AstroSat observations during prolonged hard state of 2021 outburst of GX 339-4

ATel #14455; Bhuvana G. R. (Dayananda Sagar University, Bengaluru), Radhika D. (Dayananda Sagar University, Bengaluru), Anuj Nandi (U R Rao Satellite Centre, Bengaluru)
on 12 Mar 2021; 11:52 UT
Credential Certification: Dipankar Bhattacharya (dipankar@iucaa.in)

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The outbursting black hole binary source GX 339-4 was detected to undergo a new outburst by MeerKAT observation on 2021-01-04 14:20 UTC (ATel #14336). The follow-up observations by various instruments suggested that the source is in its rising hard state (ATel #14336, #14351,#14352, #14354, #14367, #14384, #14400, #14419). Here, we report the broadband spectral and temporal properties of the source based on the continuous observation performed by AstroSat during 2021-03-02 01:03 UTC to 2021-03-04 20:20 UTC.

Preliminary modelling of the broadband (3 - 50 keV) energy spectra carried out using diskbb and nthcomp models resulted in a disk temperature of ~1.18 keV and photon index of ~1.55. In addition, we observe Fe emission line at ~6.7 keV and a possible reflection signature at higher energy. Source flux in 3 - 50 keV is ~ 6.4e-9 erg/cm²/s with percentage contribution of Comptonized flux ~ 91%.

The Low Frequency Quasi-periodic Oscillation (LFQPO) frequency (~ 0.11 Hz) in power density spectra (PDS) reported in Atel #14400 is seen to evolve in the range of 0.18 to 0.21 Hz within the 3 days period with QPO rms amplitude of ~ 9%. The PDS retained a flat top noise profile with a total rms of 23.9% in 1 mHz - 20 Hz range. Spectral and temporal characteristics indicate that the source is in a prolonged (~ 60 days) hard state since the outburst triggered. We thank the AstroSat Time Allocation Committee and Satellite operations team for the quick response to our ToO proposal, and data release. This work is part of a project funded by ISRO under AstroSat data utilization program