arXiv subjects

D. K. Sahu

Publications and source records attributed to D. K. Sahu.

At least 37 records · Page 2Linked to original sources

arXiv2023

Far-Ultraviolet to Near-Infrared Observations of SN 2023ixf: A high energy explosion engulfed in complex circumstellar material

We present early-phase panchromatic photometric and spectroscopic coverage spanning far-ultraviolet (FUV) to the near-infrared (NIR) regime of the nearest hydrogen-rich core-collapse supernova in the last 25 years, SN 2023ixf. We observe early 'flash' features in the optical spectra due to a confined dense circumstellar material (CSM). We observe high-ionization absorption lines (FeII, MgII) in the ultraviolet spectra from very early on. We also observe a multi-peaked emission profile of H-alpha in the spectrum beginning ~16 d, which indicates ongoing interaction of the SN ejecta with a pre-existing shell-shaped CSM having an inner radius of ~75 AU and an outer radius of ~140 AU. The shell-shaped CSM is likely a result of enhanced mass loss ~35-65 years before the explosion assuming a standard Red-Supergiant wind. The UV spectra are dominated by multiple highly ionized narrow absorption features and broad emission features from elements such as C, N, O, Si, Fe, and Ni. Based on early light curve models of Type II SNe, we infer that the nearby dense CSM confined to (7+-3)e14cm (~45 AU) is a result of enhanced mass loss (10^{-3.0+-0.5} Msol/yr) two decades before the explosion.

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arXiv2023

SN 2018gj: A Short-plateau Type II Supernova with Persistent Blue-shifted H-alpha Emission

We present an extensive, panchromatic photometric (UV, Optical, and NIR) and low-resolution optical spectroscopic coverage of a Type IIP supernova SN 2018gj that occurred on the outskirts of the host galaxy NGC 6217. From the V-band light curve, we estimate the plateau length to be ~ 70 +- 2 d, placing it among the very few well-sampled short plateau supernovae (SNe). With V-band peak absolute magnitude Mv < -17.0 +- 0.1 mag, it falls in the middle of the luminosity distribution of the Type II SNe. The colour evolution is typical to other Type II SNe except for an early elbow-like feature in the evolution of V-R colour owing to its early transition from the plateau to the nebular phase. Using the expanding photospheric method, we present an independent estimate of the distance to SN 2018gj. We report the spectral evolution to be typical of a Type II SNe. However, we see a persistent blue shift in emission lines until the late nebular phase, not ordinarily observed in Type II SNe. The amount of radioactive nickel (56Ni) yield in the explosion was estimated to be 0.026 +- 0.007 Msol. We infer from semi-analytical modelling, nebular spectrum, and 1-D hydrodynamical modelling that the probable progenitor was a red supergiant with a zero-age-main-sequence mass < 13 Msol. In the simulated hydrodynamical model light curves, reproducing the early optical bolometric light curve required an additional radiation source, which could be the interaction with the proximal circumstellar matter (CSM).

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arXiv2023

Post-outburst evolution of bonafide FUor V2493 Cyg: A Spectro-photometric monitoring

We present here the results of eight years of our near-simultaneous optical/near-infrared spectro-photometric monitoring of bonafide FUor candidate `V2493 Cyg' starting from 2013 September to 2021 June. During our optical monitoring period (between October 16, 2015 and December 30, 2019), the V2493 Cyg is slowly dimming with an average dimming rate of $\sim$26.6 $\pm$ 5.6 mmag/yr in V band. Our optical photometric colors show a significant reddening of the source post the second outburst pointing towards a gradual expansion of the emitting region post the second outburst. The mid infra-red colors, on the contrary, exhibits a blueing trend which can be attributed to the brightening of the disc due to the outburst. Our spectroscopic monitoring shows a dramatic variation of the H$α$ line as it transitioned from absorption feature to the emission feature and back. Such transition can possibly be explained by the variation in the wind structure in combination with accretion. Combining our time evolution spectra of the Ca II infra-red triplet lines with the previously published spectra of V2493 Cyg, we find that the accretion region has stabilised compared to the early days of the outburst. The evolution of the O I $λ$7773 Å~ line also points towards the stabilization of the circumstellar disc post the second outburst.

