Tilak Katoch (TIFR, Mumbai), H. M. Antia (CEBS & TIFR, Mumbai), Anuj Nandi (U R Rao Satellite Centre, Bengaluru), Parag Shah (TIFR, Mumbai)
ATel #16243; Discovery detection of strong QPO (~ 1. 4 Hz) at hard X-rays (> 40 keV) with AstroSat during 'unusual' peak of black hole transient Swift J1727. 8-1613
on 13 Sep 2023; 17:51 UT
Credential Certification: Tilak Katoch (tilak@tifr.res.in)
Subjects: X-ray, Black Hole
Referred to by ATel #: 16244
A recently detected new X-ray transient source Swift J1727.8-1613 on 24th August, 2023 is reported in GCN #34540 and in ATels #16205, #16206, #16209, #16210, #16211, #16215, #16216, #16217, #16219, #16225, #16228, #16231, #16232, #16235, #16237, #16235, #16238. On a subsequent observation in optical (ATel #16208), X-ray (ATel #16207), and radio (ATel #16211), the source is believed to be a low mass black hole X-ray binary. Further study for the polarization of this object in sub-millimeter wavelengths as well as in X-rays was carried out by SMA (ATel #16230) and IXPE (ATel #16242).
AstroSat is currently observing this source in multi-wavelength band as a part of the Target of Opportunity campaign. The flux of this source was around more than 6 Crab during the AstroSat observation on 8th September, 2023. In the current observation campaign, the LAXPC20 is kept switched OFF from the safety point of view due to very high expected count rate and only LAXPC10 was kept ON, since it is being operated in a low gain mode. The LAXPC10 data can be utilized for timing study purposes of the source.
Quick analysis of LAXPC observation indicates significant evolution of power spectral distribution since the first observation (off-axis pointing) of the source on 2nd September, 2023. Low-Frequency Quasi-periodic Oscillation (LFQPO) frequency (~ 1 Hz) in the power spectrum (ATel # 16235) is observed to evolve to a frequency at ~ 1.4 Hz over 7 days during the peak of the outburst. The power spectrum is fitted with multiple Lorentzian along with the power-law components.
The power spectral modeling reveals a strong detection of LFQPO at ~ 1.4 Hz (Q ~ 6.6, rms ~ 5.7%) along with weak signature of harmonic at ~ 2.8 Hz (Q ~ 1.1). The total rms variability is estimated to be ~ 20% in the frequency range of 0.01 Hz to 50 Hz. The model fitted power spectrum is shown in the Figure 1 (https://www.tifr.res.in/~astrosat_laxpc/5836-lxp-pds.jpg).
In order to investigate the energy dependence as well as the strength of the QPO detected by AstroSat, we further studied the power spectra of the source in various energy bands such as ~ 20-40 keV, 40-60 keV, 60-80 keV, and 80-100 keV. The source revealed the evolution of power spectra along with detection of QPO in all the above energy bands as shown in the Figure 2 (https://www.tifr.res.in/~astrosat_laxpc/5836-lxp-ene.jpg).
LAXPC was built by TIFR in collaboration with the Indian Space Research Organisation (ISRO). The
Indian Space Research Organisation funded, managed and facilitated the project.
