Probing spectral and timing properties of the X-ray pulsar RX J0440.9 + 4431 in the giant outburst of 2022

Probing spectral and timing properties of the X-ray pulsar RX J0440.9 + 4431 in the giant outburst of 2022–2023

Published:2023-09-12 Issue:1 Volume:526 Page:771-781
ISSN:0035-8711
Container-title:Monthly Notices of the Royal Astronomical Society
language:en
Short-container-title:

Author:

Mandal Manoj¹^ORCID,Sharma Rahul²^ORCID,Pal Sabyasachi¹^ORCID,Jaisawal G K³^ORCID,Gendreau Keith C⁴,Ng Mason⁵,Sanna Andrea⁶^ORCID,Malacaria Christian⁷^ORCID,Tombesi Francesco⁴⁸⁹¹⁰¹¹,Ferrara E C⁴⁸¹²,Markwardt Craig B⁴,Wolff Michael T¹³,Coley Joel B¹²¹⁴

Affiliation:

1. Midnapore City College , Kuturia, Bhadutala, West Bengal 721129 , India

2. Raman Research Institute, C.V. Raman Avenue, Sadashivanagar , Bengaluru, Karnataka 560080 , India

3. DTU Space, Technical University of Denmark , Elektrovej 327-328, DK-2800 Lyngby , Denmark

4. Astrophysics Science Division, NASA Goddard Space Flight Center , Greenbelt, MD 20771 , USA

5. MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology , Cambridge, MA 02139 , USA

6. Dipartimento di Fisica, Università degli Studi di Cagliari , SP Monserrato-Sestu km 0.7, I-09042 Monserrato , Italy

7. International Space Science Institute , Hallerstrasse 6, CH-3012 Bern , Switzerland

8. Department of Astronomy, University of Maryland , College Park, MD 20742 , USA

9. Physics Department, Tor Vergata University of Rome , Via della Ricerca Scientifica 1, I-00133 Rome , Italy

10. INAF – Astronomical Observatory of Rome , Via Frascati 33, I-00040 Monte Porzio Catone , Italy

11. INFN – Rome Tor Vergata , Via della Ricerca Scientifica 1, I-00133 Rome , Italy

12. Center for Research and Exploration in Space Science & Technology II (CRESST II) , NASA/GSFC, Greenbelt, MD 20771 , USA

13. Space Science Division, U.S. Naval Research Laboratory , Washington, DC 20375 , USA

14. Department of Physics and Astronomy, Howard University , Washington, DC 20059 , USA

Abstract

ABSTRACT The X-ray pulsar RX J0440.9 + 4431 went through a giant outburst in 2022 and reached a record-high flux of 2.3 Crab, as observed by Swift/BAT. We study the evolution of different spectral and timing properties of the source using NICER observations. The pulse period is found to decrease from 208 s to 205 s, and the pulse profile evolves significantly with energy and luminosity. The hardness ratio and hardness intensity diagram (HID) show remarkable evolution during the outburst. The HID turns towards the diagonal branch from the horizontal branch above a transition (critical) luminosity, suggesting the presence of two accretion modes. Each NICER spectrum can be described using a cutoff power law with a blackbody component and a Gaussian at 6.4 keV. At higher luminosities, an additional Gaussian at 6.67 keV is used. The observed photon index shows negative and positive correlations with X-ray flux below and above the critical luminosity, respectively. The evolution of spectral and timing parameters suggests a possible change in the emission mechanism and beaming pattern of the pulsar depending on the spectral transition to sub- and supercritical accretion regimes. Based on the critical luminosity, the magnetic field of the neutron star can be estimated in the order of 1012 or 1013 G, assuming different theoretical models. Moreover, the observed iron emission line evolves from a narrow to a broad feature with luminosity. Two emission lines originating from neutral and highly ionized Fe atoms are evident in the spectra around 6.4 and 6.67 keV (later is seen only in higher luminosities).

Funder

RIKEN

JAXA

NASA

Naval Research Laboratory

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

Link

https://academic.oup.com/mnras/advance-article-pdf/doi/10.1093/mnras/stad2767/51534795/stad2767.pdf

Reference64 articles.