arXiv subjects

Prasiddha Arunachalam

Publications and source records attributed to Prasiddha Arunachalam.

9 recordsLinked to original sources

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 $νL_ν \propto λ^{-2.13 \pm 0.04}$, significantly shallower than the Rayleigh-Jeans slope of $λ^{-3}$. 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.

astro-ph.HE↗

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] $λλ$7291, 7323 and weak [O I] $λλ$6300, 6364 emission lines. Simultaneous line profile modeling of [Ca II] and [O I] at nebular phases shows that an asymmetric core-collapse explosion of a low-mass ($\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.

astro-ph.HE↗

arXiv2026

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.

astro-ph.HE↗

arXiv2025

GRB 230307A Formed No Dust or Was Not a Binary Neutron Star Merger

We present a new analysis of the JWST infrared spectra of GRB 230307A (AT 2023vfi), a long gamma-ray burst (GRB) with an infrared excess and spectral lines suggestive of significant heavy $r$-process production. The spectra, taken 29 and 61~days after the GRB trigger, have blackbody-like continua with $T_{\rm eff} \approx 550$ K and an emission line near $2.1$ $μ$m, previously attributed to [Te III]. This line identification has been used as evidence for an $r$-process-powered kilonova (KN), despite no KN model producing a blackbody-like spectrum at late times. Such an infrared continuum could be emitted by newly formed dust, and we model the thermal emission to infer dust properties, including composition and mass. We find that the emission requires at least 3--$6 \times 10^{-3}$~M$_{\odot}$ of carbon or silicate dust, which is inconsistent with $r$-process yields expected from a neutron star merger. Alternatively, the continuum could be from $2\times 10^{-3}$~M$_{\odot}$ of metallic iron dust, which is mildly consistent (at 3$σ$) with KN models, but such dust is unlikely to form in the expanding ejecta. GRB 230307A's low late-time luminosity also constrains the amount of radioactive $^{56}$Ni produced to $<2.6 \times 10^{-3}$~M$_{\odot}$ (3$σ$). No KN model can simultaneously form the necessary dust for the infrared continuum and heavy elements for the [Te III] line. We conclude that the blackbody continuum is not due to dust emission, or GRB 230307A did not originate from a binary compact-object merger.

astro-ph.HE↗

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.

astro-ph.HE↗

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$20,000 K is maintained thereafter without evidence for cooling. We interpret the featureless spectra as a consequence of continuous energy injection from a central source of high-energy emission which maintains high ejecta ionization. After 35 d, faint (equivalent width <10 Å) H and He spectral features with kinematically separate velocity components centered at 0 km s$^{-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 $F_{\rmν,NIR}\proptoν^{-0.3}$ 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.

astro-ph.HE↗

arXiv2023

No Surviving SN Ia Companion In SNR 0509-67.5: Stellar Population Characterization and Comparison To Models

The community agrees that Type Ia supernovae arise from Carbon/Oxygen white dwarfs undergoing thermonuclear runaway. However, the full progenitor system and the process that prompts the white dwarf to explode remain unknown. Most current models suggest that the white dwarf explodes because of interaction with a binary companion which may survive the process and remain within the resulting remnant of the exploded star. Furthermore, both the pre-supernova interaction process and the explosion of the primary are expected to imprint a significant departure from ordinary stellar radii and temperatures onto the secondary, making the star identifiable against the unrelated stellar population. Identification of a surviving companion inside an SN Ia remnant might confirm a specific corresponding SN Ia progenitor channel based on the identity of the companion. We conducted a surviving companion search of the Type Ia remnant SNR 0509-67.5 based in the Large Magellanic Cloud. The well-constrained distance to and foreground extinction of the Large Magellanic Cloud allow for Bayesian inference of stellar parameters with low correlation and uncertainties. We present a deep catalog of fully characterized stars interior to SNR 0509-67.5 with radii, effective temperatures, and metallicities inferred using combined Hubble Space Telescope photometric observations across multiple visits. We then compile a list of surviving companion models appropriate for the age of the remnant (roughly 400 years after the explosion). We compare these predictions with the inferred stellar parameters and conclude that none of the stars are consistent with the predicted signatures of a surviving companion.

