Recent Swift Observations of the Ultra-luminous X-ray Pulsar NGC 7793 P13
ATel #13343; E. Manrow, B. Tetarenko, J. M. Miller (Univ. of Michigan)
on 10 Dec 2019; 15:23 UT
Credential Certification: Jon Miller (jonmm@umich.edu)
Subjects: X-ray, Binary, Neutron Star, Transient
We report on a recent observations of the well-known ultra-luminous X-ray source (ULX) and pulsar, NGC 7793 P13 (also see ATELs #6828, #9068), obtained with the Neil Gehrels Swift Observatory.
A 1 ks XRT exposure was obtained on 2019-10-28 (obsID 00095120034). Only 12 net photons are detected in the 0.3-10.0 keV band, preventing a spectral analysis. Assuming a fiducial absorbed power-law spectral model, with the absorption set to that expected in the Milky Way and the photon index fixed at Gamma = 2.0, we measure a flux of 6.8 E-13 erg/cm^2/s (1-sigma range: 5.6-9.7 E-13 erg/cm^2/s). This implies a source luminosity of 1.07 E+39 erg/s, which exceeds the isotropic Eddington limit for plausible neutron star masses. This value is within the range that is typical for NGC 7793 P13.
A second 1 ks observation on 2019-12-02 (obsID 00095120039) yielded 19 photons with a slightly higher flux of 7.38e-13 erg/cm^2/s (1-sigma range: 0.6-1.0 E-12 erg/cm^2/s), implying a source luminosity of 1.1 E+39 erg/s.
A later observation on 2019-12-09 (obsID 00095120040) lasted only 0.8 ks and recorded a lower flux of 1.8 E-13 erg/cm^2/s (1-sigma range: 0.7-2.4 E-13 erg/cm^2/s), implying a source luminosity of 2.8 E+38 erg/s. This luminosity is within the Eddington limit if the neutron star is particularly massive, potentially indicating that the accretion rate had changed, that geometric beaming effects had changed, or that both may have changed.
