NuSTAR observation of the new outburst of XTE J1701-462

ATel #15616; Jakob van den Eijnden (University of Oxford), Sara E. Motta (INAF-OAB, University of Oxford)
on 13 Sep 2022; 12:22 UT
Credential Certification: Jakob Van den Eijnden (a.j.vandeneijnden@uva.nl)

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XTE J1701-462 is a known neutron star X-ray binary that is currently in outburst, as initially reported by MAXI (ATel #15592) and confirmed by Swift (ATel #15594). XTE J1701-462 is active for the first time since its 2006 outburst, when it showed both atoll and Z-source states (Homan et al. 2010, ApJ, 719, 201).

NuSTAR observed XTE J1701-462 between 2022 September 11 16:45 and September 12 04:45 UTC for a total on-source exposure time of 16.9 ks. We reduced the data using standard tools, and we applied the necessary corrections to take into account the high count rate from the target, which was varying between 200 and 300 ct/s (per FPM) throughout the observation. No systematic trend in the count rate is visible in the light curve.

XTE J1701-462 is significantly detected above the background up to 30 keV. Clear reflection signatures, in particular a broad iron line between 6 and 7 keV, are clearly visible in the energy spectrum. Following the analysis of recent NICER spectra (ATel #15605) we attempt spectral fits with two different continuum models: an absorbed blackbody + power law or an absorbed disk-blackbody + blackbody continuum. In both cases, we add a simple diskline component to account for the iron line complex, although a detailed reflection analysis will be performed at a later stage.

The absorbed blackbody + power law continuum model does not provide an accurate fit, leaving significant residuals above 20 keV. On the other hand, the absorbed disk-blackbody + blackbody continuum model provides a better fit. The reduced chi squared is 1.22, dominated by remaining residuals around the iron line. We measure an absorption column density of 2.0(1)e22 cm^-2, a disk temperature of 1.85(2) keV, and a blackbody temperature of 2.71(2) keV.

The unabsorbed 0.5-10 keV model flux is 9e-9 erg/s/cm^2, which corresponds to an X-ray luminosity of 8e37 erg/s assuming a distance of 8.8 kpc (Lin et al. 2009, ApJ, 699, 60), or ~40% of the neutron star Eddington luminosity. This spectral analysis suggests that XTE J1701-462 likely remains in the soft Atoll state, increasing slowly in luminosity compared to the earlier NICER observations reported in ATel #15605.

If the source transitions to the Z-source state, we will arrange a second NuSTAR observation. Multi-wavelength monitoring of this outburst is strongly encouraged.

We thank Fiona Harisson and the NuSTAR Science Operations Centre for rapidly accepting, scheduling, and performing the requested DDT observation.