AstroSat observes XTE J1701-462 in its Z phase

ATel #15654; N. T. Thomas, S. Anirudh, L. Giridharan, S. B. Gudennavar (CHRIST - Deemed to be University), R. Misra (IUCAA), S. G. Bubbly (CHRIST - Deemed to be University)
on 10 Oct 2022; 12:10 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Shivappa Gudennavar (shivappa.b.gudennavar@christuniversity.in )

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XTE J1701−462 is a peculiar neutron star low mass X-ray binary (NS-LMXB) that was discovered when it underwent an outburst in 2006 (ATel #696) during which, it exhibited characteristics of both Z and Atoll type NS-LMXBs (Homan, J., et al., 2007, ApJ, 656, 420)

On 6^th September, 2022, MAXI/GSC detected a new X‐ray outburst from XTE J1701−462 (ATel #15592). This outburst was confirmed by SWIFT/XRT (ATels #15594 & #15602) and was followed-up with campaigns by NuSTAR (ATel #15616), ATCA (ATel #15617) and MeerKAT (ATel #15621). Continuous monitoring of the source with NICER, between 20^th and 24^th September, 2022 suggests it to trace a Cygnus-like Z track in its hardness-intensity diagram (ATels #15605 & #15627).

We report the AstroSat observation of XTE J1701−462 using the SXT and LAXPC instruments. The source was observed for ~40 ks from 25^th September, 2022 16:11:41.7 UTC to 26^th September, 2022 14:38:16.0 UTC at RA(J2000): 255.24 and Dec(J2000): −46.18. The simultaneous SXT and LAXPC spectrum of the source in 0.7 — 25.0 keV range could be adqeuately described with the model combination - constant*tbabs*(thcomp*diskbb). This yielded an inner disk temperature (kT_in) of 1.80 +0.03/−0.02 keV and a photon index (Γ) of 3.60 +/− 0.04, with the electron temperature (kT_e) frozen to 100 keV and an absorption column density (N_H) of 2.48 +/− 0.04 × 10²² cm^-2. The total unabsorbed flux in the 0.7 to 25.0 keV range was calculated to be 1.37 +0.03/−0.02 × 10^-8 erg cm^-2 s^-1. The 0.1−100 Hz power density spectrum in the 4−25 keV range showed the presence of broadband noise and a quasi periodic oscillation (QPO) peak at 36.15 +0.97/−0.93 Hz.

From the spectral and temporal parameters, we conclude XTE J1701−462 to be in the horizontal branch of Cygnus-like Z track of NS-LMXBs.

Follow-up multi-wavelength observations of XTE J1701−462 are encouraged.

Acknowledgement: We thank the AstroSat operations team for scheduling this ToO observation and the SXT and LAXPC Payload Operations Centre for the provision of data.