Understanding ultraluminous X-ray sources using the eROSITA all-sky survey
Lead Research Organisation:
DURHAM UNIVERSITY
Abstract
Ultraluminous X-ray sources (ULXs) are the most voracious accretors we know of, consuming matter at a rate relative to their mass that is far in excess of any other known, persistent systems. They comprise a compact object, a neutron star or black hole, fed via matter transfer from a companion binary star. For many years the key question for ULXs was whether they were evidence for a new class of intermediate-mass black holes (with masses between 100 and 100000 times that of the Sun), or whether they instead were stellar-mass black holes (of a few to a few tens of Solar masses) accreting in a rare "super-Eddington" state. This is where the theoretical Eddington limit, a putative maximum brightness for which the outward pressure of the intense radiation should inhibit further accretion, appears breached. However, this paradigm changed abruptly a decade ago when we found that several ULXs show pulsations. These pulsations can only occur in systems harbouring neutron stars, which for their relatively small size (less that 3 Solar masses) must be very extreme accretors, surpassing the nominal Eddington limit by factors up to ~500.
The key questions for ULXs now relate to neutron stars - what proportion of the ULX population hosts a neutron star rather than a black hole? And how is the super-Eddington accretion flow affected by the strong magnetic fields inherent to many neutron stars? Other important questions yet to be resolved include how does the substantial energy release of ULXs - both from their intense radiation, and carried in the massive energetic winds driven by super-Eddington accretion - affect the galaxies hosting them, particularly at high redshifts where large populations of ULXs are expected? Here, I present a programme which will exploit my external collaborator status on the eROSITA X-ray telescope to answer these questions.
In collaboration with eROSITA consortium colleagues, we have a ULX catalogue based on the first pass of the eROSITA all-sky survey (eRASS1) close to publication. From this we can confidently predict that the full eRASS:4 data (a combination of all 4 completed sky surveys) will provide a statistically complete and unbiased sample of all eROSITA-detectable ULXs at distances out to 10 Mpc (and beyond that at the more sensitive orbital poles). We will use a combination of the extensive available archival X-ray data (from XMM-Newton in particular, which already covers > 70% of a pilot sample) and new data to search for signatures of a neutron star in the complete sample of ULXs, including leveraging a new technique we have developed for detecting weak pulsation signals. This will be used to create the first strong limits on the proportion of the ULX population hosting a neutron star.
Any new neutron star ULX detections will be used to deepen our understanding of the accretion flow in such objects, using existing and new data with the best available models to constrain magnetic field strength and accretion geometry. The study of the compete sample will also extend to creating spectral energy distributions that extend to the infrared and optical wavebands, using the HST and JWST archives to identify counterparts to the ULXs, that can be used to constrain the energy output of a typical ULX, and so to model their effect on galaxy evolution.
The key questions for ULXs now relate to neutron stars - what proportion of the ULX population hosts a neutron star rather than a black hole? And how is the super-Eddington accretion flow affected by the strong magnetic fields inherent to many neutron stars? Other important questions yet to be resolved include how does the substantial energy release of ULXs - both from their intense radiation, and carried in the massive energetic winds driven by super-Eddington accretion - affect the galaxies hosting them, particularly at high redshifts where large populations of ULXs are expected? Here, I present a programme which will exploit my external collaborator status on the eROSITA X-ray telescope to answer these questions.
In collaboration with eROSITA consortium colleagues, we have a ULX catalogue based on the first pass of the eROSITA all-sky survey (eRASS1) close to publication. From this we can confidently predict that the full eRASS:4 data (a combination of all 4 completed sky surveys) will provide a statistically complete and unbiased sample of all eROSITA-detectable ULXs at distances out to 10 Mpc (and beyond that at the more sensitive orbital poles). We will use a combination of the extensive available archival X-ray data (from XMM-Newton in particular, which already covers > 70% of a pilot sample) and new data to search for signatures of a neutron star in the complete sample of ULXs, including leveraging a new technique we have developed for detecting weak pulsation signals. This will be used to create the first strong limits on the proportion of the ULX population hosting a neutron star.
Any new neutron star ULX detections will be used to deepen our understanding of the accretion flow in such objects, using existing and new data with the best available models to constrain magnetic field strength and accretion geometry. The study of the compete sample will also extend to creating spectral energy distributions that extend to the infrared and optical wavebands, using the HST and JWST archives to identify counterparts to the ULXs, that can be used to constrain the energy output of a typical ULX, and so to model their effect on galaxy evolution.
People |
ORCID iD |
| Timothy Roberts (Principal Investigator) | |
| Amy Knight (Researcher) |