arXiv subjects

Devendra K. Sahu

Publications and source records attributed to Devendra K. Sahu.

At least 19 recordsLinked to original sources

arXiv2026

Unveiling the Nature of C/2023 A3 (Tsuchinshan-ATLAS): A Multi-Technique Observational Approach

Comet C/2023 A3 (Tsuchinshan-ATLAS) is a non-periodic dynamically new Oort cloud comet that was discovered independently by Purple Mountain Observatory in China and Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes in South Africa. The comet passed perihelion at a distance of 0.39 AU on 27 September 2024. It was visible to the naked eye (the brightest since the comet C/1995 O1 (Hale-Bopp)) and was dubbed the great comet of 2024. In this work, we investigate the nature of this comet, which is moving in a hyperbolic orbit (e > 1), by analysing its composition using various observational techniques and tracing its orbital evolution through high-precision N-body simulations.

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arXiv2026

SN 2024dy: Dust formation in a long-lived Type IIn supernova and constraints on the dust mass

Type~IIn supernovae (SNe) are a subclass of core-collapse SNe powered by interaction between the ejecta and the dense circumstellar material. Among them, long-lived Type~IIn events are characterized by luminous, long-duration light curves with high radiative energy. Several cases of long-lived type IIn SNe exhibit substantial dust emission at late times. However, well-observed examples remain limited, and the details of their dust formation mechanisms remain poorly understood. Here we present photometric and spectroscopic observations of the Type~IIn SN~2024dy in ultraviolet, optical, and near-infrared (NIR) wavelength for $500$ days. SN~2024dy reached a peak magnitude of $M_r=-19.2$~mag with a total radiation energy of $1.9\times10^{50}$~erg. A NIR excess emerged at late phases, and the spectral energy distribution modeling indicates the presence of carbon dust with temperatures of $1300$-$1800$~K and masses of about $10^{-5}\ M_\odot$. The spectra features were typical of long-lived Type~IIn SNe. The late time H$\alpha$ profile exhibits a strong suppression of the red wing, providing evidence for newly formed dust. Our results suggest that the derived dust mass above may be underestimated due to optical depth effects. SN~2024dy provides an important observational case for understanding dust formation in Type~IIn SNe.

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arXiv2026

Gaia24ccy: An outburst followed the footsteps of its predecessor

Accretion-driven outbursts in young stellar objects remain poorly understood, largely limited by a statistically small sample of closely followed-up events. This underscores the importance of a thorough exploration of each outbursting object. We studied a peculiar outbursting system, Gaia24ccy, which exhibited two $\Delta g \sim$ 3.8 mag outbursts in 2019 and 2024. The system consists of two unresolved, nearly identical, and rapidly rotating young stars: Gaia24ccy A (1.1419 days) and Gaia24ccy B (1.7898 days). Periodogram analyses just before the onset of the outbursts suggest Gaia24ccy B to be the outbursting component. Unlike any previously known EXor sources, the two outburst profiles show a very similar evolution: both rose at the same rate for the first 15 days, followed by many 'sub-bursts' on the timescale of 10-20 days. The 2019 outburst lasted 145-255 days, while the 2024 outburst persisted for 367 days. We infer the unstable region to lie at $r_{\rm trigger} \simeq 0.019-0.047$ au ($\sim5-12.3 R_\star$). The accreted mass per event $M_{\rm acc}\sim10^{-5} M_\odot$ can be provided by a compact inner-disk reservoir. The photometric rise/decay timescales and the mid-infrared color evolution favor a thermal-viscous trigger in a hot inner disk, while the appearance of rich emission-line spectra indicates concurrent magnetospheric compression - together best described by a hybrid picture. Finally, we explain the reddening of the mid-infrared color observed during the outburst as a consequence of the competing emission from the viscous disk and the photosphere.

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arXiv2026

A Study of Two Type IIb Supernovae: SNe 2008aq and 2019gaf

We present photometric and spectroscopic studies of two core-collapse supernovae (SNe) 2008aq and 2019gaf in the optical wavelengths. Light curve and spectral sequence of both the SNe are similar to those of other Type IIb SNe. The pre-maximum spectrum of SN~2008aq showed prominent H $\alpha$ lines, the He lines started appearing in the near maximum spectrum. The near maximum spectrum of SN~2019gaf shows shallow H$\alpha$ absorption and He lines with almost similar strength. Both the SNe show transition from hydrogen-dominated spectra to helium-dominated spectra within a month after maximum brightness. The velocity evolution of SN~2008aq matches well with those of other well-studied Type IIb SNe, while SN~2019gaf shows higher velocities. Close to maximum light, the H $\alpha$ and He I line velocities of SN~2019gaf are $\sim$ 2000 km sec$^{-1}$ and $\sim$ 4000 km sec$^{-1}$ higher than other well-studied Type IIb SNe. Semi-analytical modeling indicates SN~2019gaf to be a more energetic explosion with a smaller ejecta mass than SN~2008aq. The zero-age main-sequence (ZAMS) mass of the progenitor estimated using the nebular spectra of SN~2008aq ranges between 13 to 20 M$_\odot$, while for SN~2019gaf, the inferred ZAMS mass is between 13 to 25 M$_\odot$. The [O I] to [Ca II] lines flux ratio favors a less massive progenitor star in a binary system for both the SNe.

