A new ultraluminous X-ray source in NGC 4945
ATel #14380; Murray Brightman (Caltech)
on 8 Feb 2021; 22:25 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Murray Brightman (murray@srl.caltech.edu)
Subjects: X-ray, Request for Observations, Black Hole, Neutron Star, Transient
We report on the detection of a new X-ray source serendipitously discovered in a Swift/XRT observation of NGC4945X-1 (obsID 00013908005) on 2021-02-08. The source has a position of RA=196.23359, Dec=-49.53283 (=13:04:56.06s, -49:31:58.2), with an error radius of 3.1''(90% confidence) and a 0.3--10 keV count rate of 0.017+/-0.004 cts/s. The new source is >3' from the target, and no X-ray source has been reported at or near this position before despite numerous previous Swift, Chandra, XMM-Newton, and NuSTAR observations. The source appears to be associated with NGC 4945.
The XRT spectrum is not well constrained, nevertheless it can be described by an absorbed power-law model with NH<2x10^22 cm^-2 and Gamma=1.9^+2.3_-1.1. The observed 0.3--10 keV flux is 9.4x10^-13 erg/cm^2/s, which corresponds to a luminosity of 1.5x10^39 erg/s assuming a distance of 3.7 Mpc to NGC 4945. The source is therefore likely a new ultraluminous X-ray source. Previous to this detection, NGC 4945 was observed with Swift on 2021-01-25 when the source was not detected, therefore this new ULX only became active in the last 2 weeks. Further monitoring with Swift is recommended, as well as deeper follow up with other X-ray telescopes.
