Sbarufatti, Boris (ORCID:0000000166208347)

DOE OSTI2024

Spectroscopy of a sample of unidentified gamma-ray Fermi sources

ABSTRACT The fourth-DR3 version (4FGL-DR3) of the Fermi-LAT catalogue of γ-ray sources contains ∼1000 objects at a galactic latitude |b| > 10° that are not identified with an optical counterpart. We performed a systematic study of these sources, focusing on 190 objects that have a unique X-ray counterpart in the available Swift/XRT observations. Optical counterparts were then selected, and optical spectra were found in the literature for 33 sources. We found that 21 could be classified as BL Lac objects. Among these, we were able to provide the redshift for eight of them, while for two others we established a lower limit to the redshift by detecting intervening absorption. The other 12 objects display optical spectra with prominent emission lines (0.036 < z < 1.65). These spectra are characterized by both broad and narrow emission lines, with the exception of three sources. One of these displays only broad emission lines, while the other two exclusively exhibit narrow lines. On the basis of the radio/optical flux ratio, all BL Lac objects in this study are radio-loud. Four sources out of the 12 with prominent emission lines can be classified as radio-loud, while at least five of the 12 sources with prominent lines are radio-quiet. This is somewhat unexpected, given the radio-loudness distribution of 4FGL-associated blazars.

Ulgiati, Alberto (ORCID:0009000736025276)↗

DOE OSTI2023

Heavy-element production in a compact object merger observed by JWST

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs), sources of high-frequency gravitational waves (GWs) and likely production sites for heavy-element nucleosynthesis by means of rapid neutron capture (the r -process). Here we present observations of the exceptionally bright GRB 230307A. We show that GRB 230307A belongs to the class of long-duration GRBs associated with compact object mergers and contains a kilonova similar to AT2017gfo, associated with the GW merger GW170817. We obtained James Webb Space Telescope (JWST) mid-infrared imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns, which we interpret as tellurium (atomic mass A = 130) and a very red source, emitting most of its light in the mid-infrared owing to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r -process elements across a broad atomic mass range and play a central role in heavy-element nucleosynthesis across the Universe.

79 ASTRONOMY AND ASTROPHYSICS↗