Pulsars, transients and relativistic astrophysics

Abstract:

We present the discovery of the radio afterglow of the most distant ultralong gamma-ray burst (GRB) detected to date, GRB 220627A at redshift z = 3.084. Its prompt gamma-ray light curve shows a double-pulse profile, with the pulses separated by a period of quiescence lasting ∼15 minutes, leading to early speculation it could be a strongly gravitationally lensed GRB. However, our analysis of the Fermi Gamma-ray Burst Monitor spectra taken during the time intervals of both pulses show clear differences in their spectral energy distributions, disfavouring the lensing scenario. We observed the radio afterglow from 7 to 456 days postburst: an initial, steep decay (Fν ∝ t−2) is followed by a shallower decline (Fν ∝ t−1/2) after ∼20 days. There are three scenarios that could explain these radio properties: (i) energy injection from an additional, slower ejecta component catching up to the external shock; (ii) a stratified density profile going as n ∝ r−8/3; or alternatively, (iii) the presence of a slow, wide ejecta component in addition to a fast, narrow ejecta component. We also conducted an independent test of the lensing hypothesis via very long baseline interferometry (VLBI) observations at ∼12 days postburst by searching, for the first time, for multiple images of the candidate lensed GRB afterglow. Our experiment highlighted the growing need for developments in real-time correlation capabilities for time-critical VLBI experiments, particularly as we advance towards the SKA and ngVLA era of radio astronomy.

Abstract:

We present the first broadband spectral and timing study of the Be/X-ray pulsar XTE J0111.2−7317 (SXP31.0) during the first major outburst since its discovery in 1998. This giant type II outburst, observed between April and September 2025, marks the source’s return to activity after nearly three decades of quiescence. Using NuSTAR observations together with data from Swift /XRT and SRG /ART-XC, we followed the outburst’s evolution, with the source reaching a bolometric luminosity of L bol = 3.6 × 10 38 erg s −1 . The broadband spectra are well described by an absorbed cutoff power law, two blackbody components (hot and soft), and a narrow Fe K α line. No cyclotron absorption features were detected in either the phase-averaged or phase-resolved spectra in the 5–50 keV band. Most notably, we report the discovery of a previously undetected quasiperiodic oscillation (QPO) at 0.8 ± 0.1 mHz, characterized by a fractional root-mean-square (rms) amplitude of 14% at a super-Eddington bolometric luminosity of L bol = 2.5 × 10 38 erg s −1 . In contrast, the previously reported 1.27 Hz QPO was not detected. While the 0.8 mHz QPO is present, the pulsed fraction (PF) is low in soft X-rays, which is consistent with other super-Eddington pulsars exhibiting mHz QPOs; however, it rises above 20 keV to reach 35%. The QPO vanishes in subsequent observations coinciding with a sharp increase in the PF and a distinct change in pulse profile morphology. It was not observed in any follow-up observations at luminosities above or below its initial detection, suggesting it is a transient phenomenon.

Abstract:

Abstract Early-time spectroscopy of supernovae (SNe), acquired within days of explosion, yields crucial insights into their outermost ejecta layers, facilitating the study of their environments, progenitor systems, and explosion mechanisms. Recent efforts in early discovery and follow-up of SNe have shown the potential insights that can be gained from early-time spectra. Surveys such as the Time-Domain Extragalactic Survey (TiDES), conducted with the 4-meter Multi-Object Spectroscopic Telescope (4MOST), will provide spectroscopic follow-up of transients discovered by the Legacy Survey of Space and Time (LSST). Current simulations indicate that early-time spectroscopic studies conducted with TiDES data will be limited by the current SN selection criteria. To enhance early-time SN spectroscopic studies from TiDES-like surveys, we propose a set of selection criteria focusing on young SNe (YSNe), which we define as SNe prior to −10 days before peak brightness. Utilising the Zwicky Transient Facility transient alerts, we developed criteria to select YSNe while minimising the sample’s contamination rate to 23percnt. The developed criteria were applied to LSST simulations, yielding a sample of 694 Deep Drilling Field survey SNe and 56260 Wide Fast Deep survey SNe for follow-up. We demonstrate that our criteria enables the selection of SNe at early-times, enhancing future early-time spectroscopic SN studies from TiDES-like surveys. Finally, we investigated 4MOST-like observing strategies to increase the sample of spectroscopically observed YSNe. We propose that a 4MOST-like observing strategy that follows LSST with a delay of 3 days is optimal for a TiDES-like SN survey in terms of the number of classifiable spectra obtained, while a 1 day delay is most optimal for enhancing the early-time science in conjunction with our YSN selection criteria.

Abstract:

We observed the isolated neutron star (NS) RX J2143.0+0654 with the Hubble Space Telescope (HST) in the UVOIR wavelength range (0.14–1.7 μm). The UV part is consistent with a Rayleigh–Jeans tail of a thermal spectrum, fν ∝ ν2, while a power-law spectrum, fν ∝ να with α ∼ −0.8, dominates in the near-IR–optical. A joint fit of the UVOIR and contemporaneous X-ray spectra with a two-component blackbody with possible absorption features + power-law optical spectrum yields the following temperature and apparent radius of the colder component (which gives the main contribution in the UV): kTcold ≈ 45 eV and Rcold ≈ 6d260 km, where d260 is the distance in units of 260 pc. The temperature and radius of the hotter component, kThot ≈ 106 eV and Rhot ≈ 1.5d260 km; the parameters of an absorption feature at 0.74 keV; and the properties of X-ray pulsations are the same as found in previous X-ray observations. In the near-IR images, the NS is possibly surrounded by extended emission with a characteristic size of ∼2″ and flux densities of about 1.7 and 0.9 μJy at 1.54 and 1.15 μm, respectively. Comparison with a previous HST observation in the optical 14 yr ago shows a proper motion μ ≈ 6 mas yr−1, which corresponds to a small transverse velocity of 7d260 km s−1. It is consistent with the hypothesis that the NS was born in the vicinity of the solar system about 0.5 Myr ago.