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arXiv2025

Unveiling SN 2022eyw: A Bright Member of the Type Iax Supernova Subclass

We present comprehensive photometric and spectroscopic observations of Supernova (SN) 2022eyw, a luminous member of the Type Iax SN subclass. SN 2022eyw reached a peak absolute magnitude of $M_g = -17.80\pm0.15$ mag and exhibited a rise time of $\sim$15 days, placing it among the brighter Iax events. The bolometric light curve indicates a synthesized $^{56}$Ni mass of $0.11\pm0.01~\text{M}_{\odot}$, with an estimated ejecta mass of $0.79\pm0.09~\text{M}_{\odot}$ and kinetic energy of $0.19\times10^{51}$ erg. The spectral evolution from -8 to +110 days past maximum reveals features characteristic of bright Type Iax Supernovae, including a transition from Fe III to Fe II dominance, moderate expansion velocities, and a lack of strong C III absorption. TARDIS spectral modelling of the early-phase spectra indicates a well-mixed ejecta dominated by Fe-group elements. In addition, traces of unburnt carbon are detected, pointing to incomplete burning as expected in pure deflagration models. Late-time spectral evolution shows a blend of permitted and forbidden lines. Comparison with deflagration models suggests that SN 2022eyw originated from a partial deflagration of a Chandrasekhar-mass white dwarf, with explosion properties intermediate between the N3-def and N5-def models. These observations support pure deflagration of a CO white dwarf as a viable explosion mechanism for its luminous members.

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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

Unveiling the nature of two dwarf novae: CRTS J080846.2+313106 and V416 Dra

We present the analysis of optical photometric and spectroscopic observations of two non-magnetic cataclysmic variables, namely CRTS J080846.2+313106 and V416 Dra. CRTS J080846.2+313106 has been found to vary with a period of 4.9116$\pm$0.0003 h, which was not found in earlier studies and is provisionally suggested as the orbital period of the system. In both long-period systems, the observed dominant signal at second harmonic of the orbital frequency and the orbital modulation during quiescence are suggestive of ellipsoidal variation from changing aspects of the secondary, with an additional contribution from the accretion stream or hotspot. However, during the outburst, the hotspot itself is overwhelmed by the increased brightness, which is possibly associated with the accretion disc. The mid-eclipse phase for V416 Dra occurs earlier and the width of the eclipse is greater during outbursts compared to quiescence, suggesting an increased accretion disc radius during outbursts. Furthermore, the investigation of accretion disc eclipse in V416 Dra implies that a total disc eclipse is possible during quiescence, whereas the disc seems to be partially obscured during outbursts, which further signifies that the disc may grow in size as the outburst progresses. Optical spectra of CRTS J080846.2+313106 and V416 Dra are typical of dwarf novae during quiescence, and they both show a significant contribution from the M2-4V secondary. The light curve patterns, orbital periods, and spectra observed in both systems look remarkably similar, and seem to resemble the characteristics of U Gem-type dwarf novae.

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arXiv2024

SN 2020udy: A new piece of the homogeneous bright group in the diverse Iax subclass

We present optical observations and analysis of a bright type Iax SN~2020udy hosted by NGC 0812. The light curve evolution of SN~2020udy is similar to other bright Iax SNe. Analytical modeling of the quasi bolometric light curves of SN 2020udy suggests that 0.08$\pm$0.01 M$_{\odot}$ of $^{56}$Ni would have been synthesized during the explosion. Spectral features of SN 2020udy are similar to the bright members of type Iax class showing weak Si {\sc II} line. The late-time spectral sequence is mostly dominated by Iron Group Elements (IGEs) with broad emission lines. Abundance tomography modeling of the spectral time series of SN~2020udy using TARDIS indicates stratification in the outer ejecta, however, to confirm this, spectral modeling at a very early phase is required. After maximum light, uniform mixing of chemical elements is sufficient to explain the spectral evolution. Unlike the case of normal type Ia SNe, the photospheric approximation remains robust until +100 days, requiring an additional continuum source. Overall, the observational features of SN 2020udy are consistent with the deflagration of a Carbon-Oxygen white dwarf.

