An X-ray transient in NGC 4045 detected by Swift/XRT
ATel #13336; Murray Brightman (Caltech)
on 7 Dec 2019; 17:43 UT
Credential Certification: Murray Brightman (murray@srl.caltech.edu)
Subjects: X-ray, Request for Observations, Black Hole, Neutron Star, Transient
We recently detected a new X-ray source at 12:02:42.36s (180.67650 deg), +01:58:08.5 (1.96904 deg) with Swift/XRT that appears to be associated with the galaxy NGC 4045 (z=0.0065, D=32 Mpc). The target of the Swift observation was AT2019wbg, a candidate supernova also hosted by NGC 4045, but 55" from the X-ray source and therefore not related. We do not detect any X-ray source at the position of AT2019wbg.
The source was first detected by Swift on 2019-12-04 (obsID 00012842001). The latest detection was on 2019-12-06 with a count rate of 1.4E-02+/-3.3E-03 ct/s. The spectrum can be described by a powerlaw with Gamma=1.1+/-0.7. The 0.3--10 keV flux is 6.8E-13 ergs/cm2/s, which at a distance of 32 Mpc corresponds to a luminosity of 6.3E+40 erg/s. The source is likley a new ultraluminous X-ray source (ULX). No previous X-ray source has been catalogued within several armin of this source. Observations with Swift several months prior yielded only upper limits below the current count rate. The Swift lightcurve can be found here: https://www.dropbox.com/s/c5t548q5xwebxq1/swift_ltcrv.ps?dl=0 . No source is evident in several UVOT filters.
Since ULXs are generally interpreted as super-Eddington accretion onto a black hole or neutron star, we are likely witnessing the onset of a super-Eddington event. Follow up observations with other X-ray facilities is encouraged.
Swift/XRT lightcurve