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arXiv2023

Unravelling optical and X-ray properties of the disc-dominated intermediate polar IGR J15094-6649

We present analyses of an Intermediate Polar, IGR J15094-6649, based on the archival optical data obtained from the Transiting Exoplanet Survey Satellite (TESS) and X-ray data obtained from the Suzaku, NuSTAR, and Neil Gehrels Swift Observatory (Swift). Present analysis confirms and refines the previously reported spin period of IGR J15094-6649 as 809.49584$\pm$0.00075 s. Clear evidence of a beat period of 841.67376$\pm$0.00082 s is found during the long-term TESS optical observations, which was not evident in the earlier studies. The dominance of X-ray and optical spin pulse unveils the disc-fed dominance accretion, however, the presence of an additional beat frequency indicates that part of the accreting material also flows along the magnetic field lines. The energy-dependent spin pulsations in the low (< 10 keV) energy band are due to the photoelectric absorption in the accretion flow. However, the complex absorbers may be responsible to produce low amplitude spin modulations via Compton scattering in the hard ( > 10 keV) energy band and indicate that the height of the X-ray emitting region may be negligible. The observed double-humped X-ray profiles with a pronounced dip are indicative of the photoelectric absorption in the intervening accretion stream. Analysis of the X-ray spectra reveals the complexity of the X-ray emission, being composed of multi-temperature plasma components with a soft excess, reflection, and suffers from strong absorption.

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arXiv2023

Type Ia supernovae SN 2013bz, PSN J0910+5003 and ASASSN-16ex: similar to 09dc-like?

We present optical photometric and spectroscopic studies of three supernovae (SNe) SN 2013bz, PSN J0910+5003 and ASASSN-16ex. UV-optical photometric data of ASASSN-16ex obtained with Swift-UVOT are also analyzed. These objects were initially classified as 09dc-like type Ia SNe. The decline rate parameters ($Δm_{15}(B)_{true}$) are derived as 0.92 $\pm$ 0.04 (SN 2013bz), 0.70 $\pm$ 0.05 (PSN J0910+5003) and 0.73 $\pm$ 0.03 (ASASSN-16ex). The estimated $B$ band absolute magnitudes at maximum: $-$19.61 $\pm$ 0.20 mag for SN 2013bz, $-$19.44 $\pm$ 0.20 mag for PSN J0910+5003 and $-$19.78 $\pm$ 0.20 mag for ASASSN-16ex indicate that all the three objects are relatively bright. The peak bolometric luminosities for these objects are derived as $\log L_\text{bol}^\text{max} =$ 43.38 $\pm$ 0.07 erg s$^{-1}$, 43.26 $\pm$ 0.07 erg s$^{-1}$ and 43.40 $\pm$ 0.06 erg s$^{-1}$, respectively. The spectral and velocity evolution of SN 2013bz is similar to a normal SN Ia, hence it appears to be a luminous, normal type Ia supernova. On the other hand, the light curves of PSN J0910+5003 and ASASSN-16ex are broad and exhibit properties similar to 09dc-like SNe Ia. Their spectroscopic evolution shows similarity with 09dc-like SNe, strong CII lines are seen in the pre-maximum spectra of these two events. Their photospheric velocity evolution is similar to SN 2006gz. Further, in the UV bands, ASASSN-16ex is very blue like other 09dc-like SNe Ia.

