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)
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.