NICER Monitors the Rise of the Be/X-ray Binary Swift J0243.6+6124

ATel #16121; M. Ng (MIT), Joel B. Coley (Howard University, NASA GSFC, CRESST II), G. K. Jaisawal (DTU Space), C. Malacaria (ISSI), P. Pradhan (ERAU, Prescott), K. C. Gendreau, Z. Arzoumanian (NASA GSFC), E. C. Ferrara (University of Maryland, NASA GSFC, CRESST II), on behalf of the NICER team
on 7 Jul 2023; 22:28 UT
Credential Certification: Mason Ng (masonng@mit.edu)

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The known Be/X-ray binary Swift J0243.6+6124 has been in outburst since 2023 April 8 (see MAXI/GSC, ATel #15983; Swift/XRT, ATel #15984; NICER, ATel #15987). Beginning on 2023 June 2, Swift/BAT reported the continued rise of the source (ATel #16076), with the 15-50 keV flux being 1.14 +/- 0.04 Crab on 2023 July 4. Radio emission from the source was also detected with the VLA (ATel #16101). NICER has been monitoring the outburst in recent weeks, and here we describe results from observations performed over 2023 June 17 to 2023 July 1 (ObsIDs 6050390227 to 6050390241). Given the low Sun angles in this interval, the number of enabled detectors varied across the different observations (we accounted for this variation using the NICERDAS response calculator), but we restricted our initial analysis to data obtained only during orbit-night (SUNSHINE==0).

NICER detected ~9.8 s pulsations with an H-test statistic of over 90,000 (> 300σ significance). The final pulsation frequency estimate is 0.1020(2) Hz. The folded pulse profile is roughly sinusoidal, with an ephemeral and weak secondary component that appeared toward the start of the rise and has disappeared since (after MJD 60117). This evolution was observed in the 2017 outburst of the source (Wilson-Hodge et al., 2018, ApJ, 863, 9). No orbital corrections were applied for the preliminary analysis.

We fitted the 0.8-10.0 keV spectrum for all observations with an absorbed power law plus thermal blackbody (bbodyrad in XSPEC) model, together with an additional Gaussian emission component for the neutral iron fluorescence line around 6.4 keV. The hydrogen column density was fixed to the average best-fit value (over the NICER observations) of 1.14× 10²²cm^-2. There was a slight non-monotonic increase in the blackbody temperature, from kT = 0.41 (-0.10, +0.10) keV to 0.59 (-0.05, +0.06) keV, while the blackbody normalization remained roughly constant at around 600 (weighted average) within uncertainties. On the other hand, the power-law index increased from 1.14 (-0.02, +0.02) on MJD 60112 to around 1.267 (-0.013,+0.012) on MJD 60117 and has remained constant since then. The latest value, on MJD 60126, is 1.27 (-0.02, +0.02). The Gaussian line width and equivalent widths were roughly constant with weighted averages of 0.26 keV and 0.053 keV, respectively. Finally the 0.4-10.0 keV absorbed flux has increased by 170% from 7.53 (-0.04, +0.03) × 10^-9 erg/s/cm² to 1.293 (-0.010, +0.007) × 10^-8 erg/s/cm². Uncertainties are reported at 90% confidence.

NICER continues to observe Swift J0243.6+6124. We encourage further observations with other facilities.

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.