arXiv subjects

Alexei V. Filippenko

Publications and source records attributed to Alexei V. Filippenko.

At least 73 records · Page 4Linked to original sources

arXiv2025

Rapid follow-up observations of infant supernovae with the Gran Telescopio Canarias

The first few hours of a supernova (SN) contain significant information about the progenitor system. The most modern wide-field surveys that scan the sky repeatedly every few days can discover all kinds of transients in those early epochs. At such times, some progenitor footprints may be visible, elucidating critical explosion parameters and helping to distinguish between leading explosion models. A dedicated spectroscopic classification programme using the optical spectrograph OSIRIS mounted on the Gran Telescopio Canarias was set up to try to obtain observations of supernovae (SNe) at those early epochs. With the time awarded, we obtained spectra of 10 SN candidates, which we present here. Half of them were thermonuclear SNe, while the other half were core-collapse SNe. Most (70\%) were observed within the first six days of the estimated explosion, with two being captured within the first 48\,hr. We present a characterization of the spectra, together with other public ancillary photometry from the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS). This project shows the need for an accompanying rapid-response spectroscopic programme for existing and future deep photometric wide-field surveys located at the right longitude to be able to trigger observations in a few hours after the discovery of the SN candidate.

astro-ph.SR↗

arXiv2025

FRB 20250316A: A Brilliant and Nearby One-Off Fast Radio Burst Localized to 13 parsec Precision

Precise localizations of a small number of repeating fast radio bursts (FRBs) using very long baseline interferometry (VLBI) have enabled multiwavelength follow-up observations revealing diverse local environments. However, the 2--3\% of FRB sources that are observed to repeat may not be representative of the full population. Here we use the VLBI capabilities of the full CHIME Outriggers array for the first time to localize a nearby (40 Mpc), bright (kJy), and apparently one-off FRB source, FRB 20250316A, to its environment on 13-pc scales. We use optical and radio observations to place deep constraints on associated transient emission and the properties of its local environment. We place a $5σ$ upper limit of $L_{\mathrm{9.9~\mathrm{GHz}}} < 2.1\times10^{25}~\mathrm{erg~s^{-1}~Hz^{-1}}$ on spatially coincident radio emission, a factor of 100 lower than any known compact persistent radio source associated with an FRB. Our KCWI observations allow us to characterize the gas density, metallicity, nature of gas ionization, dust extinction and star-formation rate through emission line fluxes. We leverage the exceptional brightness and proximity of this source to place deep constraints on the repetition of FRB 20250316A, and find it is inconsistent with all well-studied repeaters given the non-detection of bursts at lower spectral energies. We explore the implications of a measured offset of 190$\pm20$ pc from the center of the nearest star-formation region, in the context of progenitor channels. FRB 20250316A marks the beginning of an era of routine localizations for one-off FRBs on tens of mas-scales, enabling large-scale studies of their local environments.

astro-ph.HE↗

arXiv2025

Constraining the z $\sim$ 1 Initial Mass Function with {\it HST} and {\it JWST} Lensed Stars in MACS J0416.1-2403

Our understanding of galaxy properties and evolution is contingent on knowing the initial mass function (IMF), and yet to date, the IMF is constrained only to local galaxies. Individual stars are now becoming routinely detected at cosmological distances, where luminous stars such as supergiants in background galaxies strongly lensed by galaxy clusters are temporarily further magnified by huge factors (up to $10^{4}$) by intracluster stars, thus being detected as transients. The detection rate of these events depends on the abundance of luminous stars in the background galaxy and is thus sensitive to the IMF and the star-formation history (SFH), especially for the blue supergiants detected as transients in the rest-frame ultraviolet/optical filters. As a proof of concept, we use simple SFH and IMF models constrained by spectral energy distributions (SEDs) to see how well we can predict the {\it HST} and {\it JWST} transient detection rate in a lensed arc dubbed ``Spock'' ($z = 1.0054$). We find that demanding a simultaneous fit of the SED and the transient detection rate places constraints on the IMF, independent of the assumed simple SFH model. We conclude our likelihood analysis indicates that the data definitively prefers the ``Spock'' galaxy to have a Salpeter IMF ($α= 2.35$) rather than a Top-heavy IMF ($α= 1$) -- which is thought to be the case in the early universe -- with no clear excess of supergiants above the standard IMF.

