Kavli Affiliate: Deepto Chakrabarty
| First 5 Authors: Mason Ng, Ronald A. Remillard, James F. Steiner, Deepto Chakrabarty, Dheeraj R. Pasham
| Summary:
We report on results from a one-year soft X-ray observing campaign of the
ultraluminous X-ray pulsar NGC 300 ULX-1 by the Neutron star Interior
Composition Explorer (NICER) during 2018–2019. Our analysis also made use of
data from Swift/XRT and XMM-Newton in order to model and remove contamination
from the nearby eclipsing X-ray binary NGC 300 X-1. We constructed and fitted a
series of 5-day averaged NICER spectra of NGC 300 ULX-1 in the 0.4–4.0 keV
range to evaluate the long-term spectral evolution of the source, and found
that an absorbed power-law model provided the best fit overall. Over the course
of our observations, the source flux (0.4–4.0 keV; absorbed) dimmed from
$2times10^{-12}$ to below $10^{-13}{rm,erg,s^{-1},cm^{-2}}$ and the
spectrum softened, with the photon index going from $Gammaapprox1.6$ to
$Gammaapprox2.6$. We interpret the spectral softening as reprocessed emission
from the accretion disk edge coming into view while the pulsar was obscured by
the possibly precessing disk. Some spectral fits were significantly improved by
the inclusion of a disk blackbody component, and we surmise that this could be
due to the pulsar emerging in between obscuration episodes by partial covering
absorbers. We posit that we observed a low-flux state of the system (due to
line-of-sight absorption) punctuated by the occasional appearance of the
pulsar, indicating short-term source variability nested in longer-term
accretion disk precession timescales.
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