Correlated Optical/X-ray Timing Variations in MAXI J1820+070 found by Swift UVOT and XRT

ATel #11432; John A. Paice, Poshak Gandhi (Univ. Southampton), Kim Page (Univ. Leicester), Diego Altamirano, Jamie Court, Phil Charles (Univ. Southampton)
on 16 Mar 2018; 11:56 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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MAXI J1820+070 is a new X-ray transient discovered on 2018 March 11 12:50 UT by MAXI (ATel #11399), and associated with a bright optical transient ASASSN-18ey (ATel #11400). It has already been the focus of several observing campaigns over a very short time span, including Swift (ATel #11403, #11404, #11406, #11427), MAXI (ATel #11406), Faulkes/LCO (ATel #11418), AMI (ATel #11420), Sarah L. Krizmanich Telescope (ATel #11421), NICER (ATel #11423), SOAR (ATel #11424), Keck (ATel #11425), and Kiso Schmidt (ATel #11426). These observations have noted its rapid brightening, fast optical variability, and sub-second flaring. They also indicate a nature consistent with a black hole X-ray binary in the hard state (http://www.faulkes-telescope.com/1820).

We report now on correlated optical and X-ray observations by Neil Gehrels Swift Observatory UVOT/XRT (ObsID: 00010627003). The observation was carried out on 2018 March 15, starting at 01:39 (UT), for a duration of 1.8 ks (XRT) and 1.2 ks (UVOT). UVOT was employed in the event mode with the v-filter and XRT was in Windowed Timing Mode. Standard extraction steps were used for selecting and accumulating source counts and constructing light curves. Example light curves using 1 s binning are shown at the link below. No time frame corrections have been included since both lightcurves are from Swift.

The mean count rates are ~184 ct/s (XRT full band) and ~100 ct/s (UVOT). The X-ray data were extracted from the Swift Online Tool. The mean signal/noise in UVOT is ~7, and the fractional optical rms of the source (after removing shot noise contribution) is ~9%, typical of hard-state outbursts (e.g. Gandhi et al. 2010 MNRAS 407 2166).

A Discrete Correlation Function analysis (Edelson & Krolik, 1988 ApJ 333 646) shows the optical signal lagging the X-rays by a few seconds or shorter (see link below). This lag is also evident by eye in the light curves showing correlated flares. The bulk of the correlation appears to be concentrated below a lag of 10 s. These lags are shorter than UVOT lags reported in the case of the rise of V404 Cyg (ATel #7727). If these lags are related to canonical reprocessing of X-rays to the optical, they constrain the size of the disc to be <~0.2 x that of V404 Cyg, or an orbital period of <~15 h x (M/10Msun)^(-0.5), where M is the system mass. This assumes a disk size of ~45 light seconds and an orbital period of 6.5 days for V404 Cyg (Casares et al. 1992 Nature 355 614).

There is some evidence of additional sub-second delays, but these must be confirmed with more sensitive data. It is not uncommon for transient black hole binary outbursts to display multi-component optical variability and delays originating in separate processes (e.g. Gandhi et al. 2010 MNRAS 407 2166).

Swift BAT reports that the source is continuing to increase in X-rays (https://swift.gsfc.nasa.gov/results/transients/weak/MAXIJ1820p070/). The latest optical data shows the source currently at V~13 (ATel #11418).

More observations are currently planned for this source. ULTRACAM will attempt to observe from approximately 08:30 to 09:40 (UT) when possible until March 23, and there are also observations by VLBA, INTEGRAL, AMI, XMM and NOEMA currently scheduled. More information on these observations can be found on SMARTNet, where people are encouraged to sign in: www.isdc.unige.ch/smartnet/

In the interests of multiwavelength astronomy, and considering the bright and intriguing nature of this source, we therefore welcome any and all possible observations.

We would like to extend our profound thanks to the Neil Gehrels Swift Observatory Team for their rapid approval scheduling of these observations, in particular PI Brad Cenko. We would also like to acknowledge discussions with Andy Beardmore, Sam Oates and Dave Russell.

Swift UVOT/XRT Timing Correlations in MAXI J1820+070