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arXiv2023

Quiescence of an Outburst of a Low-Mass Young Stellar Object: LDN1415-IRS

LDN1415-IRS, a low-mass young stellar object (YSO) went into an outburst between 2001 and 2006, illuminating a surrounding nebula, LDN1415-Neb. LDN1415-Neb was found to have brightened by I=3.77 mag by April 2006. The optical light curve covering $\sim$ 15.5 years, starting from October 2006 to January 2022, is presented in this study. The initial optical spectrum indicated the presence of winds in the system but the subsequent spectra have no wind indicators. The declining light curve and the absence of the P-Cygni profile in later epoch spectra indicate that the star and nebula system is retrieving back from its outburst state. Two Herbig-Haro objects (HHOs) are positioned linearly with respect to the optical brightness peak of the nebula, probably indicating the circumstellar disk being viewed edge-on. Our recent deep near-infrared (NIR) imaging using TANSPEC has revealed the presence of a nearby star-like source, south of the LDN1415-IRS, at an angular distance of $\sim$ 5.4 arcsec.

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arXiv2022

TNO or Comet? The Search for Activity and Characterization of Distant Object 418993 (2009 MS9)

2009 MS9 is a trans-Neptunian object (TNO) whose perihelion brings it close to the distance where some long period comets are seen to become active. Knowing this, and the fact that this object appears to brighten in excess of it's predicted nucleus brightness suggests that 2009 MS9 has a delayed onset of activity brought on by the sublimation of a species more volatile than water. In this paper we characterize 2009 MS9's physical properties and investigate potential outgassing through composite images, sublimation models, and measurements of spectral reflectivity. We find that deep composite images of the object at various epochs along its orbit show no evidence of dust yet place sensitive limits to the dust production. We estimate the nucleus radius to be 11.5 km $\pm 3.5$ km using thermal IR modeling from NEOWISE data and use this and data pre-perihelion to estimate a geometric albedo of 0.25. We compare a CO-sublimation activity model to its post perihelion heliocentric light curve and find this data supports an active fractional area of $5 \times 10^{-6}$ assuming 2 $μ$m sized grains and other typical comet parameters. The spectral reflectivity of the surface materials obtained with the Gemini Observatory and CFHT at different epochs shows a reddening spectral slope. We compare the physical properties of 2009 MS9 to both TNO and comet populations, and speculate that 2009 MS9's reddening may be due to the buildup of a dust mantle on the surface and could be an explanation of why TNOs exhibit a color bimodality.

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arXiv2022

Optical studies of a bright Type Iax supernova SN 2020rea

We present optical photometric and spectroscopic analysis of a Type Iax supernova (SN) 2020rea situated at the brighter luminosity end of Type Iax supernovae (SNe). The light curve decline rate of SN~2020rea is $Δ$m$_{15}$(g) = 1.31$\pm$0.08 mag which is similar to SNe 2012Z and 2005hk. Modelling the pseudo bolometric light curve with a radiation diffusion model yields a mass of $^{56}$Ni of 0.13$\pm$0.01 M$_{\odot}$ and an ejecta mass of 0.77$^{+0.11}_{-0.21}$ M$_{\odot}$. Spectral features of SN~2020rea during the photospheric phase show good resemblance with SN 2012Z. TARDIS modelling of the early spectra of SN~2020rea reveals a dominance of Iron Group Elements (IGEs). The photospheric velocity of the Si {\sc II} line around maximum for SN~2020rea is $\sim$ 6500 km s$^{-1}$ which is less than the measured velocity of the Fe {\sc II} line and indicates significant mixing. The observed physical properties of SN~2020rea match with the predictions of pure deflagration model of a Chandrasekhar mass C-O white dwarf. The metallicity of the host galaxy around the SN region is 12+log(O/H) = 8.56$\pm$0.18 dex which is similar to that of SN 2012Z.

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arXiv2021

Intermediate Luminosity Type Iax SN 2019muj With Narrow Absorption Lines: Long-Lasting Radiation Associated With a Possible Bound Remnant Predicted by the Weak Deflagration Model

We present comprehensive spectroscopic and photometric analyses of the intermediate luminosity Type Iax supernova (SN Iax) 2019muj based on multi-band datasets observed through the framework of the OISTER target-of-opportunity program. SN 2019muj exhibits almost identical characteristics with the subluminous SNe Iax 2008ha and 2010ae in terms of the observed spectral features and the light curve evolution at the early phase, except for the peak luminosity. The long-term observations unveil the flattening light curves at the late time as seen in a luminous SN Iax 2014dt. This can be explained by the existence of an inner dense and optically-thick component possibly associated with a bound white dwarf remnant left behind the explosion. We demonstrate that the weak deflagration model with a wide range of the explosion parameters can reproduce the late-phase light curves of other SNe Iax. Therefore, we conclude that a common explosion mechanism operates for different subclass SNe Iax.

