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Swift Observations of the M51 ULX RX J133007+47110

ATel #15803; E. Raul, S. Bacon, M. Brumback, S. Hoin, J. M. Miller, M. T. Reynolds (Univ. of Michigan)
on 12 Dec 2022; 18:28 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

Subjects: X-ray, Binary

We report on recent observations of the ultra-luminous X-ray source M 51 ULX-8 (RX J133007+47110) within the face-on spiral galaxy as obtained by the Neil Gehrels Swift Observatory. This source appears to be an especially variable ULX and therefore of particular interest.

A 0.1 ks XRT exposure was taken on 2022-10-28 (obsID 00011417138). Only 1 net photon was detected in the 0.3-10.0 keV band. Using an absorption value of N_H = 3.27 E+20 cm^-2 and a photon index of Gamma = 2.0, we calculate an upper limit on the flux of F < 3.4 E-13 ergs/cm2/s and a corresponding upper limit on the source luminosity of L < 2.9 E+39 ergs/cm2/s assuming a distance of 8.4 Mpc.

A second observation, ~1 ks in duration, was conducted on 2022-11-03 (obsID 00011417139) and yielded 14 net photons. Assuming an absorbed power-law spectral model with the same model parameters as before, we find a higher flux of F = 1.0 E-12 ergs/cm2/s (1-sigma range: 0.64-1.37 E-12 erg/cm^2/s), which indicates a source luminosity of L = 5.3 E+39 ergs/cm2/s (1-sigma range: 3.4-7.2 E+39 erg/cm^2/s).

A third observation on 2022-11-09 (obsID 00011417140) that also lasted ~1 ks received 9 net photons and recorded a similarly higher flux of F = 7.1 E-13 erg/cm^2/s (1-sigma range: 4.4-9.4 E-13 erg/cm^2/s), which implies a source luminosity of L = 3.8 E+39 ergs/cm2/s (1-sigma range: 2.6-5.2 E-39 erg/cm^2/s).

All of these luminosities exceed the isotropic Eddington limit for typical neutron star masses, potentially indicating beamed emission. We encourage further observations with Swift and other telescopes in order to better determine its variability characteristics.