MAXI J1820+070 has optical period of 3.4 hours
ATel #11596; Michael Richmond (RIT)
on 2 May 2018; 18:41 UT
Credential Certification: Michael Richmond (mwrsps@rit.edu)
Subjects: Optical, X-ray, Black Hole, Transient
Referred to by ATel #: 11723
I have observed the transient source MAXI J1820+070 in the optical since
2018 March 24. In the past two weeks, the source has started to show long-term
variations in brightness of amplitude approximately 0.03 - 0.10 magnitude.
Measurements made at the RIT Observatory, Rochester, NY, USA,
using a 30-cm Meade LX200 telescope, no filter and ATIK 11000 CCD,
between UT 2018 Apr 22 and UT 2018 May 02, can be fit by a rough
sinusoid with period of about 0.1417 days = 3.4 hours.
Since the source is only visible above the horizon for about 3 hours before
dawn, it is difficult to separate intrinsic variability of a similar period
from several systematic effects, especially color-dependent extinction.
However, the increase in amplitude on UT 2018 May 02 makes the effect easier
to detect.
The period appears to be very close to one-seventh of a day, which leads
to some ambiguity; a periodogram shows strong peaks at one-sixth and one-eighth
of a day as well. In order to break this degeneracy, I added measurements made
by Stephen M Brincat (BSM), at Flarestar Observatory, Malta, using a 25-cm
Meade telescope, no filter, and Moravian G2-1600 CCD; these measurements kindly
provided by the American Association of Variable Star Observers (AAVSO).
Since this data was acquired at a longitude far from that of RIT, it distinguishes
well between these signals. The data from BSM on UT 2018 May 01,
combined with the three nights of RIT data, yield a good fit for a period
of 0.1417 days.
This period may be caused by the orbital motions of the members
of the binary system. Further tests of this variation by other observers,
and at other wavelengths, is welcome.
Further details on the measurements and data reduction from RIT Observatory,
as well as light curves and datafiles in ASCII format, can be found in the nightly report
linked below.
RIT measurements of MAXI J1820+070 on UT 2018 May 02
