NGC 2403 XMM4: evidence for a super-Eddington neutron star with a possible transient pulsation

Luangtip, Wasutep; Roberts, Timothy P

Abstract

We present a study of the X-ray source NGC 2403 XMM4 (4XMM J073702.2+653934) based on 20 yr of archival observations
with XMM–Newton, Chandra, Swift, and NuSTAR. Although it has previously been classified as an ultraluminous X-ray source
(ULX), we show that itsluminosity rarely, if ever, passesthe 1039 erg s−1 threshold luminosity for a ULX. It does, however, behave
very similarly to ULXs, with its 0.3–10 keV spectra well described by two thermal components, the softer of which behaves
consistently with the expectations for an advection-dominated disc (L ∝ T1.49±0.85), and we find tentative evidence for an extra
spectral component above 10 keV. We also find moderately significant evidence for an absorption feature in one spectrum that
could originate in an outflowing wind, although a cyclotron resonance scattering feature is also a possibility. Most intriguingly,
we find a possible transient pulsation at ∼3.32 Hz in a short segment of one observation using an accelerated pulsation search.
This evidence suggests that NGC 2403 XMM4 is displaying many of the hallmarks of super-Eddington accretion at luminosities
in the range 5–10 × 1038 erg s−1 that, when considered alongside the putative pulsation, points to the presence of a neutron star
as the accreting object this system.

Citation

Luangtip, W., & Roberts, T. P. (2024). NGC 2403 XMM4: evidence for a super-Eddington neutron star with a possible transient pulsation. Monthly Notices of the Royal Astronomical Society, 528(1), 418-431. https://doi.org/10.1093/mnras/stae023

• Journal Article Type: Article
• Acceptance Date: Jan 3, 2024
• Online Publication Date: Jan 5, 2024
• Publication Date: 2024-02
• Deposit Date: May 22, 2024
• Publicly Available Date: May 22, 2024
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 528
• Issue: 1
• Pages: 418-431
• DOI: https://doi.org/10.1093/mnras/stae023
• Public URL: https://durham-repository.worktribe.com/output/2456218