arXiv subjects

Ravjit Kaur

Publications and source records attributed to Ravjit Kaur.

13 recordsLinked to original sources

arXiv2026

JWST Reveals Large Reservoirs of Dust and Ongoing Circumstellar Interaction in SN Ibn/Icn 2023xgo over a Year Post-Explosion

We present infrared (IR) photometric and spectroscopic observations of SN 2023xgo, a recent and nearby Type Ibn/Icn supernova (SN Ibn/Icn) which shows shock interaction with a He/C-rich and H-poor circumstellar medium (CSM). Although interacting SNe are predicted to produce large amounts of dust, the rarity of SNe Ibn and Icn has resulted in few opportunities to observe these objects in the IR at late times. Here, we report observations of SN 2023xgo from JWST (NIRSpec and MIRI), WISE, and Gemini taken out to +377 days post-explosion. At +377 days, the JWST spectrum is consistent with both emission from cool (~300-600 K) silicate dust with $M \gtrsim 3 \times 10^{-2}$ M$_{\odot}$ at a radius similar to the shock radius ($2.3 \times 10^{16}$ cm), and optically thin carbonaceous dust with $M = 8 \times 10^{-3}$ M$_{\odot}$. We also detect narrow (FWHM = 520+/-130 km s$^{-1}$) He I $\lambda$2.06 micron emission at +377 days, indicating that the SN shock continues to encounter material shed from the star to this late epoch. The emission line is blueshifted from the rest frame by 340+/-40 km s$^{-1}$. The Gemini and WISE observations at ~70-100 days reveal emission from 6.8$\times$10$^{-5}$ M$_{\odot}$ of hot (~1300 K) dust, which we interpret as a lower limit of the total dust mass at that phase. Molecular gas emission is not detected in any data, though emission line profiles in the optical and NIR taken at ~70 days after explosion show progressively less redshifted emission, attributed to attenuation from dust and suggesting that some dust is rapidly forming interior to the unshocked CSM. The large dust mass and rapid onset of dust formation observed in SN 2023xgo show that the unique physical environments of SNe Ibn/Icn facilitate substantial dust formation both before and after the SN.

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arXiv2026

JWST and Keck observations of the off-nuclear tidal disruption event TDE 2025abcr: An evolving reprocessing layer

Off-nuclear tidal disruption events (TDEs) provide a rare probe of massive black holes (MBHs) outside galactic nuclei. Only a handful are known, including five X-ray-selected candidates and two optically selected events. We present observations of TDE 2025abcr, the second optically selected off-nuclear TDE, discovered at a projected offset of $9.08 \pm 0.02$ kpc from the nucleus of its host galaxy. We analyze X-ray, UV, optical, and infrared (IR) data from Swift, Keck, ZTF, and JWST. Broad H and He emission lines in the optical and IR confirm a TDE-H-He classification. From luminosity scaling relations and MOSFiT modeling, we infer a BH mass of $10^{6}$-$10^{7}\,M_{\odot}$, substantially smaller than the $10^{8.35 \pm 0.41}\,M_{\odot}$ BH inferred for the host-galaxy nucleus. We observe velocity evolution in the N III + He II emission complex, shifting from $-500$ km s$^{-1}$ at day $-7$ to $+1000$ km s$^{-1}$ by day $+29$, which we interpret as radiative transfer effects in an evolving reprocessing layer. The IR SED deviates from a thermal blackbody, with $\nu L_{\nu} \propto \lambda^{-2.13 \pm 0.04}$, significantly shallower than the Rayleigh-Jeans slope of $\lambda^{-3}$. We rule out dust as the source of this IR excess. Two possibilities remain: free-free emission from reprocessing gas, or an unresolved stellar cluster at the TDE location. Reprocessed emission provides a natural explanation for the IR excess but an underlying stellar cluster of mass $\log(M_{*}/M_{\odot}) = 7.57 \pm 0.02$ and age $<$2 Gyr is also consistent with the data. If interpreted as a stellar cluster, the inferred mass suggests a stripped remnant of a satellite galaxy. The wandering MBH most likely originated in a minor merger with a smaller galaxy, although dynamical ejection from the host nucleus cannot yet be ruled out.

