Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13

Abstract: We report the detection of coherent pulsations from the ultraluminous X-ray source NGC 7793 P13. The ~0.42s nearly sinusoidal pulsations were initially discovered in broadband X-ray observations using XMM-Newton and NuSTAR taken in 2016. We subsequently also found pulsations in archival XMM-Newton data taken in 2013 and 2014. The significant (>>5 sigma) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star with an observed peak luminosity of ~1e40 erg/s (assuming isotropy), well above the Eddington limit for a 1.4 M_sun accretor. This makes P13 the second ultraluminous X-ray source known to be powered by an accreting neutron star. The pulse period varies between epochs, with a slow but persistent spin up over the 2013-2016 period. This spin-up indicates a magnetic field of B ~ 1.5e12 G, typical of many accreting pulsars. The most likely explanation for the extreme luminosity is a high degree of beaming, however this is difficult to reconcile with the sinusoidal pulse profile.

• The paper presents the detection of 0.42-second pulsations, identifying the compact object in NGC 7793 P13 as a neutron star.
• It employs XMM-Newton and NuSTAR data to detail pulsation characteristics, including variability and a gradual spin-up indicative of a ~1.5×10^12 G magnetic field.
• The findings challenge the assumption that ULXs are powered by black holes by demonstrating that neutron stars can attain super-Eddington luminosity, likely through beaming or collimated accretion.

Discovery of Coherent Pulsations from the Ultraluminous X-ray Source NGC\,7793 P13

The paper presents a notable discovery in the field of high-energy astrophysics, namely the identification of coherent pulsations from the ultraluminous X-ray source (ULX) NGC\,7793 P13. Using data from XMM-Newton and NuSTAR, the authors observed nearly sinusoidal pulsations with a period of approximately 0.42 seconds. The detection of such pulsations signifies that the compact object in NGC\,7793 P13 is a neutron star. This finding challenges the typical assumption that most ULXs are powered by black holes due to their high luminosity.

Key Findings

• Pulsation Detection: Coherent pulsations were detected in observations from 2013, 2014, and 2016 using XMM-Newton and NuSTAR. These pulsations suggest the presence of a neutron star rather than a black hole, as traditionally assumed for such luminous sources.
• Pulsation Characteristics: The pulse period shows variability across different observational epochs, and exhibits a slow but consistent spin-up over the timeline of observations. This spin-up is indicative of a magnetic field approximately 1.5 × 10^12 G, comparable to Galactic accreting pulsars.
• High Luminosity: The neutron star in NGC\,7793 P13 displays a peak X-ray luminosity exceeding the Eddington limit for a 1.4 solar mass accretor. This necessitates explanations involving factors like beaming or super-Eddington accretion regimes to reconcile the apparent luminosity.

Implications for Neutron Star Physics

This research further establishes that neutron stars can achieve X-ray luminosities well beyond the Eddington limit through mechanisms such as magnetic collimation of accretion flows or reduced electron scattering due to high magnetic fields. NGC\,7793 P13, like the previously discovered M82 X-2, demonstrates that neutron stars can effectively mimic characteristics associated with more massive black holes.

The authors explored various theoretical models that could explain such high luminosities. These models hinge on the neutron star's magnetic field, geometry of the accretion process, and the beaming factor. The sinusoidal pulse profile observed poses challenges to current models of extreme luminosity sourcing from neutron stars, particularly regarding beaming and its effects on pulse visibility.

Broader Astrophysical Context

The discovery impacts our understanding of ULX demographics, implying a potentially larger number of ULXs with neutron star accretors than previously recognized. The sinusoidal pulse profiles, coupled with the extreme spin-up rates, underscore the variety of physical conditions under which neutron stars operate in ULXs.

Moreover, the paper highlights the need for future observations to better constrain orbital dynamics and the precise factors influencing intrinsic luminosity. While certain models offer insights into geometric and magnetic effects in neutron stars, no present model fully explains the observed phenomena in ULX pulsars.

Future Directions

Future studies should focus on:

• Continuing monitoring of the pulse period evolution to provide insights into the accretion dynamics and magnetic field estimates.
• Exploring additional ULX systems for pulsations using contemporary X-ray observatories to enrich the dataset and validate theoretical models across different environments.
• Detailed spectral analysis over pulse phases to explore further the interplay between accretion mechanics and observed emission properties.

This paper extends the landscape of neutron star research, particularly emphasizing the complex interplay of magnetic fields and accretion processes in ULXs. As more such discoveries are made, they will undoubtedly lead to a refined understanding of neutron star physics and accretion dynamics in extreme environments.