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Swift detects X-rays from XTE J1859+226; continued optical brightening.

ATel #14375; Eric C. Bellm (UW)
on 7 Feb 2021; 02:13 UT
Credential Certification: Eric Bellm (ecbellm@uw.edu)

Subjects: Optical, X-ray, Binary, Black Hole

Referred to by ATel #: 14415, 14512

We analyzed 1.4 ksec of PC-mode Swift-XRT quicklook data for the black hole LMXB XTE J1859+226, which has brightened in the optical (ATel #14372).

We detect a single source at 18h58m41.25s +22d39m35.5s in the 0.3-10 keV band, which we identify as XTE J1859+226.

The spectrum is well-fit by an absorbed power law with photon index 1.7+/-0.4 and an observed 0.3-10 keV flux of 7.9 (+2.0, -1.5) x 10-12 erg cm-2 s-1. This represents a substantial brightening relative to the quiescent 0.3-8 keV flux of 1.5 x 10-15 erg cm-2 s-1 reported by Tomsick et al. (2003).

Zwicky Transient Facility observations on UTC 2021-02-06.6 indicate that the source continues to brighten in the the optical, with the flux increasing by ~0.4 mag in the past two days (see https://antares.noirlab.edu/loci/ANT2021dn4jk).

We conclude that XTE J1859+226 is in the early stages of an outburst; multiwavelength observations are encouraged.



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. We acknowledge further support from the NSF under grants AST-1812779 and the Heising-Simons Foundation under grant 2018-0908.