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arXiv2026

SN 2024iss: A Multi-Wavelength Expos\'e of a Type IIb Supernova with an Early-Time Ultraviolet Spectrum and Shock Breakout Constraints

We present multi-wavelength observations and a comprehensive analysis of the nearby (D$\sim$14 Mpc) Type IIb supernova (SN IIb) 2024iss. Observations of SN2024iss include an early ZTF detection at $\sim$40 minutes after first light and the earliest Hubble Space Telescope UV spectrum for a SN IIb to date at 7 days after first light. With the bolometric light curve and He-star models, we estimate an ejecta mass range of $\sim 1.1-3.3~M_{\odot}$ and a $^{56}\textrm{Ni}$ mass of $0.11 \pm 0.01~M_{\odot}$. We fit shock-cooling emission models to the first peak in the light curve and estimate a progenitor radius of $100-320~R_{\odot}$ and a H-rich envelope mass of $0.07-0.46~M_{\odot}$. We also compared optical/UV spectra to binary progenitor model spectra, which indicate a stripped H-rich envelope mass of $0.19-0.28~M_{\odot}$. We use early-time X-ray detections to calculate CSM densities that are consistent with a progenitor mass-loss rate of $5\times10^{-4}~M_{\odot}$ ($v_w = 100~$km/s), corresponding to a period of significant mass ejection in the final ~2-5 years before core collapse. In the UV spectrum, we observe strong Mg II emission extending to $\sim15,000 ~$km/s as well as weak P-Cygni profiles of iron-group elements (e.g., Fe, Ti, Al, Ni) present in the outer SN ejecta during the end of shock cooling phase. We find that the overall spectroscopic evolution of SN2024iss is comparable to other SNe IIb, but that the increased brightness following the initial light curve peak is likely influenced by SN ejecta-CSM interaction. Finally, optical/NIR nebular spectroscopy of SN2024iss at $\sim 260-412~$ days reveals multi-peaked forbidden line profiles of O I and Mg I] indicative of inner ejecta asymmetry and/or clumping. We demonstrate the utility of a rich, multi-wavelength dataset for constraining the progenitor systems and explosion dynamics of SNe IIb.

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arXiv2026

An infrared echo from a circumstellar disk in the hydrogen- and helium-poor SN 2024aecx

We present near-infrared (NIR) spectroscopy of the hydrogen- and helium-poor (Type Ic) supernova (SN) 2024aecx, which displays a strong NIR excess emerging 32 days post peak. SN 2024aecx is a peculiar SN Ic that exhibited luminous shock-cooling emission at early times, suggestive of close-in circumstellar medium (CSM), unexpected for this class of SNe. Its early NIR spectra are typical for a SN Ic but with strong C I absorption features. By $\sim$32 days post peak, the spectra show a strong NIR excess, while maintaining normal optical colors, unprecedented for SNe Ic. We find that the NIR excess is well fit with a single-temperature, optically thin dust model with declining temperature, increasing mass, and roughly constant luminosity over time. The NIR excess appears too promptly for dust to have formed in the SN ejecta, indicating an IR echo from pre-existing dust in the CSM. The IR echo is likely powered by the relatively slowly evolving SN peak light, and not the brief shock cooling emission, as the latter requires unrealistically high CSM densities to explain the observed dust mass. We consider different potential CSM geometries and find that a thick face-on disk with an inner edge of around $5\times 10^{16} \rm \ cm$ can best explain the dust mass and temperature evolution. In this scenario, the SN shock should start interacting with this CSM $440\pm200$ days post explosion. CSM around SN Ic is rare, and follow-up observations of SN 2024aecx will probe the mass-loss process responsible for removing hydrogen and helium from their progenitor star.

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arXiv2026

The Double-Peaked Calcium-Strong SN 2025coe: Progenitor Constraints from Early Interaction and Ejecta Asymmetries

Supernova (SN) 2025coe at a distance of $\sim$25 Mpc is the second-closest calcium-strong (CaST) transient. It was discovered at a large projected offset of $\sim$34 kpc from its potential host galaxy NGC 3277. Multiband photometry of SN 2025coe indicates the presence of two peaks at day $\sim$2 and day $\sim$11 after explosion. Modeling the bolometric light curve, we find that the first peak can be reproduced either by shock cooling of a compact envelope ($R_\mathrm{env}$ $\approx $6-40 $R_{\odot}$; $M_\mathrm{env}$ $\approx $0.1-0.2 $M_{\odot}$) or by interaction with close-in circumstellar material (CSM; $R_{\mathrm{CSM}} \lesssim 6 \times10^{14}$ cm), or a combination of both. The second peak is dominated by radioactive decay of $^{56}$Ni ($M_{\mathrm{ej}} \approx $0.4-0.5 $M_{\odot}$; $M_{^{56}\mathrm{Ni}} \approx 1.4 \times 10^{-2}$ $M_{\odot}$). SN 2025coe rapidly evolves from the photospheric phase dominated by He I P-Cygni profiles to nebular phase spectra dominated by strong [Ca II] $\lambda \lambda$7291, 7323 and weak [O I] $\lambda \lambda$6300, 6364 emission lines. Simultaneous line profile modeling of [Ca II] and [O I] at nebular phases shows that an asymmetric core-collapse explosion of a low-mass ($\lesssim$3.3 $M_{\odot}$) He-core progenitor can explain the observed line profiles. Alternatively, lack of local star formation at the site of the SN explosion combined with a low ejecta mass is also consistent with a thermonuclear explosion due to a low-mass hybrid He-C/O white dwarf + C/O white dwarf merger.