astro-ph.CO↗

arXiv2025

Large Cold Dust Reservoir Revealed in Transitional SN Ib 2014C by James Webb Space Telescope Mid-Infrared Spectroscopy

Supernova (SN) 2014C is a rare transitional event that exploded as a hydrogen-poor, helium-rich Type Ib SN and subsequently interacted with a hydrogen-rich circumstellar medium (CSM) a few months post-explosion. This unique interacting object provides an opportunity to probe the mass-loss history of a stripped-envelope SN progenitor. Using the James Webb Space Telescope (JWST), we observed SN 2014C with the Mid-Infrared Instrument Medium Resolution Spectrometer at 3477 days post-explosion (rest frame), and the Near-Infrared Spectrograph Integral Field Unit at 3568 days post-explosion, covering 1.7 to 25 $μ$m. The bolometric luminosity indicates that the SN is still interacting with the same CSM that was observed with the Spitzer Space Telescope 40--1920 days post-explosion. JWST spectra and near-contemporaneous optical and near-infrared spectra show strong [Ne II] 12.831 $μ$m, He 1.083 $μ$m, H$α$, and forbidden oxygen ([O I] $λ$$λ$6300, 6364, [O II] $λ$$λ$7319, 7330, and [O III] $λ$$λ$4959, 5007) emission lines with asymmetric profiles, suggesting a highly asymmetric CSM. The mid-IR continuum can be explained by ~$0.036 \ M_\odot$ of carbonaceous dust at ~300 K and ~0.043 $M_\odot$ of silicate dust at ~200 K. The observed dust mass has increased tenfold since the last Spitzer observation 4 yr ago, with evidence suggesting that new grains have condensed in the cold dense shell between the forward and reverse shocks. This dust mass places SN 2014C among the dustiest SNe in the mid-IR and supports the emerging observational trend that SN explosions produce enough dust to explain the observed dust mass at high redshifts.

astro-ph.HE↗

arXiv2025

Spectropolarimetric Evolution of SN 2023ixf: an Asymmetric Explosion in a Confined Aspherical Circumstellar Medium

We present complete spectropolarimetric coverage of the Type II supernova (SN) 2023ixf ranging from 1 to 120 days after explosion. Polarimetry was obtained with the Kast double spectrograph on the Shane 3m telescope at Lick Observatory. As the ejecta interact with circumstellar material (CSM) during the first week, the intrinsic polarization of SN 2023ixf is initially high at $\lesssim$1%, dropping steeply within days down to $\sim$ 0.4% when the ejecta sweep up the optically-thick CSM. The continuum polarization stays low at $\sim$ 0.2% thereafter, until it rises again to $\sim$ 0.6% as the ejecta transition to the nebular phase. We model this evolution using a combination of archival and newly-computed 2D polarized radiative-transfer models. In this context, we interpret the early-time polarization as arising from an aspherical CSM with a pole-to-equator density contrast $\gtrsim$ 3. We propose that the surge in polarization at late times originates from an asymmetric distribution of $^{56}$Ni deep in the ejecta. The distinct sources of asymmetries at early and late times are consistent with the temporal evolution of the observed polarization and the polarization angle in SN 2023ixf.

astro-ph.HE↗

arXiv2025

SN 2021hpr: A Normal Type Ia Supernova Showing Excess Emission in the Early Rising Phase

We present extensive optical observations of a nearby Type Ia supernova (SN Ia), SN 2021hpr, located in the spiral galaxy NGC 3147 at a distance of $\sim$ 45 Mpc. Our observations cover a phase within $\sim 1-2$ days to $\sim 290$ days after the explosion. SN 2021hpr is found to be a spectroscopically normal SN Ia, with an absolute B-band peak magnitude of $M_{max}(B) \approx -19.16 \pm 0.14$ mag and a post-peak decline rate of $Δm_{15}(B)= 1.00 \pm 0.01 $ mag. Early-time light curves showed a $\sim 7.0 \%$ excess emission compared to a homogeneously expanding fireball model, likely due to SN ejecta interacting with a companion or immediate circumstellar matter. The optical spectra of SN 2021hpr are overall similar to those of normal SNe Ia, but characterized by prominent detached high-velocity features (HVFs) of Si {\sc ii} and Ca {\sc ii} in the early phase. After examining a small sample of well-observed normal SNe Ia, we find that the HVFs are likely common for the subgroup with early-excess emission. The association of early bump feature with the HVFs could be attributed to density or abundance enhancement at the outer layer of the exploding star, likely as a result of interactions with companion$/$CSM or experiencing more complete burning. Nevertheless, the redshifted Fe {\sc ii} and Ni {\sc ii} lines in the nebular-phase spectra of SN 2021hpr, contrary to the blueshift trend seen in other SNe Ia showing early bump features, indicate its peculiarity in the explosion that remains to be understood.