astro-ph.SR↗

arXiv2022

A Hydro-Based MCMC Analysis of SNR 0509-67.5 Revealing the Explosion Properties from Fluid Discontinuities Alone

Using HST narrow-band Hα images of supernova remnant 0509-67.5 taken {\sim}10 years apart, we measure the forward shock (FS) proper motions (PMs) at 231 rim locations. The average shock radius and velocity are 3.66 {\pm} 0.036 pc and 6315 {\pm} 310 km s{^{-1}}. Hydrodynamic simulations, recast as similarity solutions, provide models for the remnant's expansion into a uniform ambient medium. These are coupled to a Markov chain Monte Carlo (MCMC) analysis to determine explosion parameters, constrained by the FS measurements. For specific global parameters, the MCMC posterior distributions yield an age of 315.5 {\pm} 1.8 yr, a dynamical explosion center at 5h09m31s.16s -67{^\circ}31{^\prime}17.1{^{\prime\prime}} and ambient medium densities at each azimuth ranging from 3.7-8.0 {\times} 10{^{-25}} g cm{^{-3}}. We can detect stellar PMs corresponding to speeds in the LMC of 770 km s{^{-1}} or more using the H$α$ images. Five stars in the remnant show measurable PMs but none appear to be moving radially from the center, including a prominent red star 4.6{^{\prime\prime}} from the center. Using coronal [Fe XIV] λ5303 emission as a proxy for the reverse shock location, we constrain the explosion energy (for a compression factor of 4) to a value of E = (1.30 {\pm} 0.41) {\times} 10{^{51}} erg for the first time from shock kinematics alone. Higher compression factors (7 or more) are strongly disfavored based on multiple criteria, arguing for inefficient particle acceleration in the Balmer shocks of 0509-67.5.

astro-ph.HE↗

arXiv2021

Head-to-Toe Measurement of El Gordo: Improved Analysis of the Galaxy Cluster ACT-CL J0102-4915 with New Wide-field Hubble Space Telescope Imaging Data

We present an improved weak-lensing (WL) study of the high$-z$ $(z=0.87)$ merging galaxy cluster ACT-CL J0102-4915 (El Gordo) based on new wide-field Hubble Space Telescope (HST) imaging data. The new imaging data cover the 3.5$\times$3.5 Mpc region centered on the cluster and enable us to detect WL signals beyond the virial radius, which was not possible in previous studies. We confirm the binary mass structure consisting of the northwestern (NW) and southeastern (SE) subclusters and the 2$σ$ dissociation between the SE mass peak and the X-ray cool core. We obtain the mass estimates of the subclusters by simultaneously fitting two Navarro-Frenk-White (NFW) halos without employing mass-concentration relations. The masses are $M_{200c}^{NW} = 9.9^{+2.1}_{-2.2} \times 10^{14} M_{\sun}$ and $M_{200c}^{SE} = 6.5^{+1.9}_{-1.4} \times 10^{14} M_{\sun}$ for the NW and SE subclusters, respectively. The mass ratio is consistent with our previous WL study but significantly different from the previous strong lensing results. This discrepancy is attributed to the use of extrapolation in strong lensing studies because the SE component possesses a higher concentration. By superposing the two best-fit NFW halos, we determine the total mass of El Gordo to be $M_{200c} = 2.13^{+0.25}_{-0.23} \times 10^{15} M_{\sun}$, which is 23% lower than our previous WL result [$M_{200c} =(2.76\pm0.51) \times 10^{15} M_{\sun}$]. Our updated mass is a more direct measurement since we are not extrapolating to $R_{200c}$ as in all previous studies. The new mass is compatible with the current $Λ$CDM cosmology.

astro-ph.CO↗