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arXiv2022

Revisiting the Transit Timing Variations in the TrES-3 and Qatar-1 systems with TESS data

We present and analyze 58 transit light curves of TrES-3b and 98 transit light curves of Qatar-1b observed by Transiting Exoplanet Survey Satellite (TESS), plus two transit light curves of Qatar-1b observed by us using a ground-based 1.23\,m telescope. These light curves are combined with the best-quality light curves taken from the Exoplanet Transit Database (ETD) and literature. The precisely determined mid-transit times from these light curves enable us to obtain the refined orbital ephemerides with improved precision for both hot Jupiters. From the timing analysis, we find an indication for the presence of transit timing variations (TTVs) in both systems. Since the observed TTVs are unlikely to be short-term and periodic, the possibility of additional planets in the orbits close to TrES-3b and Qatar-1b are ruled out. Possible causes of long-term TTVs such as orbital decay, apsidal precession, the Applegate mechanism and line-of-sight acceleration are also examined. However, none of these possibilities are found to explain the observed TTV of TrES-3b. In contrast to this, the line-of-sight acceleration appears to be a plausible explanation for the observed TTV of Qatar-1b. In order to confirm these findings, further high-precision transit and RV observations of both systems would be worthwhile.

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arXiv2022

MUSSES2020J: The Earliest Discovery of a Fast Blue Ultraluminous Transient at Redshift 1.063

In this Letter, we report the discovery of an ultraluminous fast-evolving transient in rest-frame UV wavelengths, MUSSES2020J, soon after its occurrence by using the Hyper Suprime-Cam (HSC) mounted on the 8.2 m Subaru telescope. The rise time of about 5 days with an extremely high UV peak luminosity shares similarities to a handful of fast blue optical transients whose peak luminosities are comparable with the most luminous supernovae while their timescales are significantly shorter (hereafter "fast blue ultraluminous transient," FBUT). In addition, MUSSES2020J is located near the center of a normal low-mass galaxy at a redshift of 1.063, suggesting a possible connection between the energy source of MUSSES2020J and the central part of the host galaxy. Possible physical mechanisms powering this extreme transient such as a wind-driven tidal disruption event and an interaction between supernova and circumstellar material are qualitatively discussed based on the first multiband early-phase light curve of FBUTs, although whether the scenarios can quantitatively explain the early photometric behavior of MUSSES2020J requires systematical theoretical investigations. Thanks to the ultrahigh luminosity in UV and blue optical wavelengths of these extreme transients, a promising number of FBUTs from the local to the high-z universe can be discovered through deep wide-field optical surveys in the near future.

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arXiv2022

Host galaxy magnitude of OJ 287 from its colours at minimum light

OJ 287 is a BL Lacertae type quasar in which the active galactic nucleus (AGN) outshines the host galaxy by an order of magnitude. The only exception to this may be at minimum light when the AGN activity is so low that the host galaxy may make quite a considerable contribution to the photometric intensity of the source. Such a dip or a fade in the intensity of OJ 287 occurred in November 2017, when its brightness was about 1.75 magnitudes lower than the recent mean level. We compare the observations of this fade with similar fades in OJ 287 observed earlier in 1989, 1999, and 2010. It appears that there is a relatively strong reddening of the B$-$V colours of OJ 287 when its V-band brightness drops below magnitude 17. Similar changes are also seen V$-$R, V$-$I, and R$-$I colours during these deep fades. These data support the conclusion that the total magnitude of the host galaxy is $V=18.0 \pm 0.3$, corresponding to $M_{K}=-26.5 \pm 0.3$ in the K-band. This is in agreement with the results, obtained using the integrated surface brightness method, from recent surface photometry of the host. These results should encourage us to use the colour separation method also in other host galaxies with strongly variable AGN nuclei. In the case of OJ 287, both the host galaxy and its central black hole are among the biggest known, and its position in the black hole mass-galaxy mass diagram lies close to the mean correlation.

