Fading of the black hole transient MAXI J1820+070 in X-rays and UV
ATel #16200; Jeroen Homan (Eureka Scientific), M. Cristina Baglio, Payaswini Saikia, Kevin Alabarta, David M. Russell, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Maureen van den Berg (Center for Astrophysics | Harvard & Smithsonian), Richard M. Plotkin (University of Nevada, Reno), and Jeremy Hare (NASA GSFC/CUA/CRESST II)
on 23 Aug 2023; 09:58 UT
Credential Certification: Jeroen Homan (jeroenhoman@icloud.com)
Subjects: Ultra-Violet, X-ray, Black Hole, Transient
Following the observed fading of the black hole transient MAXI J1820+070 into a deeper optical quiescent state (ATel #16192) we observed the source with the X-ray Telescope onboard the Neil Gehrels Swift Observatory. Observations were performed in Photon Counting mode between August 20 07:02 UTC and August 21 21:37 UTC for a total exposure of 2.66 ks.
In the 0.5-10 keV image we detect 5 photons within a 20 arcsec extraction radius centered on the source; only 0.9+/-0.1 photons would have been expected based on the background in the vicinity of the source (annulus with ~19 arcmin^2 area). There is only a 0.2% probability of detecting 5 photons for the measured background level. The resulting background-subtracted count rate is (1.5+/-0.7)e-3 cts/s. This is a factor of ~10 lower than the average count rate observed in Swift/XRT observations made during the second half of 2022.
We used PIMMS to convert the above count rate into an unabsorbed 0.5-10 keV flux, assuming a power-law spectrum with an index of 2.1 and an nH of 1.1e21 cm^-2 (Shaw et al. 2021, ApJ, 907, 34). This yields a flux of (6+/-3)e-14 erg/s/cm^2. For a distance of 3 kpc (Atri et al. 2020, MNRAS, 493, L81) this corresponds to a luminosity of (6+/-3)e31 erg/s. Given the orbital period of MAXI J1820+070 (16.5 hr; Torres et al. 2019, ApJ, 882, L21), this value is consistent (within scatter) with the empirical relation between the quiescent X-ray luminosity and orbital period for black hole X-ray binaries (Armas Padilla et al. 2014, MNRAS, 444, 902). Observations with Chandra or XMM-Newton are required to better characterize the quiescent properties of MAXI J1820+070.
Swift/UVOT observed the source in the U and UVW2 filters, resulting in 520 s and 1.5 ks exposures in the two filters, respectively. The source is detected in both images. We analyzed the images with the HEASOFT tool uvotsource using a 5 arcsec aperture, getting the following Vega magnitudes: U=18.98+/-0.13, UVW2=20.69+/-0.28, showing that the source is getting fainter also at UV frequencies. For comparison, a U magnitude of 18.02 +/- 0.05 was measured on 1 November 2022 and UVW2 magnitude of 18.76+/-0.09 on 1 October 2022.
We thank the Swift team for approving and executing our DDT observations of MAXI J1820+070.
