XTE J1701-462 continues its rise in the soft state

Following reports (ATels #15592, #15594, #15602) of a new outburst of the neutron star transient XTE J1701-462, NICER observed the source three times on 2022 September 8 and 9 for a total exposure of 482 s. A clear rising trend in the 0.5-10 keV count rate was seen, from ~590 cts/s on Sep 8 17:12 UTC to ~650 cts/s on Sep 9 04:03 UTC. This confirms the rapid rise seen with Swift and MAXI (ATel #15602).

No type-I X-ray bursts were observed. A combined power spectrum (0.3-10 keV) of the three data segments reveals little variability in the 0.01-4000 Hz range, consistent with it being due to white noise. A fit with a power-law to power spectrum results in an 3-sigma upper limit on the power in the (0.01-100 Hz) range of 2.1% r.m.s.. No significant coherent periodicities were detected from acceleration searches over the individual segments or from averaged power spectra (32 s segments).

We used several models to fit the time-averaged 0.5-10 keV spectrum of the observation. An absorbed power-law (as reported in ATel #15602) provides a poor fit (reduced chi-squared of 2.64 for 821 degrees of freedom) with broad residuals at low and high energies. Adding a blackbody significantly improves the fit (reduced chi-squared of 1.01 for 819 d.o.f.) and results in an Nh of 2.89(3)e22 cm^-2, a power-law index of 1.71(4), and blackbody temperature of 1.34(2) keV. Such a model would indicate a spectrally hard or intermediate state in a neutron-star LMXB atoll source, but this would be inconsistent with the observed lack of rapid variability. We therefore also tried a double thermal model (absorbed disk blackbody plus blackbody). This model also performs well (reduced chi-squared of 0.98 for 819 d.o.f.) and results in an Nh of 2.60(2)e22 cm^-2, a disk temperature of 1.27(4) keV, and blackbody temperature of 1.82(8) keV. This would indicate a soft spectral atoll source state, which is supported by the variability properties. For this model, we obtain an unabsorbed 0.5-10 keV flux of ~7.1e-9 erg/cm^2/s. For a distance of 8.8 kpc (Lin et al. 2009, ApJ, 699, 60) this corresponds to a luminosity of 6.6e37 erg/s or ~34% of the Eddington luminosity for a 1.4 solar-mass neutron star. This luminosity range is also more consistent with a soft spectral state than with hard or intermediate states. If the source continues its current rapid rise, we expect it may start showing signatures of Z source behavior around a week from now (Lin et al. 2009, ApJ, 696, 1257; Homan et al. 2010, ApJ, 719, 201).

NICER will continue to observe XTE J1701-462. We encourage further observations of the source with other observatories.

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.