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arXiv2022

SN 2020jfo: A short plateau Type II supernova from a low mass progenitor

We present spectroscopic and photometric observations of the Type IIP supernova, SN 2020jfo, in ultraviolet and optical wavelengths. SN 2020jfo occurred in the spiral galaxy M61 (NGC 4303), with eight observed supernovae in the past 100 years. SN 2020jfo exhibited a short plateau lasting < 65 d, and achieved a maximum brightness in V-band of $M_V$ = -17.4 $\pm$ 0.4 mag at about 8.0 $\pm$ 0.5 d since explosion. From the bolometric light curve, we have estimated the mass of $^{56}$Ni synthesised in the explosion to be 0.033 $\pm$ 0.006 $M_\odot$. The observed spectral features are typical for a type IIP supernova except for shallow H$α$ absorption throughout the evolution and the presence of stable $^{58}$Ni feature at 7378 Å, in the nebular phase. Using hydrodynamical modelling in the MESA + STELLA framework, an ejecta mass of ~ 5 $M_\odot$ is estimated. Models also indicate SN 2020jfo could be the result of a Red Super Giant progenitor with $M_{ZAMS}$ ~ 12 $M_\odot$. Bolometric light curve modelling revealed the presence of a secondary radiation source for initial ~ 20 d, which has been attributed to interaction with a circumstellar material of mass ~ 0.2 $M_\odot$, which most likely was ejected due to enhanced mass loss about 20 years prior to the supernova explosion.

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arXiv2022

Investigating the observational properties of Type Ib supernova SN 2017iro

We report results of optical imaging and low-resolution spectroscopic monitoring of supernova (SN) 2017iro that occurred in the nearby ($\sim$\,31 Mpc) galaxy NGC 5480. The \ion{He}{1} 5876 Å\, feature present in the earliest spectrum (--\,7 d) classified it as a Type Ib SN. The follow-up observations span from --\,7 to +\,266 d with respect to the $B$-band maximum. With a peak absolute magnitude in $V$-band, ($M_{V}$)\,=\,$-17.76\pm0.15$ mag and bolometric luminosity (log$_{10}$\,L)\,=\,42.39\,$\pm$\, 0.09 erg s$^{-1}$, SN 2017iro is a moderately luminous Type Ib SN. The overall light curve evolution of SN 2017iro is similar to SN 2012au and SN 2009jf during the early (up to $\sim$100 d) and late phases ($>$150 d), respectively. The line velocities of both \ion{Fe}{2} 5169 Å\, and \ion{He}{1} 5876 Å\, are $\sim$\,9000 km s$^{-1}$ near the peak. The analysis of the nebular phase spectrum ($\sim$\,+209 d) indicates an oxygen mass of $\sim$\,0.35 M$_{\odot}$. The smaller [\ion{O}{1}]/[\ion{Ca}{2}] flux ratio of $\sim$\,1 favours a progenitor with a zero-age main-sequence mass in the range $\sim$\,13--15 M$_{\odot}$, most likely in a binary system, similar to the case of iPTF13bvn. The explosion parameters are estimated by applying different analytical models to the quasi-bolometric light curve of SN 2017iro. $^{56}$Ni mass synthesized in the explosion has a range of $\sim$\,0.05\,--\,0.10 M$_{\odot}$, the ejecta mass $\sim$1.4\,--\,4.3 M$_{\odot}$ and the kinetic energy $\sim$\,0.8\,--\,1.9\,$\times$ 10$^{51}$ erg.

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arXiv2021

The Transit Timing and Atmosphere of Hot Jupiter HAT-P-37b

The transit timing variation (TTV) and transmission spectroscopy analyses of the planet HAT-P-37b, which is a hot Jupiter orbiting an G-type star, were performed. Nine new transit light curves are obtained and analysed together with 21 published light curves from the literature. The updated physical parameters of HAT-P-37b are presented. The TTV analyses show a possibility that the system has an additional planet which induced the TTVs amplitude signal of 1.74 $\pm$ 0.17 minutes. If the body is located near the 1:2 mean motion resonance orbit, the sinusoidal TTV signal could be caused by the gravitational interaction of a sub-Earth mass planet with mass of 0.06 $M_\oplus$. From the analysis of an upper mass limit for the second planet, the Saturn mass planet with orbital period less than 6 days is excluded. The broad-band transmission spectra of HAT-P-37b favours a cloudy atmospheric model with an outlier spectrum in $B$-filter.

