arXiv subjects

Avinash Singh

Publications and source records attributed to Avinash Singh.

At least 91 records · Page 5Linked to original sources

arXiv2020

Role of orbital off-diagonal spin and charge condensates in a three orbital model for $\rm Ca_2RuO_4$ -- Coulomb renormalized spin-orbit coupling, orbital moment, and tunable magnetic order

Strongly anisotropic spin-orbit coupling (SOC) renormalization and strongly enhanced orbital magnetic moments are obtained in the fully self consistent approach including the orbital off-diagonal spin and charge condensates. For moderate tetragonal distortion as in $\rm Ca_2 RuO_4$, dominantly planar antiferromagnetic (AFM) order with small canting of moments in and about the crystal $c$ axis are obtained. For reduced tetragonal distortion, we find a tunable regime wherein the magnetic order can be tuned (AFM or FM) by the bare SOC strength and octahedral tilting magnitude. In this regime, with decreasing tetragonal distortion, AFM order is maintained by progressively decreasing octahedral tilting, as observed in $\rm Ca_{2-x}Sr_x RuO_4$. For purely planar order, the only self consistent solution is FM order along crystal $b$ axis, which is relevant for the bilayer ruthenate compound $\rm Ca_3 Ru_2 O_7$.

cond-mat.str-el↗

arXiv2020

Optical studies of two stripped envelope supernovae SN 2015ap (Type Ib) and SN 2016P (Type Ic)

We present the photometric and spectroscopic studies of a Type Ib SN 2015ap and a Type Ic SN 2016P. SN 2015ap is one of the bright (M$_{V}$ = $-$18.04 mag) Type Ib while SN 2016P lies at an average value among the Type Ic SNe (M$_{V}$ = $-$17.53 mag). Bolometric light curve modelling of SNe 2015ap and 2016P indicates that both the SNe are powered by $^{56}$Ni + magnetar model with $^{56}$Ni masses of 0.01 M$_{\odot}$ and 0.002 M$_{\odot}$, ejecta masses of 3.75 M$_{\odot}$ and 4.66 M$_{\odot}$, spin period P$_{0}$ of 25.8 ms and 36.5 ms and magnetic field B$_{p}$ of 28.39 $\times$ 10$^{14}$ Gauss and 35.3 $\times$ 10$^{14}$ Gauss respectively. The early spectra of SN 2015ap shows prominent lines of He with a "W" feature due to Fe complexes while other lines of Mg II, Na I and Si II are present in both SNe 2015ap and 2016P. Nebular phase [O I] profile indicates an asymmetric profile in SN 2015ap. The [O I]/[Ca II] ratio and nebular spectral modelling of SN 2015ap hints towards a progenitor mass between 12 $-$ 20 M$_{\odot}$.

astro-ph.HE↗

arXiv2020

Kilonova Luminosity Function Constraints based on Zwicky Transient Facility Searches for 13 Neutron Star Mergers

We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo's third observing run. We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook follow-up with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization of 4480 deg^2, median distance of 267 Mpc and false alarm rates ranging from 1.5 to 1e-25 per yr. The ZTF coverage had a median enclosed probability of 39%, median depth of 20.8mag, and median response time of 1.5 hr. The O3 follow-up by the GROWTH team comprised 340 UVOIR photometric points, 64 OIR spectra, and 3 radio. We find no promising kilonova (radioactivity-powered counterpart) and we convert the upper limits to constrain the underlying kilonova luminosity function. Assuming that all kilonovae are at least as luminous as GW170817 at discovery (-16.1mag), we calculate our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than -16.6mag (extrapolated peak magnitude of GW170817) and fade at 1 mag/day (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations, the joint probability of zero detections, assuming all kilonovae are brighter than -16.6mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, <57% (<89%) of putative kilonovae could be brighter than -16.6mag assuming flat (fading) evolution, at 90% confidence. If we further account for the online terrestrial probability for each GW trigger, we find that <68% of putative kilonovae could be brighter than -16.6mag. Comparing to model grids, we find that some kilonovae must have Mej < 0.03 Msun or Xlan>1e-4 or phi>30deg to be consistent with our limits. (Abridged)

