[vsnet-alert 22283] ASASSN-18ey = MAXI J1820+070 state change (ATEL)

[Taichi Kato]

ASASSN-18ey = MAXI J1820+070 state change (ATEL)

   Such a state change around the secondary maximum
(appearance of superhumps) was also observed in past
classical X-ray transients.

===

ATEL #11820							     ATEL #11820

Title:	A rapid state transition in MAXI J1820+070
Author:	J. Homan (Eureka Scientific and SRON), P. Uttley (Univ. of
		Amsterdam), K. Gendreau, Z. Arzoumanian (GSFC), M. Saylor (KBRwyle),
		J. F. Steiner, D. Pasham (MIT), A. L. Stevens (Michigan State Univ.),
		D. Altamirano (Southampton), E. Kara (Univ. of Maryland). A. C. Fabian
		(Univ. of Cambridge) for the NICER team
Queries:	jeroen at space.mit.edu
Posted:	5 Jul 2018; 21:59 UT
Subjects:X-ray, Black Hole, Transient

We report on ongoing NICER monitoring observations of the black hole transient
MAXI J1820+070 (ATel #11399, #11400, #11418, #11423, #11420, #11426). 

Following a fast rise upon its discovery in March 2018 and an extended
hard state plateau, the 0.2-12 keV count rate of the source began a steady
decline in mid-May (see link below for light/hardness curves). A reversal
of this behavior was observed in mid-June. The NICER X-ray Timing Instrument
(XTI) showed a broad minimum (at ~5600 cts/s) yielding to an increase in
count rate, along with a softening of the spectrum. This increase and softening
have both been accelerating in the past week. On July 5 the average 0.2-12
keV count rate was ~34300 cts/s, up from ~9300 cts/s on June 27.  

Rapid evolution has also been seen in the power density spectra. The low-frequency
QPOs, which have been reported previously (ATel #11488, #11510  #11576,
#11578), have increased in frequency from ~0.35 Hz on June 27 to ~3.0 Hz
on July 5. To emphasize the rate of evolution, we note that the QPO frequency
doubled in less than 18 hours between the July 4 and 5 observations. 

The spectrum of the July 5 observation is reasonably well described by
an absorbed continuum model consisting of a disk component, a power law,
and reflection. The slope of the power law (~2.4) is considerably steeper
than during the hard state observations of the source (1.6-1.7, ATel #11576).
The unabsorbed 0.3-10 keV flux on July 5 was 9.8e-8 erg/cm^2/s. For a distance
of 3.2 kpc (Gandhi et al. 2018, arXiv:1804.11349) this corresponds to a
luminosity of 1.2e38 erg/s.  

The above behavior suggests that MAXI J1820+070 is currently in a hard
intermediate state and transitioning, at an unusually rapid rate, toward
the soft state. Since the soft state may already be reached within a few
days, NICER observations of the source are continuing at an intensive rate
and we strongly encourage observations at all wavelengths in the coming
days to follow this state transition in detail.  

NICER is a 0.2-12 keV X-ray telescope operating on the International Space
Station. The NICER mission and portions of the NICER science team activities
are funded by NASA. 

Count rate and hardness evolution of MAXI J1820+070: https://www.dropbox.com/s/dc884hjjkqkggwe/MAXIJ1820.pdf?dl=0