A double-edged sword: extra-galactic Fast X-ray Transients

Projektinformationen

Starstruck

ID Finanzhilfevereinbarung: 101095973

DOI

10.3030/101095973

EK-Unterschriftsdatum 21 Juni 2023

Startdatum 1 September 2023

Enddatum 31 August 2028

Finanziert unter

• European Research Council (ERC)

Gesamtkosten

€ 3 000 000,00

EU-Beitrag

€ 3 000 000,00

Koordiniert durch

STICHTING RADBOUD UNIVERSITEIT
Netherlands

Periodic Reporting for period 1 - Starstruck (A double-edged sword: extra-galactic Fast X-ray Transients)

Berichtszeitraum: 2023-09-01 bis 2026-02-28

Zusammenfassung vom Kontext und den Gesamtzielen des Projekts

Fast X-ray Transients (FXTs) are explosions detected in X-ray light from outside our own Galaxy. They have been known to exist for a while (at least since 2013) but their origin is unknown. FXTs manifest as singular short flashes of X-ray photons with durations ranging from
minutes to hours. The new Einstein Probe satellite has an instrument that monitors nearly 10% of the sky instantaneously and it reports newly discovered FXTs as soon as possible after their discovery. This allows us to obtain follow-up observations of the location on the sky
where the new FXT is discovered using ground-based instrumentation. This will enable the discovery and immediate follow-up of a statistically significant sample of more than a hundred bright FXTs over the >3-yr Einstein Probe mission duration. Complemented with a comprehensive study of the FXT host galaxies, this will lead to a breakthrough in our understanding of FXTs. One leading model is that the FXTs are casued by binary neutron star (BNS) mergers. Merging neutron stars are an important proposed location for the formation of r-process elements such as rare-earth elements such as gold and iodine, and provide they provide signals allowing the Hubble constant (H0) to be measured maximizing the science output of these multi-messenger events. Unlike the highly beamed short gamma-ray burst signal associated with BNS mergers, the FXT signal is ~isotropic providing us with the means to quickly localise the merger, even for events out to the BNS merger detection horizon of the 3rd generation of GW detectors such as Einstein Telescope or Cosmic Explorer.

Arbeit, die ab Beginn des Projekts bis zum Ende des durch den Bericht erfassten Berichtszeitraums geleistet wurde, und die wichtigsten bis dahin erzielten Ergebnisse

The X-ray survey satellite Einstein Probe (EP), launched successfully on Jan. 9, 2024 and designed for rapid detection and alerting of Fast X-ray Transients (FXTs), is truly revolutionizing our understanding of this field. Einstein Probe is discovering about 100 extra-galactic transients per year.

This ERC grant enabled the PI and the team to play a leading role in this emerging field of astronomy. The team and their international collaborators have already written more than 70 General Coordinates Network Circulars, informing the field on scientific results quickly to allow the field to use their resources most efficiently to try to unravel the nature of Fast X-ray Transients. In addition, we have written 17 papers that are either accepted for publication (the peer review has successfully been finished) or published. Overall, the
progenitors of FXTs are found to be diverse. A clear sub-sample of the FXTs is associated with massive stars that collapse into a black hole at the end of their lives. Material falling into this black hole causes a lot of energy to be released and part of that is channeled
into a jet. This jet can break out of the stellar envelope causing a gamma-ray burst and at later times a particular type of supernova is detected (a type Ic broad-lined supernova). It seems like cases where the jet break-out was just successful or even unsuccessful may cause an FXT. These events are evidence that some relativistic supernovae exist, without making a gamma-ray burst, extending the parameter space of relativistic transients. Such events are called cocoon breakout, for the cocoon of emission breaking out of the star after the jet is choked by baryon loading.

For several FXTs our observations rule out the presence of a supernova. Clearly, this is another type of FXT than a collapsar FXT, providing clear evidence for multiple progenitors of FXTs.

Overall, FXTs are a heterogeneous group of (relativistic) transients, where a fraction can be used to probe the high redshift Universe, a fraction is caused by massive collapsing stripped-envelop stars, and yet another progenitor. One event that we followed is EP241103a where we obtained deep observations in the optical wavelength rapidly after the discovery, but no counterpart was found in the optical.

Fortschritte, die über den aktuellen Stand der Technik hinausgehen und voraussichtliche potenzielle Auswirkungen (einschließlich der bis dato erzielten sozioökonomischen Auswirkungen und weiter gefassten gesellschaftlichen Auswirkungen des Projekts)

Before the start of the ERC project, the nature of the FXTs was not known. Studies of FXTs lacked contemporaneous detections of emission in wavelengths other than X-rays. As a consequence thereof, the distance derived through a redshift measurement, was not known. Now with our follow-up observations and study of Einstein Probe discovered FXTs the redshift has been determined for more than 30 FXTs. Furthermore, for about 5 we now know that a relativistic supernova is associated with the FXT, while for 2 we know that the FXT is _not_ due to such a relativistic supernova. Probably those are due to the merger of two neutron stars. Going forward, we need to solidify the evidence for FXTs in the latter group and we need to find out why some relativistic supernovae do produce an FXT and a burst of gamma-ray emission, whereas others seem to produce an FXT only. Lastly, the record redshift for an FXT is a redshift of nearly 5, this is a high redshift, but there was an expectation that FXTs could be used for the detection of even higher redshift events. Why are those lacking? We will adapt our strategies to specifically search for such events.