[vsnet-alert 22294] ASASSN-18ey = MAXI J1820+070: near-IR decline? (ATEL)

[Taichi Kato]

ASASSN-18ey = MAXI J1820+070: near-IR decline? (ATEL)

ATEL #11833							     ATEL #11833

Title:	Declining near-infrared flux from the black-hole candidate
		MAXI J1820+070 (ASASSN-18ey) in transition
Author:	P. Casella (INAF-OAR), V. Testa (INAF-OAR), D. M. Russell
		(NYU Abu Dhabi), T. M. Belloni (INAF-OAB), T. J. Maccarone (TTU)
Queries:	piergiorgio.casella at inaf.it
Posted:	9 Jul 2018; 17:24 UT
Subjects:Infra-Red, Optical, Request for Observations, Binary, Black
		Hole, Transient

We report on near-infrared and optical photometry of the bright X-ray transient
MAXI J1820+070/ASASSN-18ey (ATel #11399, #11400, #11406, #11418, #11420,
, #11421, #11423, #11439, #11451, #11462, #11481), carried out with the
60-cm robotic telescope REM (La Silla, Chile). The source was reported
recently to be undergoing a fast spectral transition (#11820, #11823, #11827).

REM has been monitoring the target almost daily since 2018  Mar 22nd, observing
nearly simultaneously in optical (four simultaneous filters, g', r', i',
z') and near-infrared (cycling over three filters, J, H, K). The latest
three observations were carried out on 2018 July 2nd (from about 04:15
to 04:30 UT), 8th (03:33 - 03:46) and 9th (04:18 - 04:31), the gap being
due to bad weather in La Silla.  

The source is always detected in each single filter. Magnitudes were calibrated
against a number of standard stars in the field of view. Preliminary analysis
shows that the optical flux is roughly constant, with the griz magnitudes
being all around 13.3 +/- 0.13 during all observations. There is instead
clear evidence for a declining near-infrared flux, especially at the reddest
filter, with a K magnitude going from 11.82 +/- 0.18 on July 2nd to 12.38
+/- 0.12 on July 8th to 12.43 +/- 0.14 on July 9th. 

We assumed E(B-V)=0.163 (#11418) to deredden the observed magnitudes, obtaining
an inverted (optically thick) optical spectrum going from about 30 mJy
in g' to 20 mJy in z', with a spectral index of about 0.5. The spectrum
extended with a similar spectral index to the NIR regime, reaching a flux
of about 13 mJy in K, on July 2nd. On July 8th and 9th, while the optical
spectrum was observed to remain constant, the NIR spectrum steepened, showing
a z'-to-K spectral index of about 1 as the flux in the K filter reached
a flux of about 7 mJy. 

We note that these values must be taken with caution given the large current
uncertainties. Nevertheless, the rather smooth evolution of the magnitudes
in most filters seems to confirm the overall trend, which appears consistent
with a quenching of the compact jet during the state transition 

REM will continue to observe this target on a daily basis, weather allowing.
In particular, a long observation is scheduled for the upcoming Chilean
night, as REM is expected to stare at this source from 23:30 July 9th to
06:30 July 10th (UT). 

Multi-wavelength coverage is highly encouraged, especially but not only
at high high time resolution and/or at longer wavelengths than near-infrared.
People interested in coordinating can contact the authors or sign up to
SMARTNet (<a>www.isdc.unige.ch/smartnet</a>). 

We thank the REM team and especially Dino Fugazza for scheduling these
observations.