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1. CSS 161010: A Luminous Fast Blue Optical Transient with Broad Blueshifted Hydrogen Lines

   https://doi.org/10.3847/1538-4357/ad89a5  

   Gutiérrez, Claudia P; Mattila, Seppo; Lundqvist, Peter; Dessart, Luc; González-Gaitán, Santiago; Jonker, Peter G; Dong, Subo; Coppejans, Deanne; Chen, Ping; Charalampopoulos, Panos; et al (December 2024, The Astrophysical Journal)

   Abstract We present ultraviolet, optical, and near-infrared photometric and optical spectroscopic observations of the luminous fast blue optical transient (LFBOT) CSS 161010:045834–081803 (CSS 161010). The transient was found in a low-redshift (z= 0.033) dwarf galaxy. The light curves of CSS 161010 are characterized by an extremely fast evolution and blue colors. TheV-band light curve shows that CSS 161010 reaches an absolute peak of ${\mathit{M}}_V^{\max }=-20.66\pm 0.06$ mag in 3.8 days from the start of the outburst. After maximum, CSS 161010 follows a power-law decline ∝t^−2.8±0.1in all optical bands. These photometric properties are comparable to those of well-observed LFBOTs such as AT 2018cow, AT 2020mrf, and AT 2020xnd. However, unlike these objects, the spectra of CSS 161010 show a remarkable transformation from a blue and featureless continuum to spectra dominated by very broad, entirely blueshifted hydrogen emission lines with velocities of up to 10% of the speed of light. The persistent blueshifted emission and the lack of any emission at the rest wavelength of CSS 161010 are unique features not seen in any transient before CSS 161010. The combined observational properties of CSS 161010 and itsM_*∼ 10⁸M_⊙dwarf galaxy host favor the tidal disruption of a star by an intermediate-mass black hole as its origin. 

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2. Roaring to Softly Whispering: X-Ray Emission after ∼3.7 yr at the Location of the Transient AT2018cow and Implications for Accretion-powered Scenarios*

   https://doi.org/10.3847/2041-8213/ad2764  

   Migliori, Giulia; Margutti, R; Metzger, B D; Chornock, R; Vignali, C; Brethauer, D; Coppejans, D L; Maccarone, T; Rivera_Sandoval, L; Bright, J S; et al (February 2024, The Astrophysical Journal Letters)

   Abstract We present the first deep X-ray observations of luminous fast blue optical transient (LFBOT) AT 2018cow at ∼3.7 yr since discovery, together with the reanalysis of the observation atδt∼ 220 days. X-ray emission is significantly detected at a location consistent with AT 2018cow. The very soft X-ray spectrum and sustained luminosity are distinct from the spectral and temporal behavior of the LFBOT in the first ∼100 days and would possibly signal the emergence of a new emission component, although a robust association with AT 2018cow can only be claimed atδt∼ 220 days, while atδt∼ 1350 days contamination of the host galaxy cannot be excluded. We interpret these findings in the context of the late-time panchromatic emission from AT 2018cow, which includes the detection of persistent, slowly fading UV emission withνL_ν≈ 10³⁹erg s⁻¹. Similar to previous works (and in analogy with arguments for ultraluminous X-ray sources), these late-time observations are consistent with thin disks around intermediate-mass black holes (withM_•≈ 10³–10⁴M_☉) accreting at sub-Eddington rates. However, differently from previous studies, we find that smaller-mass black holes withM_•≈ 10–100M_☉accreting at ≳the Eddington rate cannot be ruled out and provide a natural explanation for the inferred compact size (R_out≈ 40R_☉) of the accretion disk years after the optical flare. Most importantly, irrespective of the accretor mass, our study lends support to the hypothesis that LFBOTs are accretion-powered phenomena and that, specifically, LFBOTs constitute electromagnetic manifestations of super-Eddington accreting systems that evolve to ≲Eddington over a ≈100-day timescale. 

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3. The Most Luminous Known Fast Blue Optical Transient AT 2024wpp: Unprecedented Evolution and Properties in the X-Rays and Radio

   https://doi.org/10.3847/2041-8213/ae0b4d  

   A_J, Nayana; Margutti, Raffaella; Wiston, Eli; Laskar, Tanmoy; Migliori, Giulia; Chornock, Ryan; Galvin, Timothy J; LeBaron, Natalie; Hajela, Aprajita; Christy, Collin T; et al (October 2025, The Astrophysical Journal Letters)