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arXiv2025

Einstein Probe Discovery of an X-ray Flare from K-type Star PM J23221-0301

Stellar flares are an intense stellar activity that can significantly impact the atmospheric composition of the surrounding planets and even the possible existence of life. During such events, the radiative energy of the star is primarily concentrated in the optical and X-ray bands, with the X-ray flux potentially increasing by tens or even hundreds of times. Einstein Probe (EP) detected a new X-ray transient EP J2322.1-0301 on 27 September 2024. Its spatial localization shows a high positional coincidence with the nearby high proper motion K-type star PM J23221-0301. Follow-up X-ray observations confirmed the flux enhancement of the source, while optical spectroscopic monitoring revealed time-variable features, particularly the disappearance of the H-alpha emission line. This X-ray flare is consistent with a characteristic fast-rise-exponential-decay (FRED) light curve, with a rise timescale of 1.4 ks, a decay timescale of 5.7 ks, and a total duration of about 7.1 ks. The peak luminosity in the 0.5-4.0 keV energy band reached about 1.3 x 10^31 erg s^-1, with a total energy release of about 9.1 x 10^34 erg, consistent with the empirical energy correlations observed in magnetic-reconnection-driven stellar flares, as inferred from the multitemperature plasma structure and H-alpha-X-ray energy correlation. This discovery underscores EP's capability in understanding stellar magnetic activity via observing stellar transients.

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arXiv2025

AT2025ulz and S250818k: Deep X-ray and radio limits on off-axis afterglow emission and prospects for future discovery

The first joint electromagentic (EM) and gravitational wave (GW) detection, known as GW170817, marked a critical juncture in our collective understanding of compact object mergers. However, it has now been 8 years since this discovery, and the search for a second EM-GW detection has yielded no robust discoveries. Recently, on August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported a low-significance (high false alarm rate) binary neutron star merger candidate S250818k. Rapid optical follow-up revealed a single optical candidate AT2025ulz ($z=0.08484$) that initially appeared consistent with kilonova emission. We quickly initiated a set of observations with \textit{Swift}, \textit{XMM-Newton}, \textit{Chandra}, and the Very Large Array to search for non-thermal afterglow emission. Our deep X-ray and radio search rules out that the optical rebrightening of AT2025ulz is related to the afterglow onset, reinforcing its classification as a stripped-envelope supernova (SN 2025ulz). We derive constraints on the afterglow parameters for a hypothetical binary neutron star merger at the distance of AT2025ulz ($\approx 400$ Mpc) based on our X-ray and radio limits. We conclude that our observational campaign could exclude a GW170817-like afterglow out to viewing angles of $\theta_\textrm{v}\approx 12.5$ degrees. We briefly discuss the prospects for the future discovery of off-axis afterglows.

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arXiv2025

JWST and Keck Observations of the Off-Nuclear TDE AT 2024tvd: A Massive Nuclear Star Cluster and Minor-Merger Origin for its Black Hole

We present JWST/NIRSpec and NIRCam observations of the first optically selected off-nuclear tidal disruption event (TDE), AT 2024tvd, along with Keck/KCWI integral field unit spectroscopy. The spectra show broad H and He emission lines that are characteristic of a TDE. Stellar kinematics show smooth host-galaxy morphology and ordered bulge rotation, with no evidence of disturbances in velocity, dispersion, age or metallicity space. We construct the first quasi-simultaneous spectral-energy distribution (SED) from X-rays to infrared for a TDE and decompose it into three components: the TDE accretion flow, an unresolved nuclear star cluster (NSC), and heated dust emission. The accretion component implies a black hole mass of $\log(M_\bullet/M_\odot) = 5.50\pm 0.04$, an instantaneous super-Eddington accretion rate of $\log (\dot{M}/M_{\odot} yr^{-1}) = -1.22 \pm 0.04$, and an outer disk photosphere radius of $\log(r_{out}/r_{g}) = 3.8 \pm 0.1$. The dust emission is well described by a blackbody with $T_{dust} = 873\pm 15$ K and peak luminosity $\log (L_{dust}/erg$ $s^{-1}) = 40.80\pm 0.01$, consistent with a dust echo near the sublimation radius. The SED is best fit when including additional stellar emission above the galaxy background at the TDE location, corresponding to $\log(M_{\star}/M_\odot) = 7.97^{+0.16}_{-0.26}$, which we interpret as a massive NSC or an ultra-compact dwarf galaxy. These results support a minor-merger origin for the MBH responsible for the TDE over scenarios involving gravitational recoil or dynamical ejection from the nucleus.

