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1. Ultraviolet Quasiperiodic Eruptions from Star–Disk Collisions in Galactic Nuclei

   https://doi.org/10.3847/2041-8213/ad2464  

   Linial, Itai; Metzger, Brian D (February 2024, The Astrophysical Journal Letters)

   Abstract “Quasiperiodic eruptions” (QPE) are recurrent nuclear transients with periods of several hours to almost a day, which thus far have been detected exclusively in the X-ray band. We have shown that many of the key properties of QPE flares (period, luminosity, duration, emission temperature, alternating long-short recurrence time behavior, and source rates) are naturally reproduced by a scenario involving twice-per-orbit collisions between a solar-type star on a mildly eccentric orbit, likely brought into the nucleus as an extreme mass-ratio inspiral (EMRI), and the gaseous accretion disk of a supermassive black hole (SMBH). The flare is generated by the hot shocked debris expanding outwards from either side of the disk midplane, akin to dual miniature supernovae. Here, we consider the conditions necessary for disk–star collisions to generate lower-temperature flares that peak in the ultraviolet (UV) instead of the X-ray band. We identify a region of parameter space at low SMBH massM_•∼ 10^5.5M_⊙and QPE periodsP≳ 10 hr for which the predicted flares are sufficiently luminousL_UV∼ 10⁴¹erg s⁻¹to outshine the quiescent disk emission at these wavelengths. The prospects to discover such “UV QPEs” with future satellite missions such as ULTRASAT and Ultraviolet Explorer depend on the prevalence of very low-mass SMBHs and the occurrence rate of stellar EMRIs onto them. For gaseous disks produced by the tidal disruption of stars, we predict that X-ray QPEs will eventually shut off, only to later reappear as UV QPEs as the accretion rate continues to drop. 

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2. A Volume-limited Radio Search for Magnetic Activity in 140 Exoplanets with the Very Large Array

   https://doi.org/10.3847/1538-3881/ad58be  

   Ortiz_Ceballos, Kevin N; Cendes, Yvette; Berger, Edo; Williams, Peter_K G (August 2024, The Astronomical Journal)

   Abstract We present results from a search for radio emission in 77 stellar systems hosting 140 exoplanets, predominantly within 17.5 pc using the Very Large Array (VLA) at 4–8 GHz. This is the largest and most sensitive search to date for radio emission in exoplanetary systems in the GHz frequency range. We obtained new observations of 58 systems and analyzed archival observations of an additional 19 systems. Our choice of frequency and volume limit is motivated by radio detections of ultracool dwarfs (UCDs), including T dwarfs with masses at the exoplanet threshold of ∼13M_J. Our surveyed exoplanets span a mass range of ≈10⁻³–10M_Jand semimajor axes of ≈10⁻²–10 au. We detect a single target—GJ 3323 (M4) hosting two exoplanets with minimum masses of 2 and 2.3M_⊕—with a circular polarization fraction of ≈40%; the radio luminosity agrees with its known X-ray luminosity and the Güdel–Benz relation for stellar activity suggesting a likely stellar origin, but the high circular polarization fraction may also be indicative of star–planet interaction. For the remaining sources our 3σupper limits are generallyL_ν≲ 10^12.5erg s⁻¹Hz⁻¹, comparable to the lowest radio luminosities in UCDs. Our results are consistent with previous targeted searches of individual systems at GHz frequencies while greatly expanding the sample size. Our sensitivity is comparable to predicted fluxes for some systems considered candidates for detectable star–planet interaction. Observations with future instruments such as the Square Kilometre Array and Next-Generation VLA will be necessary to further constrain emission mechanisms from exoplanet systems at GHz frequencies. 

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3. The Massive and Quiescent Elliptical Host Galaxy of the Repeating Fast Radio Burst FRB 20240209A

   https://doi.org/10.3847/2041-8213/ad9de2  

   Eftekhari, T; Dong_董, Y 雨欣; Fong, W; Shah, V; Simha, S; Andersen, B C; Andrew, S; Bhardwaj, M; Cassanelli, T; Chatterjee, S; et al (January 2025, The Astrophysical Journal Letters)

