[vsnet-alert 22289] ASASSN-18ey = MAXI J1820+070 state change (ATEL 11823)

[Taichi Kato]

ASASSN-18ey = MAXI J1820+070 state change (ATEL 11823)

   Updated information.

===

ATEL #11823                                                          ATEL #11823

Title:  Continuing NICER observations of the state transition in ASASSN-18ey/MAX
I
                J1820+070
Author: J. Homan (Eureka Scientific and SRON), P. Uttley (Univ. of
                Amsterdam), K. Gendreau, Z. Arzoumanian (GSFC), M. Saylor (KBRwy
le),
                J. F. Steiner, D. Pasham (MIT), A. L. Stevens (Michigan State Un
iv.),
                D. Altamirano (Southampton), E. Kara (Univ. of Maryland). A. C. 
Fabian
                (Univ. of Cambridge), S. Eikenberry (Univ. of Florida) for the N
ICER
                team
Queries:        jeroen at space.mit.edu
Posted: 6 Jul 2018; 23:16 UT
Subjects:X-ray, Black Hole, Transient

Following the onset of a rapid state transition in the black hole transient
ASASSN-18ey/MAXI J1820+070 (ATel #11820), NICER has been observing the
source with a very high cadence (see left panel in figure linked at the
end of this telegram). The new NICER observations show that in the past
24 hours, the 0.2-12 keV count rate (scaled to 52 active Focal Plane Modules)
has increased from ~41000 cts/s around July 5 22:00 UTC to 66000 cts/s
(>6 Crab in rate) around July 6 19:20 UTC. During this period the spectral
hardness decreased smoothly. 

The (type C) low-frequency QPO that we reported in ATel #11820 increased
in frequency from ~3.8 Hz on July 5 22:00 UTC to 6.4 Hz on July 6 11:45
UTC, while it significantly weakened. Around 14:45 UTC the QPO was only
marginally detected around 8 Hz, although its harmonic at ~16 Hz could
still be seen clearly.   

A rapid jump in count rate (from ~57000 cts/s to ~63000 cts/s) occurred
during the time interval of July 06 16:05-16:16 UTC. This sudden increase
in count rate was not accompanied by a strong change in hardness. However,
after the count rate jump a type-B QPO appeared in the power spectrum,
around 4.4 Hz. This type-B QPO was still present in our most recent observation
(July 6 19:20 UTC), but had decreased in frequency to 3.4 Hz.   

The power-spectral and spectral evolution suggest that the source has now
moved to the soft intermediate state.  This is confirmed by quantifying
the 5-10 keV power-spectral shape in terms of ratios between integrated
power in different frequency bands (see right panel in linked figure and
Heil et al. MNRAS, 448, 3339 (2015) for details), for comparison with black
hole transients observed by RXTE.  Based on past observations of transitions
between the hard and soft intermediate states in black hole transients,
we expect that significant evolution should be taking place in the radio
through optical, as the result of changes in the jet emission, including
the possible launching of ballistic jets (see Fender et al., MNRAS, 396,1370
(2009)). 

NICER plans to make observations this coming weekend at the times provided
(https://www.dropbox.com/s/d25xapanoab7a8m/nicerobservationtimes.txt?dl=0).
Coordinated observations are encouraged. 

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 

Light/hardness curves and power-color diagram: https://www.dropbox.com/s/ty84yqx
dwdgfuw6/atel_figure.pdf?dl=0