The high energy X-ray probe (HEX-P): studying extreme accretion with ultraluminous X-ray sources

Abstract

Introduction: Ultraluminous X-ray sources (ULXs) represent an extreme class of accreting compact objects: from the identification of some of the accretors as neutron stars to the detection of powerful winds travelling at 0.1–0.2 c, the increasing evidence points towards ULXs harbouring stellar-mass compact objects undergoing highly super-Eddington accretion. Measuring their intrinsic properties, such as the accretion rate onto the compact object, the outflow rate, the masses of accretor/companion-hence their progenitors, lifetimes, and future evolution-is challenging due to ULXs being mostly extragalactic and in crowded fields. Yet ULXs represent our best opportunity to understand super-Eddington accretion physics and the paths through binary evolution to eventual double compact object binaries and gravitational-wave sources. Methods: Through a combination of end-to-end and single-source simulations, we investigate the ability of HEX-P to study ULXs in the context of their host galaxies and compare it to XMM-Newton and NuSTAR, the current instruments with the most similar capabilities.Results: HEX-P’s higher sensitivity, which is driven by its narrow point-spread function and low background, allows it to detect pulsations and broad spectral features from ULXs better than XMM-Newton and NuSTAR.Discussion: We describe the value of HEX-P in understanding ULXs and their associated key physics, through a combination of broadband sensitivity, timing resolution, and angular resolution, which make the mission ideal for pulsation detection and low-background, broadband spectral studies.

Return to the Forgotten Ultraluminous X-Ray Source: A Broadband NICER+NuSTAR Study of NGC 4190 ULX-1

“…However, a full picture of the X-ray behavior of this source can only be obtained with broadband X-ray data. While coordination of a soft X-ray observatory with NuSTAR is invaluable with present-day capabilities, this source illustrates the value of a future dedicated broadband observatory such as the High Energy X-ray Probe in the study of ULXs (Bachetti et al 2023;Madsen et al 2023).…”

Theoretical Analysis of the RX J0209.6−7427 X-Ray Spectrum during a Giant Outburst

“…In particular, ULXPs share spectral similarities with BeXRBs (e.g., Koliopanos et al 2017;Walton et al 2018), which have so far mainly been explored using empirical and phenomenological models. Next-generation X-ray telescopes, such as the High Energy X-ray Probe (Madsen et al 2024), will have higher effective areas extending over larger energy ranges than current detectors, which will facilitate much more detailed studies of super-Eddington bursts (see simulations by Bachetti et al 2023;Ludlam et al 2023).…”

Timing (Pulsating) ULX-ray sources – lessons learned from M82

“…), a mission concept recently submitted for the NASA Astrophysical Probe Explorer Announcement of Opportunity, the will be able to make much sharper images over the 0.3-30 keV, and allow for sensitive pulsar searches in crowded fields like those typical of ULXs. In particular, [48][49][50] show different examples of how the narrow PSF of the low-energy telescope (LET; 2.5 ′′ HEW) and the two high-energy telescopes (HET; ∼ 18 ′′ HEW) carried by the mission, together with the large effective areas comparable to XMM-Newton and NuSTAR respectively, will lead to better sensitivity to pulsations than both XMM-Newton and NuSTAR.…”