Optical observations of MAXI J1820+070 suggest it is a black hole X-ray binary

ATel #11418; M. Cristina Baglio, Dave M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 14 Mar 2018; 18:15 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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We report on optical observations of the newly discovered transient MAXI J1820+070 (ATel #11399, #11400, #11403, #11404, #11406). We performed optical (g', i') observations of the field with the Las Cumbres Observatory (LCO) 1-m robotic telescope located at Cerro Tololo Inter-American Observatory, Chile, on March 13th, 2018 (MJD 58190.38) obtaining one 200-second integration image of the field for each filter. The optical counterpart is clearly detected with the following AB magnitudes:

g'=13.14 +/- 0.03
i'=13.08 +/- 0.05

These values are not corrected for Galactic extinction (nH=1.12E21 cm^-2 in the direction of the source; Dickey & Lockman, 1990, ARAA, 28, 215; Kalberla et al. 2005, A&A, 440, 775), that corresponds to Av=0.51 +/- 0.02 according to the NH/Av relation of Guver & Ozel 2009.

The calibration of the magnitudes was performed against a group of PanStarrs bright stars in the field of the target. In the PanStarrs catalogue, moreover, the magnitudes of a faint object at a position that is coincident with that of the source are tabulated (g'=19.381 +/- 0.073, i'= 18.039 +/- 0.101). This faint target is therefore likely to be the optical counterpart of MAXI J1820+070 during quiescence. From this, we could infer the amplitude of the outburst, that so far is 6.24 +/- 0.08 mag and 4.96 +/- 0.11 mag in the g' and i' bands, respectively. According to the empirical relation between outburst amplitude and orbital period for LMXBs (Shahbaz & Kuulkers 1998, MNRAS, 295, L1), this implies the orbital period is likely to be < 11.6 hours.

Assuming the source is near the far side of this Galactic dust, we corrected for the reddening (E(B-V)=0.163 +/- 0.007), and obtained the unabsorbed optical fluxes per unit frequency. Considering the 2-10 keV unabsorbed flux measured by MAXI on the same day (3.2839 +- 0.3893 E-9 erg/cm2/s), we could place MAXI J1820+070 on the optical/X-ray correlation plot for BH- and NS- X-ray binaries (Russell et al. 2006, MNRAS, 371, 1334; 2007, MNRAS, 379, 1108), adopting various possible distances for the source (0.3, 1, 3, 8, 20 kpc); see link below. We find that MAXI J1820+070 lies in a region of the diagram that is consistent with a black hole X-ray binary, for any reasonable distance. Only for distances << 1 kpc could the optical/X/-ray ratio be consistent with a typical neutron star X-ray binary. We can rule out a high-mass X-ray binary due to the 6-mag amplitude of the outburst, and the source is likely too X-ray bright to be an accreting white dwarf (it is still rising according to the MAXI light curve). We therefore tentatively claim that MAXI J1820+070 is a new black hole candidate low-mass X-ray binary, although confirmation (ideally dynamical, which may be possible in quiescence) will be required.

Further multiwavelength observations are strongly encouraged in order to prove the nature of the source. The LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO).

LCO observations of MAXI J1820+070