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arXiv2021

A Comprehensive Study of the Young Cluster IRAS 05100+3723: Properties, Surrounding Interstellar Matter, and Associated Star Formation

We present a comprehensive multiwavelength investigation of a likely massive young cluster `IRAS 05100+3723' and its environment with the aim to understand its formation history and feedback effects. We find that IRAS 05100+3723 is a distant ($\sim$3.2 kpc), moderate mass ($\sim$500 \msun), young ($\sim$3 Myr) cluster with its most massive star being an O8.5V-type. From spectral modeling, we estimate the effective temperature and log $g$ of the star as $\sim$33,000 K and $\sim$3.8, respectively. Our radio continuum observations reveal that the star has ionized its environment forming an HII region of size $\sim$2.7 pc, temperature $\sim$5,700 K, and electron density $\sim$165 cm$^{-3}$. However, our large-scale dust maps reveal that it has heated the dust up to several parsecs ($\sim$10 pc) in the range 17$-$28 K and the morphology of warm dust emission resembles a bipolar HII region. From dust and $^{13}$CO gas analyses, we find evidences that the formation of the HII region has occurred at the very end of a long filamentary cloud around 3 Myr ago, likely due to edge collapse of the filament. We show that the HII region is currently compressing a clump of mass $\sim$2700 \msun at its western outskirts, at the junction of the HII region and filament. We observe several 70 $μ$m point sources of intermediate-mass and class 0 nature within the clump. We attribute these sources as the second generation stars of the complex. We propose that the star formation in the clump is either induced or being facilitated by the compression of the expanding HII region onto the inflowing filamentary material.

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arXiv2021

SN 2020sck: deflagration in a carbon-oxygen white dwarf

We present optical UBVRI photometry and low-to-medium resolution spectroscopic observations of type Iax SN 2020sck spanning -5.5 d to +67 d from maximum light in the B-band. From the photometric analysis we find $Δm_{\rm{B}}$(15) = 2.03$\pm$0.05 mag and $M_{\rm{B}}$=-17.81$\pm$0.22 mag. Radiation diffusion model fit to the quasi-bolometric light curve indicates 0.13$\pm$0.02 $M_{\odot}$ of $^{56}$Ni and 0.34 $M_{\rm \odot}$ of ejecta are synthesized in the explosion. Comparing the observed quasi-bolometric light curve with angle-averaged bolometric light curve of three-dimensional pure deflagration explosion of $M_{\rm{ch}}$ carbon-oxygen white dwarf, we find agreement with a model in which 0.16 $M_{\odot}$ of $^{56}$Ni and 0.37 $M_{\odot}$ of ejecta is formed. By comparing the +1.4 day spectrum of SN 2020sck with synthetic spectrum generated using SYN++, we find absorption features due to C II, C III and O I. These are unburned materials in the explosion and indicate a C-O white dwarf. One dimensional radiative transfer modeling of the spectra with TARDIS shows higher density in the ejecta near the photosphere and a steep decrease in the outer layers with an ejecta composition dominated mostly by C, O, Si, Fe, and Ni. The star formation rate of the host galaxy computed from the luminosity of the H$α$ ($λ$6563) line is 0.09 $M_{\odot}$ yr$^{-1}$ indicating a relatively young stellar environment.

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arXiv2021

Photometric, polarimetric, and spectroscopic studies of the luminous, slow-decaying Type Ib SN 2012au

