star observed simultaneously in a second ULTRACAM window, and also corrected for Galactic extinction of A(g')=0.76 mag and A(i')=0.39 mag (Schlafly & Finkbeiner 2011 ApJ 737 103). The comparison star is URAT1 486-270264 (Zacharias et al. 2015 ApJ 150 4) at RA=18h20m26.43s, Dec=+07d10m11.8s.

Illustrative light curve segments are shown at the link below. The observation reveals spectacular flaring activity over a range of timescales. Strong sub-second variations with some flares unresolved on the native cycle time of ~13.8 ms are apparent (see also ATel #11426). As a prominent example, the strongest i' flare peaks a factor of ~4.3 in flux (Delta mag ~1.6) above the adjacent mean flux level, has a duration of only ~0.3 s and shows clear shorter sub-structure. The median signal-to-noise per frame is 24 (i') and 19 (g').

The sub-second flaring is strongly red, with a maximum colour change (g'-i') of ~1.24 mag above the median(g'-i') colour of ~ -0.20. There is a clear trend of the source being redder when brighter (see link below). Power spectra show that the variability is dominated by a strong and broad hump spanning ~0.1-10 Hz and peaking just above 1 Hz, stronger in the red. There is also evidence of a weaker high frequency component around ~20 Hz, and possible quasi-periodicities at low frequencies between ~0.01-0.05 Hz. The fractional rms variability amplitude (Vaughan et al. 2003 MNRAS 345 1271) is 0.22 and 0.12 in i' and g', respectively.

These characteristics are strongly reminiscent of those seen in the hard state of the black hole binary GX 339-4, which also showed red flaring with a power spectral peak around 1 Hz in the optical and near-infrared (Gandhi et al. 2010 MNRAS 407 2166, Casella et al. 2010 MNRAS 404 L21) whose origin has been associated with compact jet activity. MAXI J1820+070 has recently displayed a bright radio counterpart consistent with the presence of a jet (ATel #11420). These facts suggest that we are likely seeing optical synchrotron activity in MAXI J1820+070 with a variability spectrum peaking in the red. The short optical flaring timescales point to a compact emission zone.

The origin of these flares can be further tested by searching for sub-second optical/infrared delays with respect to X-rays, and we encourage rapid simultaneous multiwavelength observations to detect and monitor any evolution in such delays (e.g. Gandhi et al. 2017 Nat. Astron. 1 859). Very few such observations exist during outburst rise and the present bright outburst presents an excellent opportunity in this regard.

ULTRACAM will attempt to observe the source at every opportunity over the coming week, typically near the end of Chilean night (~08:30UT [or earlier] into morning twilight). Anyone interested in coordination should contact the authors, or sign up to SMARTNet (www.isdc.unige.ch/smartnet). PG thanks Phil Uttley for discussions.

ULTRACAM observations of MAXI J1820+070