Optical brightening of Aql X-1 detected by ZTF

ATel #14437; A. K.H. Kong (NTHU) on behalf of the Zwicky Transient Facility Collaboration
on 4 Mar 2021; 17:32 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

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We report an optical brightening of the recurrent neutron star low-mass X-ray binary Aql X-1 with the Zwicky Transient Facility (ZTF; Bellm et. al 2019, Graham et. al 2019). ZTF detected the source (ZTF name: ZTF18accedau) as a transient on 2021-02-22 with a r magnitude of ~17.6 in four observations. The source was seen again in three observations on 2021-02-27 with similar brightness. ZTF has been monitoring the field of Aql X-1 frequently since 2021-02-05 and the source was not detected. The last optical outburst of Aql X-1 detected by ZTF was in 2020 September/October with a peak r-band magnitude of about 16.4.

Public ZTF data of Aql X-1 can be obtained from https://lasair.roe.ac.uk/object/ZTF18accedau/

The 2020 September optical outburst of Aql X-1 also had an X-ray outburst detected by Swift/XRT (ATel #13961) and a superburst detected by MAXI (ATel #14079). Public MAXI data indicates that Aql X-1 has brightened since 2021-03-02 and the X-ray flux is increasing based on inspection of daily images (http://maxi.riken.jp/pubdata/v7lrkn/J1911+005/index.html). This strongly suggests that Aql X-1 is undergoing an outburst.

Multi-wavelength follow-up 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.

ZTF light curve of Aql X-1