ZTF observations of AMXP MAXI J1957+032

ATel #15455; Yuankun Wang (UW), Eric C. Bellm (UW), and Amruta Jaodand (Caltech) on behalf of the Zwicky Transient Facility Collaboration
on 22 Jun 2022; 23:39 UT
Credential Certification: Eric Bellm (ecbellm@uw.edu)

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Following the report of an outburst of the accreting millisecond pulsar MAXI J1957+032 in X-rays (ATel #15440) and optical (ATel #15448), we obtained forced photometry measurements on images taken by the Zwicky Transient Facility (ZTF; Bellm et al. 2019). We find 13 detections of the optical counterpart in the g-band at quiescent levels of m_g ~ 19.5 mag and 15 detections in the r-band at m_r ~ 18.9 mag in the 50 days prior to the outburst. The last observation at quiescent levels was on 2022-06-13. ZTF first observed optical brightening from the source at 2022-06-18 08:49 UTC, when the source was at m_r = 18.57 +/- 0.03 mag. A g-band observation later the same night at 09:32 UTC found the source at m_g = 18.89 +/- 0.06 mag. This optical brightening preceded the MAXI trigger at 17:26 UTC (Atel #15440). A second set of ZTF observations on 2022-06-20 yielded detections at m_g = 18.75 +/- 0.04 mag and m_r = 18.43 +/- 0.04 mag. The most recent observation by ZTF on 2022-06-22 10:00 UTC found the source at m_r = 18.71 +/- 0.02 mag, indicating that the source may be beginning to fade.

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: Graham). We acknowledge further support from the NSF under grant AST-1812779 and the Heising-Simons Foundation under grant 2018-0908.