NOEMA Sub-millimetre Detection of MAXI J1820+070

ATel #11440; A. J. Tetarenko (Alberta), M. Bremer (IRAM), J. Bright (Oxford), G. R. Sivakoff (Alberta), J. C.A. Miller-Jones (Curtin), T. D. Russell (UvA), and the JACPOT XRB collaboration
on 19 Mar 2018; 16:19 UT
Credential Certification: Alexandra Tetarenko (tetarenk@ualberta.ca)

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We performed target-of-opportunity observations of the newly-discovered candidate black hole X-ray binary, MAXI J1820+070 (ATel #11399, #11400, #11404, #11406, #11418, #11420, #11421, #11423, #11424, #11425, #11426, #11427, #11432, #11437, #11439), with the Northern Extended Millimetre Array (NOEMA), as part of the JACPOT XRB project.

Our NOEMA observations occurred on 2018-03-16, with scans on source from 05:25 to 07:44 UTC (MJD=58193.2257-58193.3222), in the 3mm (97.5 GHz) band. MAXI J1820+070 was significantly detected as a point source, with a preliminary flux density of 30 +/- 3 mJy, measured in the UV-plane (where the uncertainty on the flux density represents a 10% error on the absolute flux scale). Adverse weather conditions prevented us from obtaining observations of MAXI J1820+070 in the 2mm (140 GHz) and 1mm (230 GHz) bands.

With these data, we measure the position of the source to be,
RA(J2000) = 18h20m21.94 +/- 0.001s (0.02")
DEC(J2000) = +07d11'07.24 +/- 0.06"

This position lies within the X-ray error circle (1.33 arcsec separation; ATel #11406), and is consistent with the reported optical position (0.05 arcsec separation from a star in archival Gaia DR1 data; ATel #11424, and 0.6 arcsec separation from ASASSN-18ey; ATel #11400).

Sub-mm detections of black hole X-ray binaries typically arise from a compact synchrotron-emitting jet in the hard state, characterized by a flat to slightly inverted, optically thick spectrum (S_ν ∝ nu^α, spectral index α≥0), originating from overlapping self-absorbed synchrotron contributions from different regions along the jet. Given our sub-mm flux, and the radio flux from a new AMI radio observation (that was taken simultaneously with our NOEMA observations) of 29 +/- 3 mJy, we derive a radio through sub-mm spectral index of 0.02+/-0.09 (consistent with the flat radio spectrum reported in ATel #11439). This AMI flux corresponds to a radio luminosity of (3.32 +/- 0.34)E31*(d/8kpc)^2 erg/s).

Swift/XRT observed MAXI J1820+070 on 2018-03-16 between 03:01:32 and 03:17:56 UTC. We extracted the data using the online XRT pipeline (Evans et al. 2009) and fit with an absorbed power-law model. We find a best fit with an N_H of (3.0 +/- 0.5)E20 cm^-2 and a photon index of 1.50 +/- 0.02, providing an unabsorbed 0.5-10 keV X-ray flux of (8.96 +/- 0.08)E-9 erg/s/cm^2, corresponding to a 0.5-10 keV X-ray luminosity of (6.87 +/- 0.15)E37*(d/8kpc)^2 erg/s (all errors are 1-sigma). The X-ray and radio luminosities reported here are consistent with evidence at other wavelengths that suggest the source is a black hole X-ray binary in the rising hard state (ATel #11427, #11432).

We will continue to monitor the source at sub-mm frequencies.

We thank the IRAM NOEMA staff for rapidly scheduling our observations and the AMI scheduling team for their continued monitoring of the source.