March 19, 2018: Drs. Gregory Sivakoff (University of Alberta), Poshak Gandhi (University of Southampton), and Diego Altamirano (University of Southampton) have requested AAVSO observers' assistance in monitoring the new black hole X-ray binary candidate ASASSN-18ey = MAXI J1820+070.
The X-ray transient MAXI J1820+070 was discovered on 2018 March 11.5347 UT by MAXI (Monitor of All-sky X-ray Image; ATel #11399). It was matched with the optical counterpart ASASSN-18ey (ATel #11400), which was discovered by the ASAS-SN project on 2018 Mar. 06.59 UT (list of ASAS-SN Transients).
Since then at least 12 observing campaigns have been carried out using a variety of space- and ground-based instruments (references include ATel #11403, #11404, #11406, #11418, #11420, #11421, #11423, #11424, #11425, #11426, #11427, #11432, #11437, #11439).
In support of multiwavelength observing campaigns underway, being planned, and in development, AAVSO observers are requested to monitor this active black hole X-ray binary.
Dr. Gandhi writes: "At present, we find that the source shows red flaring on timescales apparently down to milli-seconds. So my primary recommendation would be to use redder filters and as short exposures as possible. The source appears to be approaching V404 Cyg levels of brightness, so that could be a useful comparison. It is not obvious that it displays the spectacular variations that V404 Cyg showed on ~minutes to hours and days. But there is clear and strong variability on timescales of ~seconds and less.
"Latest observations now find [g'~12.03 mag and i'~12.24 (ATel #11439)], so the outburst continues to brighten (quickly) for now, and we may expect a transition in the source properties soon, assuming this is a canonical black hole binary. Observations over the next ~weeks (or until the source fades, if that happens earlier) would be very useful to catch the long-term evolution. Unfiltered [photometry] is acceptable as long as there is a consistent dataset."
ASASSN-18ey = MAXI J1820+070 has a published optical range of 12.5 CV - 18.3: V. The most recent observations submitted to the AAVSO International Database are:
2018 Mar. 18.31398 UT, 12.502 CV +/-0.025 (F.-J. Hambsch, Mol, Belgium);
18.31418, 12.386 CV +/-0.025 (Hambsch);
18.31436, 12.501 CV +/-0.026 (Hambsch);
18.31455, 12.454 CV +/-0.025 (Hambsch);
18.31473, 12.468 CV +/-0.025 (Hambsch);
18.31492, 12.474 CV +/-0.025 (Hambsch);
18.31510, 12.494 CV +/-0.025 (Hambsch);
18.31529, 12.690 CV +/-0.030 (Hambsch);
18.31547, 12.518 CV +/-0.028 (Hambsch);
19.17500, 12.33 (M. Deconinck, Artignosc sur Verdon, France);
Coordinates (2000.0): R.A. 18 20 21.95 Dec. +07 11 07.3
Charts with a comparison star sequence for ASASSN-18ey may be created using the AAVSO Variable Star Plotter (VSP). You may use either ASASSN-18ey or MAXI J1820+070 as the name when creating a chart.
Please submit observations to the AAVSO International Database using the name ASASSN-18ey.
This observing campaign is being followed on the AAVSO Time Sensitive Alerts online forum at https://www.aavso.org/asassn-18ey-suspected-black-hole-binary-bright-outburst
This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.
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