Near Infrared JHKs observations of the transient MAXI J1820+070 / ASASSN-18ey

ATel #11458; A. K. Mandal (Christ Deemed University, Bengaluru), A. Singh, C. S. Stalin (Indian Institute of Astrophysics, Bengaluru), S. Chandra (CSR, NWU, South Africa), P. Gandhi (U. Southampton)
on 21 Mar 2018; 17:16 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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We report near-infrared JHKs band photometric observations of the newly discovered X-ray transient MAXI J1820+070 (Atel #11399, #11403, #11406), associated with ASASSN-18ey which began brightening in the optical on March 06 (ATel #11400).

These observations were carried out on 2018 March 18 between 23:33 and 23:49 UT using the 2 m Himalayan Chandra Telescope (HCT), situated at Hanle and operated by the Indian Institute of Astrophysics, Bangalore. Imaging was in dithered mode and consisted of five exposures each in three dithered positions, with each exposure 20 seconds long.

The transient can be associated with the 2MASS object (2MASS 18201471+0710574) having Vega magnitudes in J, H and Ks bands of 15.87 +/- 0.07, 15.44 +/- 0.10 and 15.12 +/- 0.12 respectively. Reduction of the data were carried out using PSF photometry. The derived instrumental magnitudes when calibrated against two stars in the same field namely 2MASS 18201851+0710328 and 2MASS 18202643+0710117 gave the following brightness of the source in the near-IR bands:

J = 11.12 +/- 0.08
H = 10.24 +/- 0.04
Ks = 9.54 +/- 0.04

These magnitudes have been not corrected for Galactic extinction. Comparison of our observations with the 2MASS magnitudes indicates that the source has brightened by 4.75, 5.20 and 5.58 mag in J, H and Ks bands.

Ks-band monitoring with HAWK-I/VLT and the above 2MASS association have also been reported by Casella et al. (ATel #11451), with our HCT observations preceding theirs by about 9 hours. Our Ks band measurement is about 0.7 mag brighter than the average mag reported by Casella et al., likely indicative of strong variations on min-hours timescales, in addition to the faster variability seen in HAWK-I.

The above 2MASS mags correspond to flux densities of ~57 (J), 82 (H) and 102 mJy (Ks; Cohen, Wheaton & Megeath 2003 AJ 126 1090). Galactic extinction corrections are likely to be <17%, <10% and <7%, respectively, based upon the range of optical extinction estimates reported so far (ATel #11418, #11437, #11445) and for a typical Galactic reddening law (Fitzpatrick 1999 PASP 111 755). This implies that the near-infrared flux density rises to longer wavelengths, possibly consistent with optically-thin jet emission. There is multiwavelength spectral and timing evidence supporting the presence of jet activity (e.g. ATel #11420, #11426, #11437, #11439, #11440, #11451).

X-ray spectral modeling of a Swift Window Timing mode XRT observation from the same day (ObsID 00010627007 with an observation time of 2018-03-18T02:52:56) is consistent with an approximate power law fit of photon-index 1.52 +/- 0.03 (68% confidence error) over the 3-9 keV range, with chi^2/dof=641/581 and Flux(3-9)~1.3x10^{-8} erg/s/cm^2. The H-band and 3-9 keV fluxes place the source close to the hard state outburst rise track of the black hole binary GX 339-4 (Coriat et al. 2009 MNRAS 400 123).

We will attempt to carry out further observations of this ongoing transient.