Continued Swift Monitoring of the Galactic Black-Hole Binary MAXI J1820+070

ATel #12688; D. Vozza, S. Ali, M. Balakrishnan, J. Chen, N. Kebebe, J. M. Miller, M. Reynolds, B. E. Tetarenko (Univ. of Michigan)
on 25 Apr 2019; 13:45 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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MAXIJ1820+070 is a galactic black-hole candidate and low-mass X-ray binary. This transient source was discovered when it went into outburst for approximately 8 months from March-October 2018 (ATEL #11399, #11403,#12064).

On 13 March 2019, this source was found by Swift to again be in outburst. We have been following the progression of the rebrightening since 17 March 2019. Analyzing five of the most recent observations between 17 March and 17 April, it appears that the rebrightening has stopped and the object has begun a return to quiescence.

All five recent windowed timing mode observations were well modeled by a tbabs*pegpwrlw model in xspec (holding N_H = 1.5 E+21 cm^-2 as per ATEL #11423) with reduced chi-squared values of 0.90/203, 0.93/247, 1.07/175, and 1.05/240 respectively. A power-law fit was found to dominate each spectrum with little evidence of any low-temperature thermal component.

OBSID Date Flux (ergs cm^-2 s^-1) Photon Index
00010627141 17/03/19 1.04(+2.0/-0.7)E-10 1.93 +/-2.8E-2
00010627150 29/03/19 5.22(+0.1/-0.1)E-10 1.78 +/-2.0E-2
00010627156 04/04/19 3.18(+0.1/-0.1)E-10 1.89 +/-2.8E-2
00010627162 15/04/19 8.16(+0.3/-0.3)E-11 1.99 +/-4.5E-2
00010627163 17/04/19 8.23(+2.7/-2.0)E-11. 3.39 +/-9.5E-1

A steadily decreasing flux trend seems to indicate that the March 2019 rebrightening may be over, having lasted a much shorter time than its previous full outburst. Multi-wavelength observations of the object are encouraged to both confirm that the object is now dimming in all wavelengths and to track any subsequent flux variations.

We thank the Neil Gehrels Swift Observatory for executing these observations.