Swift J073006.8-193709: An X-ray transient likely associated with an historical optical outburst.

ATel #15404; P. A Evans (U. Leicester), D. B. Malesani (Radboud U. and DAWN/NBI)
on 30 May 2022; 15:56 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Phil Evans (pae9@star.le.ac.uk)

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Swift-XRT has detected an X-ray transient at RA, Dec = 112.5284, -19.6193 which corresponds to:

RA (J2000): 07h 30m 06.82s
Dec (J2000): -19d 13' 09.3"

with an uncertainty of 5.7 arcsec (90% confidence radius). The transient was detected in an observation from 08:13:32 UT to 08:39:29 UT on 2022 May 30, with a 0.3-10 keV count-rate of 1.6 (+0.5, -0.4) x 10^-2 ct/sec. The 3-sigma upper limit derived by summing all previous observations of this position is 3.5×10^-3 ct/sec.

There are too few events to allow an accurate characterisation of the X-ray spectrum, however a simple power-law fit gives a photon index of 1.6 (+1.5, -1.2) and an absorption of <10²² cm^-2, from which we deduce a 0.3-10 keV observed flux of 7.5 (+11.4, -4.6) x 10^-13 erg cm^-2 s^-1.

This position is coincident with a source reported in the PanSTARRS-1 and Gaia catalogues. The mean Gaia magnitudes are G=20.95, BP=20.62, R=19.93. The PS1 magnitudes are generally similar, (g=21.8, i=20.9, z=20.8) but with singificant outliers: r=16.7 and y=17.8. An inspection of the PS1 archive shows that this object underwent a strong outburst on 2013 June 24-25: the PS1 images showing the outburst have MJD range 56316.45144 to 56317.41653 and there are no further images in the archive until MJD 56676.34316 (2014 Jan 19) thus we have no strong constraint on the duration of the outburst.

Given the XRT detection, it is likely that this source has erupted again, and follow-up observations are encouraged. Unfortunately, the object lay outside of the UVOT field of view.

This source was detected in near real-time by a new automated XRT transient-searching system operating at the University of Leicester.