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Detection of X-ray pulsations at the lowest observed luminosity of Be/X-ray binary pulsar EXO 2030+375 with AstroSat

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Abstract

We present the results obtained from timing and spectral studies of Be/X-ray binary pulsar EXO 2030+375 using observations with the Large Area Xenon Proportional Counters and Soft X-ray Telescope of AstroSat, at various phases of its Type-I outbursts in 2016, 2018, and 2020. The pulsar was faint during these observations as compared to earlier observations with other observatories. At the lowest luminosity of 2.5\(\times \)10\(^{35}\) erg s\(^{-1}\) in 0.5–30 keV energy range, \(\approx \)41.3 s pulsations were clearly detected in the X-ray light curves. This finding establishes the first firm detection of pulsations in EXO 2030+375 at an extremely low mass accretion rate to date. The shape of the pulse profiles is complex due to the presence of several narrow dips. Though pulsations were detected up to \(\sim \)80 keV when the source was brighter, pulsations were limited up to \(\sim \)25 keV during the third AstroSat observation at lowest source luminosity. A search for quasi-periodic oscillations in \(2\times 10^{-4}\) Hz to 10 Hz yielded a negative result. Spectral analysis of the AstroSat data showed that the spectrum of the pulsar was steep with a power-law index of \(\sim \)2. The values of photon-indices at observed low luminosities follow the known pattern in sub-critical regime of the pulsar.

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Notes

  1. http://astrosat-ssc.iucaa.in/.

  2. http://www.swift.ac.uk/user_objects/.

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Acknowledgements

We thank the anonymous reviewer for suggestions on the paper. This publication uses the data from AstroSat mission of the ISRO, archived at the Indian Space Science Data Centre. We thank members of SXT and LAXPC instrument teams for their contribution to the development of the instruments and analysis software. The SXT and LAXPC Payload Operations Centers (POCs) at TIFR are acknowledged for verifying and releasing the data via the ISSDC data archive and providing the necessary software tools for data analyses. We also acknowledge the contributions of the AstroSat project team at ISAC and IUCAA. This research has made use of data obtained through HEASARC Online Service, provided by the NASA/GSFC, in support of NASA High Energy Astrophysics Programs. This work used the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA).

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Correspondence to Gaurava K. Jaisawal.

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This article is part of the Special Issue on “AstroSat: Five Years in Orbit”.

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Jaisawal, G.K., Naik, S., Gupta, S. et al. Detection of X-ray pulsations at the lowest observed luminosity of Be/X-ray binary pulsar EXO 2030+375 with AstroSat. J Astrophys Astron 42, 33 (2021). https://doi.org/10.1007/s12036-021-09699-2

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  • DOI: https://doi.org/10.1007/s12036-021-09699-2

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