astro-ph.HE↗

arXiv2020

Perturbations in Tachyon Dark Energy and their Effect on Matter Clustering

A non-canonical scalar tachyon field is a viable candidate for dark energy and has been found to be in good agreement with observational data. Background data alone cannot completely rule out degeneracy between this model and others. To further constrain the parameters, apart from the distance measurements, we study perturbations in the tachyon scalar field and how they affect matter clustering. We consider two tachyon potentials for this study, an inverse square potential and an exponential potential. We study the evolution of the gravitational potential, matter density contrast, and dark energy density contrast, and compare them with the evolution in the $ΛCDM $ model. Although perturbations in dark energy at sub-Hubble scales are negligible in comparison with matter perturbations, they cannot be ignored at Hubble and super-Hubble scales ($λ_p > 1000$ Mpc). We also study the evolution of growth function and growth rate of matter, and find that the growth rate is significantly suppressed in dark energy dominated era with respect to the growth rate for $ΛCDM$ model. A comparison of these models with Redshift Space Distortion growth rate data is presented by way of calculating $fσ_8(z)$. There is a tension of $2.9σ$ ($2.26σ$ ) between growth rate data and Planck-2015 (Planck-2018) Cosmic Microwave Background Radiation data for $ΛCDM$ model. We present constraints on free parameters of these models and show that perturbations in the tachyon scalar field reduce this tension between different data sets.

astro-ph.CO↗

arXiv2020

Correlated motion of particle-hole excitations across the renormalized spin-orbit gap in $\rm Sr_2 Ir O_4$

The high-energy collective modes of particle-hole excitations across the spin-orbit gap in $\rm Sr_2IrO_4$ are investigated using the transformed Coulomb interaction terms in the pseudo-spin-orbital basis constituted by the $J=1/2$ and $3/2$ states arising from spin-orbit coupling. With appropriate interaction strengths and renormalized spin-orbit gap, these collective modes yield two well-defined propagating spin-orbit exciton modes, with energy scale and dispersion in excellent agreement with resonant inelastic X-ray scattering (RIXS) measurements.

cond-mat.str-el↗

arXiv2019

Flash ionization signatures in the type Ibn supernova SN 2019uo

We present photometric and spectroscopic observations of the type Ibn supernova (SN) 2019uo, the second ever SN Ibn with flash ionization (He II, C III, N III) features in its early spectra. SN 2019uo displays a rapid post-peak luminosity decline of 0.1 mag d$^{-1}$ similar to most of the SNe Ibn, but is fainter ($M^V_{max} = -18.30 \pm 0.24$ mag) than a typical SN Ibn and shows a color evolution that places it between SNe Ib and the most extreme SNe Ibn. SN 2019uo shows P-cygni He I features in the early spectra which gradually evolves and becomes emission dominated post peak. It also shows faster evolution in line velocities as compared to most other members of the type Ibn subclass. The bolometric light curve is fairly described by a $^{56}$Ni + circumstellar interaction model.

astro-ph.HE↗

arXiv2019

Magnetic order and anisotropic interactions induced by mixing between the $J=1/2$ and $3/2$ sectors in spin-orbit coupled honeycomb-lattice compounds

Novel magnetic ordering on the honeycomb lattice due to emergent weak anisotropic interactions generated by the mixing between the $J=1/2$ sector and the magnetically inactive 3/2 sector is investigated in a three-orbital interacting electron model in the absence of Hund's coupling. Self-consistent determination of magnetic order yields anisotropic Néel and zigzag orders for different parameter regimes, highlighting the effect of the emergent single-ion anisotropy. Study of magnon excitations shows extremely small magnon energy scale compared to the hopping energy scale, and enhancement of anisotropy effects for smaller spin-orbit coupling. These results account for several features of the honeycomb lattice compounds such as $\rm Na_2 Ir O_3$ and $\rm Ru Cl_3$, where the leading order anisotropic interactions within the magnetically active $J=1/2$ sector are completely quenched due to the edge-sharing octahedra.

cond-mat.str-el↗

arXiv2019

SN 2018hna: 1987A-like supernova with a signature of shock breakout

High cadence ultraviolet, optical and near-infrared photometric and low-resolution spectroscopic observations of the peculiar Type II supernova (SN) 2018hna are presented. The early phase multiband light curves exhibit the adiabatic cooling envelope emission following the shock breakout up to ~14 days from the explosion. SN~2018hna has a rise time of $\sim$\,88 days in the V-band, similar to SN 1987A. A $\rm^{56}Ni$ mass of ~0.087$\pm$0.004 $\rm M_{\odot}$ is inferred for SN 2018hna from its bolometric light curve. Hydrodynamical modelling of the cooling phase suggests a progenitor with a radius ~50 $\rm R_{\odot}$, a mass of ~14-20 $\rm M_{\odot}$ and explosion energy of ~1.7-2.9$\rm \times$ $\rm 10^{51}\ erg$. The smaller inferred radius of the progenitor than a standard red supergiant is indicative of a blue supergiant progenitor of SN 2018hna. A sub-solar metallicity (~0.3 $\rm Z_{\odot}$) is inferred for the host galaxy UGC 07534, concurrent with the low-metallicity environments of 1987A-like events.