   Abstract We present X-ray (0.3–79 keV) and radio (0.25–203 GHz) observations of the most luminous fast blue optical transient (LFBOT) AT 2024wpp atz= 0.0868, spanning 2–280 days after first light. AT 2024wpp shows luminous (L_X≈ 1.5 × 10⁴³erg s⁻¹), variable X-ray emission with a Compton hump peaking atδt≈ 50 days. The X-ray spectrum evolves from a soft (F_ν∝ν^−0.6) to an extremely hard state (F_ν∝ν^1.26) accompanied by a rebrightening atδt≈ 50 days. The X-ray emission properties favor an embedded high-energy source shining through asymmetric expanding ejecta. We detect radio emission peaking atL_{9 GHz}≈ 1.7 × 10²⁹erg s⁻¹Hz⁻¹atδt≈ 73 days. The spectral evolution is unprecedented: the early millimeter fluxes rise nearly an order of magnitude duringδt≈ 17–32 days, followed by a decline in spectral peak fluxes. We model the radio emission as synchrotron radiation from an expanding blast wave interacting with a dense environment ( $\dot{M}\sim 10^{-3}{M}_{\odot }{\mathrm{yr}}^{-1}$ forv_w= 1000 km s⁻¹). The inferred outflow velocities increase from Γβc≈ 0.07cto 0.42cduringδt≈ 32–73 days, indicating an accelerating blast wave. We interpret these observations as a shock propagating through a dense shell of radius ≈10¹⁶cm and then accelerating into a steep density profileρ_CSM(r) ∝r^−3.1. All radio-bright LFBOTs exhibit similar circumstellar medium (CSM) density profiles (ρ_CSM∝r⁻³), suggesting similar progenitor processes. The X-ray and radio properties favor a progenitor involving super-Eddington accretion onto a compact object launching mildly relativistic disk wind outflows. 

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4. A Luminous and Hot Infrared through X-Ray Transient at a 5 kpc Offset from a Dwarf Galaxy

   https://doi.org/10.3847/1538-4357/ae1501  

   Somalwar, Jean J; Ravi, Vikram; Margutti, Raffaella; Chornock, Ryan; Natarajan, Priyamvada; Lu, Wenbin; Angus, Charlotte; Graham, Matthew J; Hammerstein, Erica; Nathan, Edward; et al (December 2025, The Astrophysical Journal)

   Abstract We are searching for hot, constant-color, offset optical flares in the Zwicky Transient Facility data stream that are >10″ from any galaxy in public imaging data from the PanSTARRS survey. Here, we present the first discovery from this search: AT 2024puz, a luminous multiwavelength transient offset by 5 kpc from a ∼10⁸M_⊙galaxy atz= 0.356 with a low–moderate star formation rate (0.01 ± 0.003M_⊙yr⁻¹). It produced luminous 10^44.79±0.04erg s⁻¹optical/UV emission that evolved on a ∼20 day timescale, as well as 10^44.12±0.03erg s⁻¹X-ray emission with a photon-index $\mathrm{\Gamma }=1.73_{-0.09}^{+0.10}$ . No associated radio or millimeter emission was detected. We show that the early time optical emission is likely powered by reprocessing of high-energy, accretion-powered radiation, with a possible contribution from a shock in a dense circumtransient medium. If the shock is dominant at early times, the circumtransient medium has a mass ∼0.1–1M_⊙, a radius 10¹⁵cm, and a density profile shallower than ∼r⁻¹. A near-infrared excess appears at late-times and is suggestive of reprocessing within a wind or other circumtransient medium. The X-rays are most consistent with a central engine. We suggest that AT 2024puz may be associated with an accretion event onto a 50–10⁵M_⊙black hole, where the lower masses are preferred based on the large projected offset from the host galaxy. AT2024puz exhibits properties similar to both luminous, fast, blue optical transients and tidal disruption events, but is intermediate between them in its energetics and evolution timescale. This highlights the need for broader exploration of the landscape of hot optical transients to trace their origins. 

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5. The Luminosity Phase Space of Galactic and Extragalactic X-Ray Transients Out to Intermediate Redshifts

   https://doi.org/10.3847/1538-4357/acf765  

   Polzin, Ava; Margutti, Raffaella; Coppejans, Deanne L; Auchettl, Katie; Page, Kim L; Vasilopoulos, Georgios; Bright, Joe S; Esposito, Paolo; Williams, Peter_K G; Mukai, Koji; et al (December 2023, The Astrophysical Journal)

   Abstract We present a detailed compilation and analysis of the X-ray phase space of low- to intermediate-redshift (0 ≤z≤ 1) transients that consolidates observed light curves (and theory where necessary) for a large variety of classes of transient/variable phenomena in the 0.3–10 keV energy band. We include gamma-ray burst afterglows, supernovae, supernova shock breakouts and shocks interacting with the environment, tidal disruption events and active galactic nuclei, fast blue optical transients, cataclysmic variables, magnetar flares/outbursts and fast radio bursts, cool stellar flares, X-ray binary outbursts, and ultraluminous X-ray sources. Our overarching goal is to offer a comprehensive resource for the examination of these ephemeral events, extending the X-ray duration–luminosity phase space (DLPS) to show luminosity evolution. We use existing observations (both targeted and serendipitous) to characterize the behavior of various transient/variable populations. Contextualizing transient signals in the larger DLPS serves two primary purposes: to identify areas of interest (i.e., regions in the parameter space where one would expect detections, but in which observations have historically been lacking), and to provide initial qualitative guidance in classifying newly discovered transient signals. We find that while the most luminous (largely extragalactic) and least luminous (largely Galactic) part of the phase space is well populated att> 0.1 days, intermediate-luminosity phenomena (L_X= 10³⁴–10⁴²erg s⁻¹) represent a gap in the phase space. We thus identifyL_X= 10³⁴–10⁴²erg s⁻¹andt= 10⁻⁴to 0.1 days as a key discovery phase space in transient X-ray astronomy. 

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