The energy spectra could be reasonably well described with an absorbed disk plus power-law continuum model. The absorption column density is measured at 3.2 x 1021 cm-2. Between the two sets of observations we further observe the disk temperature to decrease from 0.34 +/- 0.01 keV to 0.29 +/- 0.01 keV, while the power law photon index increases from 1.56 +/- 0.02 to 1.63 +/- 0.02. Strong residuals are observed between 6-8 keV likely associated with Fe emission lines. Further investigation of these features is under way. The 1-10 keV flux was 2.5 x 10-9 and 3.3 x 10-9 erg/s/cm2, respectively.
Considering the temporal variability in the 0.5-10 keV band, we find that on May 2nd the power spectrum showed band limited noise below 10 Hz with an amplitude of about 23% rms. Additionally, we see a sharp QPO at ~0.13 Hz. By May 4th, the QPO frequency had increased to ~0.26 Hz, while the amplitude of the noise component decreased to 18% rms.
The spectral and temporal properties are more in line with those of a hard state accreting stellar-mass black hole than with those of an accreting neutron star. However, further observations are needed to unambiguously identify the nature of the compact object.
NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
|Publication date||5 May 2021|
|Publication status||Published - 5 May 2021|
|Series||The Astronomer's telegram|