Blasts from the Past: Supernova Shock Breakouts among X-Ray Transients in the XMM-Newton Archive

Abstract

The first electromagnetic signal from a supernova (SN) is released when the shock crosses the progenitor surface. This shock breakout (SBO) emission provides constraints on progenitor and explosion properties. Observationally, SBOs appear as minute- to hour-long extragalactic X-ray transients. They are challenging to detect and only one SBO has been observed to date. Here, we search the XMM-Newton archive and find 12 new SN SBO candidates. We identify host galaxies to nine of these at estimated redshifts of 0.1–1. The SBO candidates have energies of ∼1046 erg, timescales of 30–3000 s, and temperatures of 0.1–1 keV. They are all consistent with being SN SBOs, but some may be misidentified Galactic foreground sources or other extragalactic objects. SBOs from blue supergiants agree well with most of the candidates. However, a few could be SBOs from Wolf–Rayet stars surrounded by dense circumstellar media, whereas two are more naturally explained as SBOs from red supergiants. The observations tentatively support non-spherical SBOs and are in agreement with asymmetries predicted by recent three-dimensional SN explosion simulations. eROSITA may detect ∼2 SBOs per year, which could be detected in live analyses and promptly followed up.

The EXTraS project: Exploring the X-ray transient and variable sky

“…The same transient has independently been discovered in a systematic search for supernova shock break-out candidates in XMM-Newton archival data (Alp & Larsson 2020). The other 11 candidates listed in Alp & Larsson (2020) could not have been detected by the algorithm described in this section because they were either already included in the 3XMM-DR5 catalogue or occurred in more recent observations that were not covered by our analysis.…”

“…On the other hand, the position of the shortest EXTraS transient (EXMM J023135.0-603743, with BIN0=315 s) is consistent with a galaxy at redshift z = 0.092 and has been interpreted as the X-ray flare of a core collapse supernova (Novara et al 2020). The same transient has independently been discovered in a systematic search for supernova shock break-out candidates in XMM-Newton archival data (Alp & Larsson 2020). The other 11 candidates listed in Alp & Larsson (2020) could not have been detected by the algorithm described in this section because they were either already included in the 3XMM-DR5 catalogue or occurred in more recent observations that were not covered by our analysis.…”

The EXOD search for faint transients in XMM-Newton observations: Method and discovery of four extragalactic Type I X-ray bursters

“…They associate the transient with a host galaxy located at redshift z = 0.62. Although the presence of four galaxies within a 15 radius from the position of the X-ray source 12 casts some doubts on the identification of the host galaxy, Alp & Larsson (2020) find the flux and spectral properties of the source to be in good agreement with blue supergiant SBO predictions. In general, we too argue that this is likely an extragalactic source because it is located well outside the galactic plane and there is no clear bright counterpart other than background galaxies.…”

“…The two sources referred as source 6 and 25 in this paper that were independently detected by Alp & Larsson (2020) and identified as SBOs while this paper was under review, represent exceptional examples of EXOD's extragalactic transient discovery potential.…”

“…The burst spectrum appears to be fairly soft, although this is based on the joint fit of the persistent and burst emission due to the low number of counts. Alp & Larsson (2020) presented also this source (under name XT 151219) as an SBO while our paper was under review. They associate the transient with a host galaxy located at redshift z = 0.62.…”