Recent Swift Observations of NGC 5907 ULX-1
ATel #15809; E. Raul, S. Bacon, M. Brumback, S. Hoin, J. M. Miller, M. T. Reynolds (Univ. of Michigan)
on 14 Dec 2022; 16:36 UT
Credential Certification: Jon Miller (jonmm@umich.edu)
Subjects: X-ray, Binary, Neutron Star
We report on four recent Neil Gehrels Swift Observatory exposures of
the ultra-luminous X-ray source (ULX) NGC 5907 ULX-1. This source has
been previously noted as an extremely short term pulsating ULX, making
it a potentially significant target for faint pulsar observations
(Israel et al. 2017).
A ~1.9 ks duration exposure was carried out on 2022-11-11 (obsID
00096749035) using the X-Ray Telescope (XRT), from which 5 net photons
were received. Using a power-law spectral model with an absorption of
N_H = 2.1 E+20 cm^-2 and a photon index of gamma = 2.0, we find an
unabsorbed flux of F = 9.1 E-14 erg/cm^2/s (1-sigma range: 0.5-1.3
E-13 erg/cm^2/s) in the 0.3-10.0 keV band. Using a distance of 17.2
Mpc, we derive an equivalent luminosity of L = 3.2 E+39 ergs/cm2/s
(1-sigma range: 1.8-4.7 E+39 erg/cm^2/s).
A subsequent ~1.8 ks observation was conducted on 2022-11-20 (obsID
00096749036), which detected 9 net photons. Utilizing the same model
and parameters as before, we record a characteristic flux of F = 1.2
E-12 erg/cm^2/s (1-sigma range: 0.7-1.8 E-12 erg/cm^2/s), one order of
magnitude higher than the previous observation. We obtain a similarly
larger source luminosity of L = 4.3 E+40 ergs/cm2/s (1-sigma range:
2.4-6.3 E+40 erg/cm^2/s).
A ~1.7 ks observation on 2022-11-25 (obsID 00096749037) collected 8
net photons in the same energy band. From this observation we once
again acquire a relatively higher flux of F = 9.2 E-13 erg/cm^2/s
(1-sigma range: 0.5-1.4 E-12 erg/cm^2/s), from which we compute a
luminosity of L = 3.3 E+40 ergs/cm2/s (1-sigma range: 1.7-5.1 E+40
erg/cm^2/s).
Lastly, a fourth observation was taken on 2022-12-02 (obsID
00096749038) for approximately ~2.1 ks. This yielded 3 net photons and
an associated flux value of F = 4.0 E-13 erg/cm^2/s (1-sigma range:
2.1-6.7 E-13 erg/cm^2/s), from which we calculate a source luminosity
of L = 1.4 E+40 ergs/cm2/s (1-sigma range: 0.7-2.4 E+40 erg/cm^2/s).
Despite the apparent increase and decrease in luminosity throughout
the four observations, it is difficult to comment on the exact
variability from the source due to the large errors. We recommend
deeper observations with Swift and other X-ray observatories to
further characterize the variability of this source.
References
Israel G. L., et al., 2017a, Science, 355, 817