   Abstract The discovery and localization of FRB 20240209A by the Canadian Hydrogen Intensity Mapping Fast Radio Burst (CHIME/FRB) experiment marks the first repeating FRB localized with the CHIME/FRB Outriggers and adds to the small sample of repeating FRBs with associated host galaxies. Here we present Keck and Gemini observations of the host that reveal a redshiftz = 0.1384 ± 0.0004. We perform stellar population modeling to jointly fit the optical through mid-IR data of the host and infer a median stellar mass log(M_*/M_⊙) = 11.35 ± 0.01 and a mass-weighted stellar population age  ~11 Gyr, corresponding to the most massive and oldest FRB host discovered to date. Coupled with a star formation rate  <0.31M_⊙yr⁻¹, the specific star formation rate  <10^−11.9yr⁻¹classifies the host as quiescent. Through surface brightness profile modeling, we determine an elliptical galaxy morphology, marking the host as the first confirmed elliptical FRB host. The discovery of a quiescent early-type host galaxy within a transient class predominantly characterized by late-type star-forming hosts is reminiscent of short-duration gamma-ray bursts, Type Ia supernovae, and ultraluminous X-ray sources. Based on these shared host demographics, coupled with a large offset as demonstrated in our companion Letter, we conclude that preferred sources for FRB 20240209A include magnetars formed through merging binary neutron stars/white dwarfs or the accretion-induced collapse of a white dwarf, or a luminous X-ray binary. Together with FRB 20200120E localized to a globular cluster in M81, our findings provide strong evidence that some fraction of FRBs may arise from a process distinct from the core collapse of massive stars. 

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4. FUMES. IV. Optical and Far-ultraviolet Spectra of a Flare on the M Dwarf GJ 4334

   https://doi.org/10.3847/1538-3881/adff74  

   Duvvuri, Girish M; Pineda, J Sebastian; García_Soto, Aylin; Berta-Thompson, Zachory K; Youngblood, Allison; France, Kevin; Newton, Elisabeth R; Stassun, Keivan G (October 2025, The Astronomical Journal)

   Abstract On 2017 September 20, we observed GJ 4334, an M5V dwarf rotating with a period of 23.5 days, simultaneously with both the Space Telescope Imaging Spectrograph aboard Hubble (1160–1710 Å) and the Dual Imaging Spectrograph mounted on the 3.5 m telescope at Apache Point Observatory (3750–5050; 5800–6950 Å) as part of a larger survey of intermediately active M dwarfs. GJ 4334 flared during the observation, starting with a rise in the flux of optical chromospheric emission lines, followed by the rapid rise and decay of multiple far-ultraviolet emission lines formed in the transition region, followed by the slow decay of the optical lines. We find significant broadening and asymmetries in the optical emission lines that are potentially from bulk plasma motion, a postflare elevated flux in both the optical and far-ultraviolet, and trends in the rise and decay timescales of the Balmer series such that higher-order lines rise earlier and decay faster than lower-order lines. The equivalent durations of the flare in individual lines range from 800 to 3 × 10⁴s, mapping to flare energies of 1 × 10²⁸–3 × 10²⁹erg for each line. To contextualize GJ 4334’s flare behavior, we measure and compare its optical flare frequency distribution with TESS to EV Lacertae, a similar mass but faster rotating M dwarf, and find that GJ 4334 has an excess of large flares relative to the power law established by the majority of its smaller flares. This data set is a rare opportunity to characterize flares near a critical transition in stellar magnetic activity. 

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5. The Thermal Emission Spectrum of the Nearby Rocky Exoplanet LTT 1445A b from JWST MIRI/LRS

   https://doi.org/10.3847/1538-3881/adc990  

   Wachiraphan, Patcharapol; Berta-Thompson, Zachory K; Diamond-Lowe, Hannah; Winters, Jennifer G; Murray, Catriona; Zhang, Michael; Xue, Qiao; Morley, Caroline V; Rosario-Franco, Marialis; Duvvuri, Girish M (May 2025, The Astronomical Journal)

   Abstract The nearby transiting rocky exoplanet LTT 1445A b presents an ideal target for studying atmospheric retention in terrestrial planets orbiting M dwarfs. It is cooler than many rocky exoplanets yet tested for atmospheres, receiving a bolometric instellation similar to Mercury’s. Previous transmission spectroscopy ruled out a light H/He-dominated atmosphere but could not distinguish between a bare-rock, a high-MMW, nor a cloudy atmosphere. We present new secondary eclipse observations using JWST’s MIRI/LRS, covering the 5–12μm range. From these observations, we detect a broadband secondary eclipse depth of 41 ± 9 ppm and measure a mid-eclipse timing consistent with a circular orbit (at 1.7σ). From its emission spectrum, the planet’s dayside brightness temperature is constrained to 525 ± 15 K, yielding a temperature ratio relative to the maximum average dayside temperature from instant thermal reradiation by a rocky surfaceR= $T_{\mathrm{day,obs}}/T_{\max }$ = 0.952 ± 0.057, consistent with emission from a dark rocky surface. From an energy balance perspective, such a warm dayside temperature disfavors thick atmospheres, excluding ∼100 bar atmospheres with Bond albedo >0.08 at the 3σlevel. Furthermore, forward modeling of atmospheric emission spectra disfavor simple 100% CO₂atmospheres with surface pressures of 1, 10, and 100 bar at 4.2σ, 6.6σ, and 6.8σconfidence, respectively. These results suggest that LTT 1445A b lacks a very thick CO₂atmosphere, possibly due to atmospheric erosion driven by stellar activity. However, the presence of a moderately thin atmosphere (similar to those on Mars, Titan, or Earth) remains uncertain. 

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