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arXiv2025

The Most Luminous Known Fast Blue Optical Transient AT 2024wpp: Unprecedented Evolution and Properties in the Ultraviolet to the Near-Infrared

We present an extensive photometric and spectroscopic ultraviolet-optical-infrared campaign on the luminous fast blue optical transient (LFBOT) AT 2024wpp over the first ~100 d. AT 2024wpp is the most luminous LFBOT discovered to date, with $L_{\rm{pk}}\approx(2-4)\times10^{45}$ erg s$^{-1}$ (5-10 times that of the prototypical AT 2018cow). This extreme luminosity enabled the acquisition of the most detailed LFBOT UV light curve thus far. In the first ~45 d, AT 2024wpp radiated $>10^{51}$ erg, surpassing AT 2018cow by an order of magnitude and requiring a power source beyond the radioactive $^{56}$Ni decay of traditional supernovae. Like AT 2018cow, the UV-optical spectrum of AT 2024wpp is dominated by a persistently blue thermal continuum throughout our monitoring, with blackbody parameters at peak of T>30,000 K and $R_{\rm{BB}}/t\approx0.2-0.3c$. A temperature of $\gtrsim$20,000 K is maintained thereafter without evidence for cooling. We interpret the featureless spectra as a consequence of continuous energy injection from a central source of high-energy emission which maintains high ejecta ionization. After 35 d, faint (equivalent width <10 {\AA}) H and He spectral features with kinematically separate velocity components centered at 0 km s$^{-1}$ and -6400 km s$^{-1}$ emerge, implying spherical symmetry deviations. A near-infrared excess of emission above the optical blackbody emerges between 20-30 d with a power-law spectrum $F_{\rm\nu,NIR}\propto\nu^{-0.3}$ at 30 d. We interpret this distinct emission component as either reprocessing of early UV emission in a dust echo or free-free emission in an extended medium above the optical photosphere. LFBOT asphericity and multiple outflow components (including mildly relativistic ejecta) together with the large radiated energy are naturally realized by super-Eddington accretion disks around neutron stars or black holes and their outflows.

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arXiv2025

The Type II SN 2025pht in NGC 1637: A Red Supergiant with Carbon-rich Circumstellar Dust as the First JWST Detection of a Supernova Progenitor Star

We present follow-up imaging and spectroscopy and pre-explosion imaging of supernova (SN) 2025pht located in NGC 1637 at 12 Mpc. Our spectroscopy shows that SN 2025pht is a Type II SN with broad lines of hydrogen and with minimal line-of-sight extinction inferred from Na I D absorption. NGC 1637 was the target of several epochs of Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) imaging covering the site of SN 2025pht from 31 to 0.7 yr prior to discovery. Using a follow-up HST image of SN 2025pht aligned to these data, we demonstrate that there is a credible progenitor candidate detected in multiple epochs of HST imaging and in JWST imaging from 1.3-8.7 microns, the first JWST counterpart to a SN and the longest wavelength detection of a SN progenitor star. Fitting this source to red supergiant (RSG) spectral-energy distributions (SEDs), we show that it is consistent with a log(L/Lsun) = 5.0 RSG heavily reddened by circumstellar dust. The JWST photometry enable strong constraints on the nature of the circumstellar medium, and we find that the SED favors graphite-rich dust and an optical circumstellar extinction of Av = 5.3 mag as opposed to silicate-rich dust. We discuss the implications of a pre-SN RSG enshrouded in carbon-rich dust and this finding for the overall population of progenitor stars to Type II SN as well as the future of SN progenitor star discovery with JWST.