Optical, near-infrared (NIR) photometric and spectroscopic studies, along with the optical imaging polarimetric results for SN 2012au, are presented in this article to constrain the nature of the progenitor and other properties. Well-calibrated multiband optical photometric data (from $-$0.2 to +413 d since $B$-band maximum) were used to compute the bolometric light curve and to perform semi-analytical light-curve modelling using the $\texttt{MINIM}$ code. A spin-down millisecond magnetar-powered model explains the observed photometric evolution of SN 2012au reasonably. Early-time imaging polarimetric follow-up observations ($-$2 to +31 d) and comparison with other similar cases indicate signatures of asphericity in the ejecta. Good spectral coverage of SN 2012au (from $-$5 to +391 d) allows us to trace the evolution of layers of SN ejecta in detail. SN 2012au exhibits higher line velocities in comparison with other SNe Ib. Late nebular phase spectra of SN 2012au indicate a Wolf$-$Rayet star as the possible progenitor for SN 2012au, with oxygen, He-core, and main-sequence masses of $\sim$1.62 $\pm$ 0.15 M$_\odot$, $\sim$4$-$8 M$_\odot$, and $\sim$17$-$25 M$_\odot$, respectively. There is a clear absence of a first overtone of carbon monoxide (CO) features up to +319 d in the $K$-band region of the NIR spectra. Overall analysis suggests that SN 2012au is one of the most luminous slow-decaying Type Ib SNe, having comparatively higher ejecta mass ($\sim$4.7$-$8.3 M$_\odot$) and kinetic energy ($\sim$[4.8 $-$ 5.4] $\times$ 10$^{51}$ erg). Detailed modelling using $\texttt{MESA}$ and the results obtained through $\texttt{STELLA}$ and $\texttt{SNEC}$ explosions also strongly support spin-down of a magnetar with mass of around 20 M$_\odot$ and metallicity Z = 0.04 as a possible powering source of SN 2012au.

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arXiv2021

The optical properties of three type II supernovae: 2014cx, 2014cy and 2015cz

We present the photometric and spectroscopic analysis of three Type II SNe: 2014cx, 2014cy and 2015cz. SN 2014cx is a conventional Type IIP with a shallow slope (0.2 mag/50d) and an atypical short plateau ($\sim$86 d). SNe 2014cy and 2015cz show relatively large decline rates (0.88 and 1.64 mag/50d, respectively) at early times before settling to the plateau phase, unlike the canonical Type IIP/L SN light curves. All of them are normal luminosity SN II with an absolute magnitude at mid-plateau of M$_{V,14cx}^{50}$=$-$16.6$\pm$0.4$\,\rm{mag}$, M$_{V,14cy}^{50}$=$-$16.5$\,\pm\,$0.2$\,\rm{mag}$ and M$_{V,15cz}^{50}$=$-$17.4$\,\pm\,$0.3$\,\rm{mag}$. A relatively broad range of $^{56}$Ni masses is ejected in these explosions (0.027-0.070 M$_\odot$). The spectra show the classical evolution of Type II SNe, dominated by a blue continuum with broad H lines at early phases and narrower metal lines with P Cygni profiles during the plateau. High-velocity H I features are identified in the plateau spectra of SN 2014cx at 11600 km s$^{-1}$, possibly a sign of ejecta-circumstellar interaction. The spectra of SN 2014cy exhibit strong absorption profile of H I similar to normal luminosity events whereas strong metal lines akin to sub-luminous SNe. The analytical modelling of the bolometric light curve of the three events yields similar radii for the three objects within errors (478, 507 and 608 R$_\odot$ for SNe 2014cx, 2014cy and 2015cz, respectively) and a range of ejecta masses (15.0, 22.2 and 18.7 M$_\odot$ for SNe 2014cx, 2014cy and 2015cz), and a modest range of explosion energies (3.3 - 6.0 foe where 1 foe = 10$^{51}$ erg).

