Monitoring observations of SMC X-1’s excursions (MOOSE)–I. Programme description and initial high state spectral results

Abstract

SMC X-1 has exhibited three super-orbital period excursions since the onset of X-ray monitoring beginning with RXTE’s launch in 1995. NICER has recently probed a fourth observed excursion beginning in 2021 with our program Monitoring Observations of SMC X-1’s Excursions (MOOSE). These sensitive new MOOSE data probe different super-orbital periods and phases within them. Spectral fits to the high-state continuum during April 2021 to January 2022 show that the intrinsic spectral shapes are characterised by a soft (kT∼0.19 keV) disc component and a hard (Γ ∼0.7) power-law tail. When the 2021-2022 NICER observations, taken during an excursion, are compared to 2016 XMM-Newton observations (outside of an excursion), we find little evidence for intrinsic spectral variability across the high-states, but find evidence for a >3 σ change in the absorption, although we caution that there may be calibration differences between the two instruments. Thus, over different lengths of super-orbital periods, we see little evidence for intrinsic spectral changes in the high-state. Upcoming studies of the pulse profiles may shed light on the mechanism behind the excursions.

Constraining the Evolution of the Unstable Accretion Disk in SMC X-1 with NICER

“…Our 16 phase-averaged spectral fits produced an average photon index of 0.49 and an average N H of 1.4 × 10 21 cm −2 . These values are both smaller than the average photon index and N H found by Dage et al (2022), 0.69 and 2.48 × 10 21 cm −2 , respectively, whose NICER observations of SMC X-1 occurred later during the 2020-2022 superorbital excursion. The difference in these values could arise from differences in the spectral modeling; Dage et al (2022) restricted the cutoff energy in their fits to vary between 12 and 20 keV, and we used a range of 8-15 keV.…”

“…These values are both smaller than the average photon index and N H found by Dage et al (2022), 0.69 and 2.48 × 10 21 cm −2 , respectively, whose NICER observations of SMC X-1 occurred later during the 2020-2022 superorbital excursion. The difference in these values could arise from differences in the spectral modeling; Dage et al (2022) restricted the cutoff energy in their fits to vary between 12 and 20 keV, and we used a range of 8-15 keV. The cutoff energy is poorly constrained in both our data set (see large fluctuations in Table 2) and in Dage et al (2022, see their Section 3.1).…”

“…The cutoff energy is poorly constrained in both our data set (see large fluctuations in Table 2) and in Dage et al (2022, see their Section 3.1). Additionally, Dage et al (2022) used Gaussian emission lines to model the soft emission-line blends, where we used the collisionally ionized plasma model apec.…”

“…The difference in these values could arise from differences in the spectral modeling; Dage et al (2022) restricted the cutoff energy in their fits to vary between 12 and 20 keV, and we used a range of 8-15 keV. The cutoff energy is poorly constrained in both our data set (see large fluctuations in Table 2) and in Dage et al (2022, see their Section 3.1). Additionally, Dage et al (2022) used Gaussian emission lines to model the soft emission-line blends, where we used the collisionally ionized plasma model apec.…”

“…Previous and ongoing NICER studies of SMC X-1 suggest that the blackbody temperature in SMC X-1 is a more constant spectral parameter than N H (Dage et al 2022). In keeping with these findings, we limited the degeneracy between N H and blackbody temperature by only allowing both parameters to be free in the brightest observation from each epoch (Observations 105,204,304,and 402).…”