NuSTAR and Swift observations of MAXI J1820+070 indicate further dimming in X-rays

ATel #12732; John A. Tomsick (UCB/SSL) and Jeroen Homan (Eureka Scientific and SRON)
on 6 May 2019; 00:33 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

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After a large outburst in 2018 and then approaching quiescence in 2019 February (ATEL#12534), the black hole transient MAXI J1820+070 rebrightened, reaching an X-ray flux of ~5e-10 erg/cm2/s on 2019 March 29 (ATEL#12688). After that, the source declined to ~8e-11 erg/cm2/s on April 15 and April 17 (ATEL#12688), and we observed it with NuSTAR between May 1, 9.6h UT and May 2, 11.1h UT, obtaining an exposure time of 41 ks. We analyzed the NuSTAR data, and we find that the count rate is 0.159+/-0.002 c/s (both modules combined). We produced an energy spectrum in the 3-40 keV band, and fit it with an absorbed power-law model with the column density fixed to 1.5e21 cm^-2 (ATEL#11423). The quality of the fit is acceptable (chi2=179 for 182 degrees of freedom), the power-law index is Gamma = 1.99+/-0.05, and the 3-40 keV flux is (6.5+/-0.2)e-12 erg/cm2/s.

Swift observed MAXI J1820+070 with XRT twice more after the last Swift observation reported in ATel #12688: on April 28, 16.5h UT and on May 4, 0.0h UT. Fitting the spectra of these observations with an absorbed power-law (Nh fixed to 1.5e21 cm^-2 and power-law index fixed to 2) gives 0.3-10 keV fluxes of (3.3+/-0.4)e-11 erg/cm2/s and (4.7+/-0.6)e-12 erg/cm2/s, respectively.

Combined with the NuSTAR observation, the last two Swift observations indicate a sudden increase in the decay rate. Fits with an exponential decay to the Swift 0.5-10 keV count rates between April 1 and April 28 yield a decay time scale of 9.4+/-0.2 days, while a similar fit to data between April 28 and May 4 (including the NuSTAR data converted to a Swift count rate using WebPIMMS) yields a decay time scale of 3.0+/-1.8 days.