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arXiv2021

SN 2017hpa: A carbon-rich type Ia supernova

We present the optical (UBVRI) and ultraviolet (Swift-UVOT) photometry, and optical spectroscopy of Type Ia supernova SN 2017hpa. We study broadband UV+optical light curves and low resolution spectroscopy spanning from $-13.8$ to $+108$~d from the maximum light in $B$-band. The photometric analysis indicates that SN 2017hpa is a normal type Ia with $Δm_{B}(15) = 0.98\pm0.16$ mag and $M_{B}=-19.45\pm0.15$ mag at a distance modulus of $μ= 34.08\pm0.09$ mag. The $(uvw1-uvv)$ colour evolution shows that SN 2017hpa falls in the NUV-blue group. The $(B-V)$ colour at maximum is bluer in comparison to normal type Ia supernovae. Spectroscopic analysis shows that the Si II 6355 absorption feature evolves rapidly with a velocity gradient, $\dot{v}=128\pm 7$ km s$^{-1}$ d$^{-1}$. The pre-maximum phase spectra show prominent C II 6580 Å absorption feature. The C II 6580 Å line velocity measured from the observed spectra is lower than the velocity of Si II 6355 Å, which could be due to a line of sight effect. The synthetic spectral fits to the pre-maximum spectra using syn++ indicate the presence of a high velocity component in the Si II absorption, in addition to a photospheric component. Fitting the observed spectrum with the spectral synthesis code TARDIS, the mass of unburned C in the ejecta is estimated to be $\sim 0.019$~$M_{\odot}$. The peak bolometric luminosity is $L^{bol}_{peak} = 1.43\times10^{43}$ erg s$^{-1}$. The radiation diffusion model fit to the bolometric light curve indicates $0.61\pm0.02$ $M_\odot$ of $^{56}$Ni is synthesized in the explosion.

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arXiv2021

SN 2020ank: a bright and fast-evolving H-deficient superluminous supernova

We investigate the observational properties of a hydrogen-deficient superluminous supernova (SLSN) SN 2020ank (at z = 0.2485), with the help of early phase observations carried out between $-$21 and +52 d since $g$-band maximum. Photometrically, SN 2020ank is one of the brightest SLSN ($M_{g,peak}$ $\sim$ $-$21.84 $\pm$ 0.10 mag), having fast pre-peak rising and post-peak decaying rates. The bolometric light curve of SN 2020ank exhibits a higher peak luminosity ($L_{max}$) of $\sim$(3.9 $\pm$ 0.7) $\times$ 10$^{44}$ erg s$^{-1}$ and appears to be symmetric around the peak with $L^{rise}_{max}$/e $\approx$ $L^{fall}_{max}$/e $\approx$ 15 d. The semi-analytical light-curve modelling using the MINIM code suggests a spin down millisecond magnetar with $P_i$ $\sim$2.2 $\pm$ 0.5 ms and $B$ $\sim$(2.9 $\pm$ 0.1) $\times$ $10^{14}$ G as a possible powering source for SN 2020ank. The possible magnetar origin and excess ultraviolet flux at early epochs indicate a central-engine based powering source for SN 2020ank. Near-peak spectra of SN 2020ank are enriched with the W-shaped O II features but with the weaker signatures of C II and Fe III. Using the estimated rise time of $\sim$27.9 d and the photospheric velocity of $\sim$12050 km s$^{-1}$, we constrain the ejecta mass to $\sim$7.2 $M_{\odot}$ and the kinetic energy of $\sim$6.3 $\times$ 10$^{51}$ erg. The near-peak spectrum of SN 2020ank exhibits a close spectral resemblance with that of fast-evolving SN 2010gx. The absorption features of SN 2020ank are blueshifted compared to Gaia16apd, suggesting a higher expansion velocity. The spectral similarity with SN 2010gx and comparatively faster spectral evolution than PTF12dam (a slow-evolving SLSN) indicate the fast-evolving behavior of SN 2020ank.

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arXiv2020

Are There Transit Timing Variations for the Exoplanet Qatar-1b ?

Motivated by the unsettled conclusion on whether there are any transit timing variations (TTVs) for the exoplanet Qatar-1b, 10 new transit light curves are presented and the TTV analysis with a baseline of 1400 epochs are performed. Because the linear model provides a good fitting with reduced chi-square = 2.59 and the false-alarm probabilities of possible TTV frequencies are as large as 35 %, our results are consistent with a null-TTV model. Nevertheless, a new ephemeris with the reference time T_0 = 2455647.63360\pm 0.00008 (BJD) and the period P= 1.4200236\pm 0.0000001 (day) is obtained. In addition, the updated orbital semi-major axis and planetary radius in unit of stellar radius are being provided, and the lower limit of modified stellar tidal quality factor is also determined.

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