ZTF optical non-detection of Chandra Fast X-ray Transient XRT 210423

ATel #14600; Igor Andreoni (Caltech), Mansi Kasliwal (Caltech), Matthew Graham (Caltech)
on 5 May 2021; 04:14 UT
Credential Certification: Igor Andreoni (igor.andreoni@gmail.com)

Tweet

The Zwicky Transient Facility (ZTF; Bellm et al., 2019; Graham et al., 2019) observed the field where the Chandra fast transient XRT 210423 (Lin et al., ATel #14599) was found 1588 times, on 820 distinct nights, from the beginning of the survey on 2018 March 17 until 2021 May 4.

We explored ZTF data at the location of XRT 210423 using the ZTF point-spread-function (PSF) forced photometry service (Masci et al., 2018). No source was detected in individual images and no significant detection was found by binning nightly data in each g-r-i band.

On 2021 April 23, which is the date of the Chandra discovery of XRT 210423, ZTF observations of the field were not performed. A total of 43 images, taken between 2021 April 23 and 2021 May 4, were successfully processed (19 in g-band, 20 in r-band, 4 in i-band). The table below presents the closest upper limits (5 sigma) available, in each band, before and after 2021 April 23. These upper limits were computed via nightly binning of the flux measured in individual images, processed through the ZTF reduction and image subtraction pipelines at IPAC (Masci et al., 2019).

---------------------------------
Date (UT) | filter | UL (mag AB)
---------------------------------
2021-04-20 | i | >20.8
2021-04-21 | g | >21.5
2021-04-21 | r | >21.7
2021-04-24 | g | >20.5
2021-04-24 | r | >20.9
2021-05-01 | i | >19.6
---------------------------------

The coordinates of XRT 210423 reported by Lin et al. (ATel #14599) are accurate within 1.0'', which is comparable with the ZTF pixel scale. Therefore it is possible that the result of the PSF forced photometry is affected by the imprecise position of the centroid. To minimize the risk of missing a possible optical source, we visually inspected the difference images acquired after April 23 2021. We did not identify any likely point source within 3'' from the nominal coordinates in individual images, images nightly stacks, and stacks of all the images available.


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.