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arXiv2025

Photometry and Spectroscopy of SN 2024pxl: A Luminosity Link Among Type Iax Supernovae

We present extensive ultraviolet to optical photometric and optical to near-infrared (NIR) spectroscopic follow-up observations of the nearby intermediate-luminosity ($M_V = -16.81\pm0.19$~mag) Type Iax supernova (SN) 2024pxl in NGC 6384. SN~2024pxl exhibits a faster light curve than the high-luminosity members of this class, and slower than low-luminosity events. The observationally well-constrained rise time of $\sim$11 days and an estimated synthesized $^{56}$Ni mass of 0.03\, M$_\odot$, based on analytical modeling of the integrated spectral energy distribution light curve, are consistent with models of the weak deflagration of a carbon-oxygen white dwarf. Our optical spectral sequence of SN~2024pxl shows weak \ion{Si}{2} lines and spectral evolution similar to other high-luminosity Type Iax SNe, but also a prominent early-time \ion{C}{2} line, like lower-luminosity Type Iax SNe. The late-time optical spectrum of SN~2024pxl closely matches that of SN~2014dt, and its NIR spectral evolution aligns with that of other well-studied, high-luminosity Type Iax SNe. The spectral-line expansion velocities of SN~2024pxl are at the lower end of the Type Iax SN velocity distribution, and the velocity distribution of iron-group elements compared to intermediate-mass elements suggests that the ejecta are mixed on large scales, as expected in pure deflagration models. SN~2024pxl exhibits characteristics intermediate between those of high-luminosity and low-luminosity Type~Iax SNe, further establishing a link across this diverse class.

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arXiv2025

JWST and Ground-based Observations of the Type Iax Supernovae SN 2024pxl and SN 2024vjm: Evidence for Weak Deflagration Explosions

We present panchromatic optical $+$ near-infrared (NIR) $+$ mid-infrared (MIR) observations of the intermediate-luminosity Type Iax supernova (SN Iax) 2024pxl and the extremely low-luminosity SN Iax 2024vjm. JWST observations provide unprecedented MIR spectroscopy of SN Iax, spanning from $+$11 to $+$42 days past maximum light. We detect forbidden emission lines in the MIR at these early times while the optical and NIR are dominated by permitted lines with an absorption component. Panchromatic spectra at early times can thus simultaneously show nebular and photospheric lines, probing both inner and outer layers of the ejecta. We identify spectral lines not seen before in SN Iax, including [Mg II] 4.76 $\mu$m, [Mg II] 9.71 $\mu$m, [Ne II] 12.81 $\mu$m, and isolated O I 2.76 $\mu$m that traces unburned material. Forbidden emission lines of all species are centrally peaked with similar kinematic distributions, indicating that the ejecta are well mixed in both SN 2024pxl and SN 2024vjm, a hallmark of pure deflagration explosion models. Radiative transfer modeling of SN 2024pxl shows good agreement with a weak deflagration of a near-Chandrasekhar-mass white dwarf, but additional IR flux is needed to match the observations, potentially attributable to a surviving remnant. Similarly, we find SN 2024vjm is also best explained by a weak deflagration model, despite the large difference in luminosity between the two supernovae. Future modeling should push to even weaker explosions and include the contribution of a bound remnant. Our observations demonstrate the diagnostic power of panchromatic spectroscopy for unveiling explosion physics in thermonuclear supernovae.

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arXiv2024

Detecting prompt and afterglow jet emission of gravitational wave events from LIGO/Virgo/KAGRA and next generation detectors

Following the wealth of new results enabled by multimessenger observations of the binary neutron star (BNS) merger GW170817, the next goal is increasing the number of detections of electromagnetic (EM) counterparts to gravitational wave (GW) events. We study the detectability of the prompt emission and afterglows produced by the relativistic jets launched by BNS mergers that will be detected by LIGO-Virgo-KAGRA during their fifth observing run (O5), and by next generation (XG) GW detectors (Einstein Telescope and Cosmic Explorer). We quantify the impact of various BNS merger and jet afterglow parameters on the likelihood of detection, focusing on the impact of the observer's viewing angle and the jet's core half-opening angle. We explore detectability over a wide range of current state-of-the-art facilities (e.g., the James Webb Space Telescope, Chandra X-ray Observatory) as well as upcoming next-generation facilities (e.g., AXIS, NewAthena, ngVLA, SKA). We find that a few GW events (~0-4) per year may have a detectable afterglow component in O5, with the largest detection rates expected with SKA in the radio and JWST in the near-infrared. In the XG era, hundreds of multimessenger detections of afterglows per year may be possible with a range of instruments, such as NewAthena in the X-ray and ngVLA in the radio. While zero to a few GW events per year are expected to be accompanied by a detectable prompt emission in O5, dozens per year may be detectable in XG.

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