Rubin-LSST

Abstract:

Abstract Fast X-ray transients (FXTs) are brief, luminous bursts of soft X-ray emission whose physical origins remain uncertain. The Einstein Probe (EP) mission has recently enabled prompt discovery of these events, providing opportunities for rapid multi-wavelength follow-up. We present a coordinated radio observing campaign targeting 20 FXTs detected by the EP in 2024. The core consists of 59 epochs with the Allen Telescope Array (ATA), sampling post-burst timescales from ~1 to ~65 days and reaching typical 3σ sensitivities of 0.6–1.5 mJy across 1–8 GHz. We additionally incorporate higher-sensitivity observations from MeerKAT, the VLA, ATCA, eMERLIN, and AMI-LA. Two FXTs—EP240315a and EP241021a—have radio counterparts. For EP241021a, AMI-LA monitoring at 15.5 GHz reveals a light curve peaking at ~30 days with Fν ≈ 1.0 mJy; equipartition analysis implies a Newtonian equipartition energy of ~3 × 1049 erg and a mildly relativistic on-axis solution with Γon ≈ 1.3. Multi-frequency modelling of EP240315a, combining new VLA measurements with published MeerKAT, ATCA, and eMERLIN data, indicates highly relativistic early emission (Γ ≳ 3) that decelerates with time, consistent with jetted outflow. The detections and ATA non-detections span luminosities overlapping the brightest GRB and relativistic tidal disruption event (TDE) afterglows, suggesting FXTs comprise a heterogeneous population including both relativistic and non-relativistic explosions. Under a GRB-like redshift prior, our 3σ ATA limits correspond to observer-frame specific-luminosity thresholds of ~1032–1033 erg s−1 Hz−1; for nearby events (z ≲ 0.2), Lν ≲ 1030–31 erg s−1 Hz−1. Planned ATA upgrades and next-generation arrays (SKA, DSA, ngVLA) will enable sensitive, population-level radio studies of the FXT radio sky.

Abstract:

Abstract Remnant radio galaxies, whose lobes are no longer replenished by jets from the active galactic nucleus (AGN), offer key constraints on AGN duty cycles and the timescales of radio jets. We present a spectral-ageing study of 14 candidate remnant radio galaxies in the XMM−LSS field, combining new broad-band data from the MeerKAT MIGHTEE (L-band) and uGMRT superMIGHTEE (band-3 and band-4) surveys with complementary observations from LOFAR, GMRT, and JVLA, covering 144 MHz−1.5 GHz. Spectral modeling confirms 12 sources as genuine remnants, while two are reclassified as active, emphasising the importance of sensitive, multi-frequency coverage for robust remnant identification. Pixel-based spectral age maps yield results (~3-43 Myr) broadly consistent with integrated estimates, revealing relatively short spectral ages (~8-42 Myr). These ages likely reflect enhanced inverse-Compton losses at higher redshifts (0.35 ≤ z ≤ 2.85; median z = 1.25) and possible rapid lobe expansion in low-density environments. The ratios of remnant to total source ages (tOFF/ts) span 0.04-0.83, indicating that the sample traces a broad range of evolutionary stages. Our findings reveal a previously underrepresented population of faint, rapidly fading remnants, suggesting that the remnant phase may be shorter and more dynamic than previously thought. This study highlights the crucial role of MIGHTEE and superMIGHTEE surveys in reliably classifying genuine remnants and provides a framework for constraining AGN life cycles in preparation for forthcoming SKA surveys.