AstroSat and NICER timing view of the Z−type neutron star X-ray binary GX 340 + 0

Author:

Pahari Mayukh1ORCID,Suman Shree1,Bhargava Yash2ORCID,Weston Alexander3,Zhang Liang4ORCID,Bhattacharyya Sudip2ORCID,Misra Ranjeev5ORCID,McHardy Ian3

Affiliation:

1. Department of Physics, Indian Institute of Technology Hyderabad , Hyderabad, Kandi, 502285 Sangareddy , India

2. Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research , 1 Homi Bhabha Road, Colaba, 400005 Mumbai , India

3. School of Physics & Astronomy, University of Southampton , Highfield, Southampton SO17 1BJ , UK

4. Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences , 100049 Beijing , China

5. Inter-University Centre for Astronomy and Astrophysics , 411007 Pune , India

Abstract

ABSTRACT The timing properties of the Z-type low-mass X-ray binaries provide insights into the emission components involved in producing the unique Z-shaped track in the hardness–intensity diagrams of these sources. In this work, we investigate the AstroSat and NICER observations of the GX 340 + 0 covering the complete ‘Z’-track from the horizontal branch (HB) to the extended flaring branch (EFB). For the first time, we present the Z-track as seen in soft X-rays using the AstroSat/Soft X-ray Telescope and NICER (the soft colour is defined as a ratio of 3–6 to 0.5–3 keV). The shape of the track is distinctly different in soft X-rays, strongly suggesting the presence of additional components active in soft X-rays. The detailed timing analysis revealed significant quasi-periodic oscillation throughout the HB and the normal branch (NB) using large area X-ray proportional counter and the first NICER detection of 33.1 ± 1.1 Hz HB oscillation (HBO) in 3–6 keV. The oscillations at the HB/NB vertex are observed to have higher frequencies (41–52 Hz) than the HBOs (16–31 Hz) and NB oscillations (6.2–8 Hz) but significantly lower rms (∼1.6 per cent). The HBO is also limited to the energy range 3–20 keV, indicating an association of HBO origin with the non-thermal component. It is also supported by earlier studies that found the strongest X-ray polarization during HB.

Funder

ISRO

TIFR

NASA

Royal Astronomical Society

University of Southampton

NSFC

Publisher

Oxford University Press (OUP)

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