Investigating Swift Observations of NGC 5907 ULX-1 and Estimating its Luminosity

ATel #16015; Matthew Wanink, McKinley Brumback, Anna Gaishin, Elise Kesler, Nathan Morrow (University of Michigan)
on 27 Apr 2023; 14:26 UT
Credential Certification: McKinley Brumback (brumback@umich.edu)

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NGC 5907 ULX1 is an ultraluminous X-ray source residing in the Knife Edge Galaxy (NGC 5907) about 56 million light years away. This source is itself a ULX pulsar which shows variations in its luminosity with time (Fuerst et al. 2023). We analyzed four Swift XRT observations from March and April 2023, although we found that it is difficult to fit a model in a robust way due to the low number of photons in any observation. Therefore in order to gain a good estimate of the fluxes and luminosities across our observations, we utilized WebPIMMS to estimate our flux. In order to do this, we utilized this tool with an assumed absorbed power law spectral shape with N_H = 1.96x10^20 cm^-2 and a photon index of 2.0.

We first began by investigating a 2.1 ks XRT exposure collected on 17 March 2023 for this source (obsID 00096749055). In this observation, we find a total of 15 net photons in the 0.5-10.0 keV band. Using this information along with our assumed model shape stated above, we were able to use the WebPIMMS tools to estimate a flux of (2.6 +/- 0.7)x10^-13 erg/cm^2/s. Using the estimated distance to this source, we find this corresponds to a luminosity value of (9.0 +/- 2.3)x10^39 erg/s.

We then also investigated a 1.7 ks XRT exposure collected on 24 March 2023 for this source (obsID 00096749056). In this observation, we find a total of 16 net photons in the 0.5-10.0 keV band. Using this count rate data and WebPIMMS with the same spectral values described above, we estimate a flux of (3.3 +/- 0.8)x10^-13 erg/cm^2/s and a luminosity value of (1.2 +/- 0.3)x10^40 erg/s.

From here, we found some newer observations taken of this object from April 2023 and we began by looking at the first of these 2 new observations, a 2.3 ks XRT exposure from 7 April 2023 (obsID 00097096001). In this observation, we find a total of 15 net photons in the 0.5-10.0 keV band. Using the count rate data from this observation and WebPIMMS with the same spectral values described above again, we estimate a flux of (2.3 +/- 0.6)x10^-13 erg/cm^2/s and a luminosity value of (8.3 +/- 2.1)x10^39 erg/s.

We finally conclude our investigation with the last of these new observations where we investigate a 1.6 ks XRT exposure for the source from 14 April 2023 (obsID 00097096002). In this observation, we find a total of 11 net photons in the 0.5-10.0 keV band. Using the count rate data from this observation and WebPIMMS with the same spectral values described above again, we estimate a flux value of (2.4 +/- 0.7)x10^-13 erg/cm^2/s and a luminosity value of (8.5 +/- 2.6)x10^39 erg/s.

While we do note some increases and decreases across the flux and luminosity estimates for each of these observations, there are also large errors associated with all of our flux estimates and thus the fluxes for all the observations we analyzed are consistent within these errors. In addition to this, the low number of photons mentioned above also makes it difficult to say that we have assumed the best spectral model for our source and thus, in general, deeper Swift and other X-ray observatory observations are recommended to properly catalog the variability of the source as well as the spectral shape.

References
Fuerst, F., Walton, D., Israel, G., et al. (2023). A&A, 672, A140.