astro-ph.HE↗

arXiv2025

Very Late-Time JWST and Keck Spectra of the Oxygen-Rich Supernova 1995N

We present new {\it JWST}/MIRI MRS and Keck spectra of SN 1995N obtained in 2022--2023, more than 10,000 days after the supernova (SN) explosion. These spectra are among the latest direct detections of a core-collapse SN, both through emission lines in the optical and thermal continuum from infrared dust emission. The new infrared data show that dust heating from radiation produced by the ejecta interacting with circumstellar matter is still present, but greatly reduced from when SN 1995N was observed by the {\it Spitzer Space Telescope} and {\it WISE} in 2009/2010 and 2018, when the dust mass was estimated to be 0.4 M(Sun). New radiative-transfer modeling suggests that the dust mass and grain size may have increased between 2010 and 2023. The new data can alternatively be well fit with a dust mass of 0.4 M(Sun) and a much reduced heating source luminosity. The new late-time spectra show unusually strong oxygen forbidden lines, stronger than the H-alpha emission. This indicates that SN 1995N may have exploded as a stripped-envelope SN which then interacted with a massive H-rich circumstellar shell, changing it from intrinsically Type Ib/c to Type IIn. The late-time spectrum results when the reverse shock begins to excite the inner H-poor, O-rich ejecta. This change in the spectrum is rarely seen, but marks the start of the transition from SN to SN remnant.

astro-ph.SR↗

arXiv2025

Search for Axions in Magnetic White Dwarf Polarization at Lick and Keck Observatories

We present the most sensitive search to date for light axion-like particles with masses below a micro-eV, using spectropolarimetric data collected from the Lick and Keck Observatories. The conversion of optical photons emitted from the surface of a magnetic white dwarf (MWD) into axions in the strong magnetic field around the star induces a nearly wavelength-independent linear polarization in the observed starlight. We analyze the Stokes parameters $(U, Q, I)$ measured with the Kast spectrograph at the Lick Observatory toward the MWDs SDSS J033320+000720 and ZTF J190132+145807, and with the LRISp-ADC instrument at the Keck Observatory toward ZTF J190132+145807, SDSS J002129+150223, and SDSS J100356+053825 to search for this effect. The data show no evidence of axion-induced linear polarization, and we set world-leading constraints on the axion-photon coupling $|g_{aγγ}| \lesssim 1.7 \times 10^{-12} \,\mathrm{GeV}^{-1}$ at the $95\%$ confidence level for masses $m_a \lesssim 2 \times 10^{-7}\,\mathrm{eV}$.

hep-ph↗

arXiv2025

Spectroscopy of AT 2016blu's recurring supernova impostor outbursts

We present spectra of the supernova (SN) impostor AT 2016blu spanning over a decade. This transient exhibits quasiperiodic outbursts with a $\sim$113 d period, likely triggered by periastron encounters in an eccentric binary system where the primary star is a luminous blue variable (LBV). The overall spectrum remains fairly consistent during quiescence and eruptions, with subtle changes in line-profile shapes and other details. Some narrow emission features indicate contamination from a nearby H~{\sc ii} region in the host galaxy, NGC 4559. Broader H$α$ profiles exhibit Lorentzian shapes with full width at half-maximum intensity (FWHM) values that vary significantly, showing no correlation with photometric outbursts or the 113 d phase. At some epochs, H$α$ exhibits asymmetric profiles with a stronger redshifted wing, while broad and sometimes multicomponent P Cygni absorption features occasionally appear, but are again uncorrelated with brightness or phase. These P Cygni absorptions have high velocities compared to the FWHM of the H$α$ emission line, perhaps suggesting that the absorption component is not in the LBV's wind, but is instead associated with a companion. The lack of phase dependence in line-profile changes may point to interaction between a companion and a variable or inhomogeneous primary wind, in an orbit with only mild eccentricity. Recent photometric data indicate that AT 2016blu experienced its \nth{21} outburst around 2023 May/June, as predicted based on its period. This type of quasiperiodic LBV remains poorly understood, but its spectra and erratic light curve resemble some pre-SN outbursts like those of SN 2009ip.

