NICER and Swift observations of the new outburst of MAXI J1820+070

ATel #14495; J. Homan (Eureka Scientific and SRON), P. Saikia, M. C. Baglio, D. M. Russell, D. M. Bramich (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), P. Roche (Cardiff University), D. Altamirano (University of Southampton), A. Fabian (University of Cambridge), J. F. Steiner (SAO, CfA), K. Gendreau (NASA/GSFC)
on 30 Mar 2021; 20:09 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

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Following reports of renewed optical activity in the black hole X-ray transient MAXI J1820+070 (ATel #14492), we observed the source with NICER and Swift on March 29.

NICER started observing MAXI J1820+070 around 19:44 UTC for ~670 s. The source was detected significantly above the background between ~0.4 and ~2 keV, with a 0.4-2 keV source count rate of 0.49+/-0.03 cts/s. We fit the spectrum between 0.4 and 2 keV using an absorbed power law, with the nH fixed to 1e21 cm^-2 (Shaw et al. 2021, ApJ, 907, 34). The fit yields a power-law index of 2.5+/-0.2 and an (extrapolated) unabsorbed 0.5-10 keV flux of (8+/-1)e-13 erg/cm^2/s. We note that, due to uncertainties introduced by the NICER background modeling, the quoted errors on the power-law index and flux should be taken as lower limits.

Swift/XRT observed the source in Photon Counting mode, starting around 21:05 UTC and for ~1700 s. The 0.3-10 keV count rate was 0.020+/-0.003 cts/s. The 0.3-10 keV spectrum was also fit with an absorbed power law, with nH fixed to the same value as above. We find a power-law index of 1.8+/-0.3. The unabsorbed 0.5-10 keV flux was (1.0+/-0.2)e-12 erg/cm^2/s, consistent with the flux measured with NICER. Both the NICER and Swift/XRT fluxes are well above the unabsorbed 0.5-10 keV flux measured with Swift/XRT in February 2021: (2.6+/-0.6)e-13 erg/cm^2/s. The increase in the 0.5-10 keV flux by a factor of ~3-4 indicates that the outburst of MAXI J1820+070 has started in the X-rays as well. At a distance of 3 kpc (Atri et al. 2020, MNRAS, 493, 81) the newly measured fluxes correspond to a luminosity of ~1e33 erg/s. This is among the lowest luminosities at which the start of a black hole-X-ray transient outburst has been caught in X-rays.

Swift also observed MAXI J1820+070 with the UVOT in the V filter for ~1700 s. We measured a magnitude of 17.22+/-0.09 (AB system), indicating a brightening by ~0.5 Mag compared to March 27 (ATel #14492).

Swift will attempt to observe the source on a daily basis until April 17. NICER visibility for the source is poor until April 5, but observations may resume soon after.

We thank the Swift and NICER teams for rapidly approving, scheduling, and performing these observations.