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arXiv2021

Characterizing the Manx Candidate A/2018 V3

Manx objects approach the inner solar system on long-period comet (LPC) orbits with the consequent high inbound velocities, but unlike comets, Manxes display very little to no activity even near perihelion. This suggests that they may have formed in circumstances different from typical LPCs; moreover, this lack of significant activity also renders them difficult to detect at large distances. Thus, analyzing their physical properties can help constrain models of solar system formation as well as sharpen detection methods for those classified as NEOs. Here, we focus on the Manx candidate A/2018 V3 as part of a larger effort to characterize Manxes as a whole. This particular object was observed to be inactive even at its perihelion at $q$ = 1.34 au in 2019 September. Its spectral reflectivity is consistent with typical organic-rich comet surfaces with colors of $g'-r'= 0.67\pm0.02$, $r'-i' = 0.26\pm0.02$, and $r'-z' = 0.45\pm0.02$, corresponding to a spectral reflectivity slope of $10.6\pm 0.9$ %/100nm. A least-squares fit of our constructed light curve to the observational data yields an average nucleus radius of $\approx$2 km assuming an albedo of 0.04. This is consistent with the value measured from NEOWISE. A surface brightness analysis for data taken 2020 July 13 indicated possible low activity ($\lesssim0.68$ g $\rm s^{-1}$), but not enough to lift optically significant amounts of dust. Finally, we discuss Manxes as a constraint on solar system dynamical models as well as their implications for planetary defense.

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arXiv2019

Disintegration of Active Asteroid P/2016 G1 (PANSTARRS)

We report on the catastrophic disintegration of P/2016 G1 (PANSTARRS), an active asteroid, in April 2016. Deep images over three months show that object is constituted by a central concentration of fragments surrounded by an elongated coma, and presents previously unreported sharp arc-like and a narrow linear features. The morphology and evolution of these characteristics independently point toward a brief event on 2016 March 6. The arc and the linear feature can be reproduced by large particles on a ring, moving at ~2.5 m/s. The expansion of the ring defines a cone with a ~40deg half-opening. We propose that G1 was hit by a small object which caused its (partial or total) disruption, and that the ring corresponds to large fragments ejected during the final stages of the crater formation.}

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arXiv2019

The Sporadic Activity of (6478) Gault: A YORP-driven event?

On 2019 January 5 a streamer associated with the 4--10 km main-belt asteroid (6478)~Gault was detected by the ATLAS sky survey, a rare discovery of activity around a main-belt asteroid. Archival data from ATLAS and Pan-STARRS1 show the trail in early December 2018, but not between 2010 and January 2018. The feature has significantly changed over one month, perfectly matching predictions of pure dust dynamical evolution and changes in observing geometry for a short release of dust around 2018 October 28. Follow-up observations with HST show a second narrow trail corresponding to a brief release of dust on 2018 December 30. Both releases occurred with negligible velocity. We find the dust grains to be fairly large, with power-law size distributions in the $10^{-5} - 10^{-3}$~m range and power-law indices of $\sim -1.5$. Three runs of ground-based data find a signature of $\sim 2\,\rm h$ rotation, close to the rotational limit, suggesting that the activity is the result of landslides or reconfigurations after YORP spin-up.

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arXiv2016

Temperate Earth-sized planets transiting a nearby ultracool dwarf star

Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as ultracool dwarfs. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disk, there should be a large but hitherto undetected population of terrestrial planets orbiting them - ranging from metal-rich Mercury-sized planets to more hospitable volatile-rich Earth-sized planets. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system.

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arXiv2014

iPTF13bvn: The First Evidence of a Binary Progenitor for a Type Ib Supernova

The recent detection in archival HST images of an object at the the location of supernova (SN) iPTF13bvn may represent the first direct evidence of the progenitor of a Type Ib SN. The object's photometry was found to be compatible with a Wolf-Rayet pre-SN star mass of ~11 Msun. However, based on hydrodynamical models we show that the progenitor had a pre-SN mass of ~3.5 Msun and that it could not be larger than ~8 Msun. We propose an interacting binary system as the SN progenitor and perform evolutionary calculations that are able to self-consistently explain the light-curve shape, the absence of hydrogen, and the pre-SN photometry. We further discuss the range of allowed binary systems and predict that the remaining companion is a luminous O-type star of significantly lower flux in the optical than the pre-SN object. A future detection of such star may be possible and would provide the first robust identification of a progenitor system for a Type Ib SN.

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