astro-ph.SR↗

arXiv2025

Optical and Near-infrared Observations of SN 2023ixf for over 600 days after the Explosion

Context.We present a comprehensive photometric and spectroscopic study of the nearby Type II supernova (SN) 2023ixf, with our extensive observations spanning the phases from ~3 to over 600 days after the first light.\\ Aims.The aim of this study is to obtain key information on the explosion properties of SN\,2023ixf and the nature of its progenitor.\\ Methods.The observational properties of SN\,2023ixf are compared with those of a sample of Type IIP/L SNe to investigate commonalities and diversities. We conduct a detailed analysis of temporal evolution of major spectral features observed throughout different phases of the SN\,2023ixf explosion. Several interpretations are addressed through a comparison between the data and the model spectra for progenitor stars within a range of zero-age main sequence (ZAMS) masses.\\ Results.Our observations indicate that SN\,2023ixf is a transitional SN that bridges the gap between Type IIP and IIL subclasses of H-rich SNe, characterized by a relatively short plateau ($\lesssim 70$\,d) in the light curve. It shows a rather prompt spectroscopic evolution toward the nebular phase; emission lines of Na, O, H, and Ca in nebular spectra all exhibit multipeak profiles, which might be attributed to bipolar distribution of the ejecta. In particular, the H$α$ profile can be separated into two central peaked components (with a velocity of about 1500\,km\,s$^{-1}$) that is likely due to nickel-powered ejecta and two outer peak/box components (with a velocity extending up to ~8000 km\,s$^{-1}$) that can arise from interaction of the outermost ejecta with a circumstellar shell at a distance of $\sim6.2\times10^{15}$cm. The nebular-phase spectra of SN\,2023ixf show good agreement with those predicted by model spectra for progenitor stars with a ZAMS mass ranging from 15 to 19\,M${_\odot}$. A distance $D = 6.35^{+0.31}_{-0.39}$\,Mpc is estimated for M101.

astro-ph.HE↗

arXiv2025

GRB 170519A: Thermal Radiation in an X-ray Flare and Decaying Magnetic Fields for the Early-Time Afterglow

GRB 170519A was discovered by \emph{Swift}/BAT, and then observed by \emph{Swift}/XRT, \emph{Swift}/UVOT, and ground-based telescopes. We report Lick/KAIT observations of GRB 170519A, and make temporal analysis and spectral joint fits of its multiwavelength light curves. The observations present a relatively complete afterglow structure, including two X-ray flares (Flares I and II), optical onset (Slice 1), normal decay (Slices 2 and 3), and a possible jet break. The spectrum of the bright X-ray flare (Flare II) indicates that a thermal component exists at $t = 190$--240~s. \textbf{The blackbody emits in the photospheric radius $R_{\rm ph}\sim 10^{11}$ cm,} and its temperature ($kT$) decreases with time from \textbf{1.08 to 0.37 keV, its Lorentz factor of blackbody ($Γ_{\rm BB}$) decreases with time from 67.71 to 46.70. The luminosity of the blackbody ($L_{\rm BB}$), $kT$ and $Γ_{\rm BB}$ follow the relations $\bf L_{\rm BB} \propto kT^{2.49\pm 0.03}$ and $Γ_{\rm BB}\propto L_{\rm BB}^{0.27}$ (estimated from \cite{fan2012}).} In the optical light curves, there is an onset bump in the early-time afterglow, rising with an index $α_{O,1} \approx -0.43$ and peaking $\sim1174.9$ s since the BAT trigger. The bump then decays with $\bf α_{O,2} \approx 0.88$ in the normal decay phase, and the X-ray flux decays with a similar index of $\bf α_{X,1} \approx 0.95$. There is no obvious spectral evolution in the normal decay phases, with photon index $\hatΓ = 1.86$ and 1.92 in Slices 2 and 3, respectively. We find that the multiwavelength light curves of the GRB 170519A afterglow can be well fitted by an external shock with time-dependent $ε_B$. In the early afterglow, the value of $ε_B$ decays rapidly from $\bf 4.29\times10^{-2}$ to $\bf 8.23\times10^{-3}$.

