NICER continued to monitor U Sco in X-rays

ATel #15508; M. Orio (INAF-Padova and U Wisconsin) and K. Gendreau (NASA-Goddard)
on 10 Jul 2022; 12:30 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Marina Orio (orio@astro.wisc.edu)

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Following the recent outburst, U Scorpii (ATel #15502, #15499, #15465, #15451, #15499, #15428, #15436, #15435, #15423, #15422, #15420, #15417) has been monitored again with NICER starting on 2022 June 24 at UT 15:36:08 in over 25 short exposures (lasting a little over 1000 s each) until 2022 July 8. We already examined exposures taken until 2022 July 5 and fond that the average count rate steadily increased every day, from 9.22±0.09 counts/s to 64.762±0.032 counts/s, with occasional large fluctuations on time scales of minutes. Approximate estimates of the absorbed flux vary from 7.3 x 10^-12 erg/cm²/s to 5.3 10^-11 erg/cm²/s in the 0.2-10 keV range, with almost no flux above 2 keV. The spectrum was always very soft and complex, likely produced by at least three different components. For most observations, it can be fitted with a white dwarf atmosphere at temperature close to a million K and two soft components of thermal plasma (e.g. APEC in XSPEC), one at kT=40-100 eV and the second at kT=0.2-0.4 keV. The spectrum of the last exposures became moderately "harder", although the increase in flux was evident in the whole energy range.