astro-ph.HE↗

arXiv2025

A Multiwavelength Autopsy of the Interacting IIn Supernova 2020ywx: Tracing its Progenitor Mass-Loss History for 100 Years before Death

While the subclass of interacting supernovae with narrow hydrogen emission lines (SNe IIn) consists of some of the longest-lasting and brightest SNe ever discovered, their progenitors are still not well understood. Investigating SNe IIn as they emit across the electromagnetic spectrum is the most robust way to understand the progenitor evolution before the explosion. This work presents X-Ray, optical, infrared, and radio observations of the strongly interacting Type IIn SN 2020ywx covering a period $>1200$ days after discovery. Through multiwavelength modeling, we find that the progenitor of 2020ywx was losing mass at $\sim10^{-2}$--$10^{-3} \mathrm{\,M_{\odot}\,yr^{-1}}$ for at least 100 yrs pre-explosion using the circumstellar medium (CSM) speed of 120 km/s measured from our optical and NIR spectra. Despite the similar magnitude of mass loss measured in different wavelength ranges, we find discrepancies between the X-ray and optical/radio-derived mass-loss evolution, which suggest asymmetries in the CSM. Furthermore, we find evidence for dust formation due to the combination of a growing blueshift in optical emission lines and near-infrared continuum emission which we fit with blackbodies at $\sim$ 1000 K. Based on the observed elevated mass loss over more than 100 years and the configuration of the CSM inferred from the multiwavelength observations, we invoke binary interaction as the most plausible mechanism to explain the overall mass-loss evolution. SN 2020ywx is thus a case that may support the growing observational consensus that SNe IIn mass loss is explained by binary interaction.

astro-ph.HE↗

arXiv2025

Detection of [C I] Emission in Nebular Spectra of a Peculiar Type Ia Supernova 2022pul

SN 2022pul gains special attention due to its possible origin of a super-Chandarsekhar-mass white dwarf explosion (or called a 03fg-like type Ia supernova), which shows prominent [O I], [Ne II], and [Ca II] lines in its late-time spectra taken at $\sim+$300 days after the peak brightness. In this paper, we present new optical observations for this peculiar object, extending up to over 500 days after the peak brightness. In particular, in the $t\approx+515$ days spectrum, we identified for the first time the presence of narrow emission from [C I] $λ\lambda9824, 9850$, which appears asymmetric and quite similar to the accompanied [O I] $\lambda6300$ line in strength and profile. Based on the violent merger model that accounts well for previous observations but leaves little carbon in the center of the ejecta, this carbon line can be reproduced by increasing the degree of clumping in the ejecta and setting the carbon mass the same as that of oxygen ($\sim$0.06 $M_{\odot}$) in the innermost region ($\lesssim 2000$ km s$^{-1}$). In principle, the central carbon could come from the secondary white dwarf (WD) if it is ignited when hit by the shockwave of the explosion of the primary WD and explodes as a Ca-rich supernova, whereas pure deflagration of a super-Chandarsekhar-mass WD can account for such unburnt carbon more naturally.

astro-ph.HE↗

arXiv2025

SN 2023ixf in the Pinwheel Galaxy M101: From Shock Breakout to the Nebular Phase

We present photometric and spectroscopic observations of SN 2023ixf covering from day one to 442 days after explosion. SN 2023ixf reached a peak $V$-band absolute magnitude of $-18.2 \pm 0.07$, and light curves show that it is in the fast-decliner (IIL) subclass with a relatively short ``plateau'' phase (fewer than $\sim 70$ days). Early-time spectra of SN 2023ixf exhibit strong, very narrow emission lines from ionized circumstellar matter (CSM), possibly indicating a Type IIn classification. But these flash/shock-ionization emission features faded after the first week and the spectrum evolved in a manner similar to that of typical Type II SNe, unlike the case of most genuine SNe~IIn in which the ejecta interact with CSM for an extended period of time and develop intermediate-width emission lines. We compare observed spectra of SN 2023ixf with various model spectra to understand the physics behind SN 2023ixf. Our nebular spectra (between 200-400 d) match best with the model spectra from a 15 $\rm M_{\odot}$ progenitor which experienced enhanced mass loss a few years before explosion. A last-stage mass-loss rate of $\dot{M} = 0.01 \rm M_{\odot} yr^{-1}$ from the r1w6 model matches best with the early-time spectra, higher than $\dot{M} \approx 2.4 \times 10^{-3} \rm M_{\odot} yr^{-1}$ derived from the ionized H$α$ luminosity at 1.58 d. We also use SN 2023ixf as a distance indicator and fit the light curves to derive the Hubble constant by adding SN 2023ixf to the existing sample; we obtain H$_{0}=73.1^{+3.68}_{-3.50}$ km s$^{-1}$ Mpc$^{-1}$, consistent with the results from SNe~Ia and many other independent methods.

astro-ph.GA↗

arXiv2025

JWST/MIRI detects the dusty SN1993J about 30 years after explosion

Core-collapse supernovae (CCSNe) have long been considered to contribute significantly to the cosmic dust budget. New dust cools quickly and is therefore detectable at mid-infrared (mid-IR) wavelengths. However, before the era of the James Webb Space Telescope (JWST), direct observational evidence for dust condensation was found in only a handful of nearby CCSNe, and dust masses (~10$^{-2}-10^{-3} M_{\odot}$, generally limited to <5 yr and to >500K temperatures) have been 2-3 orders of magnitude smaller than either theoretical predictions or dust amounts found by far-IR/submm observations of Galactic SN remnants and in the very nearby SN 1987A. The combined angular resolution and mid-IR sensitivity of JWST finally allow us to reveal hidden cool (~100-200K) dust reservoirs in extragalactic SNe beyond SN 1987A. Our team received JWST/MIRI time for studying a larger sample of CCSNe to fill the currently existing gap in their dust formation histories. The first observed target of this program is the well-known Type IIb SN~1993J appeared in M81. We generated its spectral energy distribution (SED) from the current JWST/MIRI F770W, F1000W, F1500W, and F2100W fluxes. We fit single- and two-component silicate and carbonaceous dust models to the SED. We found that SN 1993J still contains a significant amount (~0.01 $M_{\odot}$) of dust ~30 yr after explosion. Comparing these results to those of the analysis of earlier {Spitzer Space Telescope data, we see a similar amount of dust now that was detected ~15-20 yr ago, but at a lower temperature. We also find residual background emission near the SN site (after point-spread-function subtraction on the JWST/MIRI images) that may plausibly be attributed to an IR echo from more distant interstellar dust grains heated by the SN shock-breakout luminosity or ongoing star formation in the local environment.

astro-ph.SR↗

arXiv2025

The Extinction Law in SN Ia Hosts from Background Galaxy Measurements: Toward a 1% Determination of $H_0$

In the most precise distance ladder determination of $H_0$, the observed near-infrared (NIR) fluxes of Cepheids are corrected for dust, assuming that the extinction law in large, star-forming spiral hosts of Type Ia supernovae (SN Ia) is similar to the Milky Way's average value of $R_V \approx 3.1$. Intriguingly, studies of SNe Ia often point to lower values for their hosts ($R_V \sim 2$). Ambiguities related to $R_V$ may limit future efforts to measure $H_0$ beyond $\sim 1\%$ precision. To better resolve extragalactic extinction laws, we directly measure the wavelength-dependent absorption of background galaxies seen in HST and JWST images (0.5--2.7 $μ$m). We take the following steps: (i) subtract foreground stars to measure accurate photometry of background galaxies with a tool, $\texttt{SPHOT}$; (ii) measure their redshifts and spectroscopic features with Keck/DEIMOS; (iii) determine their intrinsic spectral energy distributions from the empirical templates which match the absorption lines and breaks in observed spectroscopic features, and (iv) measure $R_V$ by fitting the extinction model to the difference between the template and the observed SEDs. The above steps are tested with artificial datasets to insure they accurately recover the input $R_V$. We apply this set of steps to a first case, NGC 5584, a SN Ia host and a calibrator of the Hubble constant. The estimated value of $R_V$ for NGC 5584, $R_V=3.59^{+0.99}_{-0.62}(\text{stat})\pm0.19(\text{syst})$, is consistent with the MW-like extinction law, and it is $\gtrsim 3.5σ$ away from $R_V=2$ as favored by SN Ia. If additional hosts show similar results, it would suggest that SN Ia extinction may not be solely due to mean interstellar dust. We are now undertaking a statistical study of 5-10 SH0ES hosts to determine the distribution of host extinction laws.

astro-ph.CO↗

arXiv2025

EP240801a/XRF 240801B: An X-ray Flash Detected by the Einstein Probe and Implications of its Multiband Afterglow

We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on August 1, 2024 by the Einstein Probe (EP) satellite, with a weak contemporaneous signal also detected by Fermi/GBM. Optical spectroscopy of the afterglow, obtained by GTC and Keck, identified the redshift of $z = 1.6734$. EP240801a exhibits a burst duration of 148 s in X-rays and 22.3 s in gamma-rays, with X-rays leading by 80.61 s. Spectral lag analysis indicates the gamma-ray signal arrived 8.3 s earlier than the X-rays. Joint spectral fitting of EP/WXT and Fermi/GBM data yields an isotropic energy $E_{γ,\rm{iso}} = (5.57^{+0.54}_{-0.50})\times 10^{51}\,\rm{erg}$, a peak energy $E_{\rm{peak}} = 14.90^{+7.08}_{-4.71}\,\rm{keV}$, a fluence ratio $\rm S(25-50\,\rm{keV})/S(50-100\,\rm{keV}) = 1.67^{+0.74}_{-0.46}$, classifying EP240801a as an X-ray flash (XRF). The host-galaxy continuum spectrum, inferred using Prospector, was used to correct its contribution for the observed outburst optical data. Unusual early $R$-band behavior and EP/FXT observations suggest multiple components in the afterglow. Three models are considered: two-component jet model, forward-reverse shock model and forward-shock model with energy injection. Both three provide reasonable explanations. The two-component jet model and the energy injection model imply a relatively small initial energy and velocity of the jet in the line of sight, while the forward-reverse shock model remains typical. Under the two-component jet model, EP240801a may resemble GRB 221009A (BOAT) if the bright narrow beam is viewed on-axis. Therefore, EP240801a can be interpreted as an off-beam (narrow) jet or an intrinsically weak GRB jet. Our findings provide crucial clues for uncovering the origin of XRFs.

astro-ph.HE↗

arXiv2025

The Luminous, Slow-Rising Orphan Afterglow AT2019pim as a Candidate Moderately Relativistic Outflow

Classical gamma-ray bursts (GRBs) have two distinct emission episodes: prompt emission from ultrarelativistic ejecta and afterglow from shocked circumstellar material. While both components are extremely luminous in known GRBs, a variety of scenarios predict the existence of luminous afterglow emission with little or no associated high-energy prompt emission. We present AT2019pim, the first spectroscopically confirmed afterglow with no observed high-energy emission to be identified. Serendipitously discovered during follow-up observations of a gravitational-wave trigger and located in a contemporaneous TESS sector, it is hallmarked by a fast-rising (t ~ 2 hr), luminous (M_UV,peak ~ -24.4 mag) optical transient with accompanying luminous X-ray and radio emission. No gamma-ray emission consistent with the time and location of the transient was detected by Fermi-GBM or by Konus, placing constraining limits on an accompanying GRB. We investigate several independent observational aspects of the afterglow in the context of constraints on relativistic motion and find all of them are consistent with an initial Lorentz factor of Gamma_0 ~ 10-30 for the on-axis material, significantly lower than in any well-observed GRB and consistent with the theoretically predicted "dirty fireball" scenario in which the high-energy prompt emission is stifled by pair production. However, we cannot rule out a structured jet model in which only the line-of-sight material was ejected at low-Gamma, off-axis from a classical high-Gamma jet core, and an on-axis GRB with below-average gamma-ray efficiency also remains a possibility. This event represents a milestone in orphan afterglow searches, demonstrating that luminous optical afterglows lacking detected GRB counterparts can be identified and spectroscopically confirmed in real time.

astro-ph.HE↗