astro-ph.HE↗

arXiv2019

Octahedral tilting induced isospin reorientation transition in iridate heterostructures

Iridate heterostructures are gaining interest as their magnetic properties are much more sensitive to structural distortion compared to pure spin systems due to spin-orbital entanglement induced by strong spin-orbit coupling. While bulk monolayer and bilayer iridates show $ab$-plane canted and $c$-axis antiferromagnetic (AFM) order, recent experiments on layered iridate superlattices (SL) have revealed striking properties, especially in the bilayer SL. A spin model is presented including the tilting induced Kitaev type interactions, which illustrates the proclivity towards $ab$-plane canted AFM order. A realistic Hubbard model including spin-dependent hopping terms arising from octahedral rotation and tilting is constructed for the bilayer SL in isospin space, and magnetic excitations are investigated in the self-consistently determined magnetic state. The Hubbard model analysis confirms the spin model results and shows strongly reduced magnon energy gap and an isospin reorientation transition from $c$-axis to $ab$-plane canted AFM order with increasing tilting.

cond-mat.str-el↗

arXiv2019

On the observational behaviour of the highly polarized Type IIn supernova SN 2017hcc

We present the results based on photometric ($Swift$ UVOT), broad-band polarimetric ($V$ and $R$-band) and optical spectroscopic observations of the Type IIn supernova (SN) 2017hcc. Our study is supplemented with spectropolarimetric data available in literature for this event. The post-peak light curve evolution is slow ($\sim$0.2 mag 100 d$^{-1}$ in $b$-band). The spectrum of $\sim$+27 d shows a blue continuum with narrow emission lines, typical of a Type IIn SN. Archival polarization data along with the $Gaia$ DR2 distances have been utilized to evaluate the interstellar polarization (ISP) towards the SN direction which is found to be $P_{ISP}$ = 0.17 $\pm$ 0.02 per cent and $θ_{ISP}$ = 140$^{\circ}$ $\pm$ 3$^{\circ}$. To extract the intrinsic polarization of SN 2017hcc, both the observed and the literature polarization measurements were corrected for ISP. We noticed a significant decline of $\sim$3.5 per cent ($V$-band) in the intrinsic level of polarization spanning a period of $\sim$2 months. In contrast, the intrinsic polarization angles remain nearly constant at all epochs. Our study indicates a substantial variation in the degree of asymmetry in either the ejecta and/or the surrounding medium of SN 2017hcc. We also estimate a mass-loss rate of $\dot M$ = 0.12 M$_{\odot}$ yr$^{-1}$ (for $v_w$ = 20 km s$^{-1}$) which suggests that the progenitor of SN 2017hcc is most likely a Luminous Blue Variable.

astro-ph.HE↗

arXiv2019

Observational signature of circumstellar interaction and $\rm ^{56}$Ni-mixing in the Type II Supernova 2016gfy

The optical and ultra-violet broadband photometric and spectroscopic observations of the Type II supernova (SN) 2016gfy are presented. The $V$-band light curve (LC) shows a distinct plateau phase with a slope, $s_2$ $\sim$ 0.12 mag / 100 d and a duration of 90 $\pm$ 5 d. Detailed analysis of SN 2016gfy provided a mean $\rm^{56}Ni$ mass of 0.033 $\pm$ 0.003 $\rm M_{\odot}$, a progenitor radius of $\sim$ 350-700 $\rm R_{\odot}$, a progenitor mass of $\sim$ 12-15 $\rm M_{\odot}$ and an explosion energy of 0.9-1.4$\rm \times 10^{51}\ erg\ s^{-1}$. The P-Cygni profile of H$\rm α$ in the early phase spectra ($\sim$ 11-21 d) shows a boxy emission. Assuming that this profile arises from the interaction of the SN ejecta with the pre-existing circumstellar material (CSM), it is inferred that the progenitor underwent a recent episode (30-80 years prior to the explosion) of enhanced mass loss. Numerical modeling suggests that the early LC peak is reproduced better with an existing CSM of 0.15 $\rm M_{\odot}$ spread out to $\sim$ 70 AU. A late-plateau bump is seen in the $VRI$ LCs during $\sim$ 50-95 d. This bump is explained as a result of the CSM interaction and/or partial mixing of radioactive $\rm ^{56}Ni$ in the SN ejecta. Using strong-line diagnostics, a sub-solar oxygen abundance is estimated for the supernova H II region (12 + log(O/H) = 8.50 $\pm$ 0.11), indicating an average metallicity for the host of a Type II SN. A star formation rate of $\sim$ 8.5 $\rm M_{\odot}\ yr^{-1}$ is estimated for NGC 2276 using the archival $GALEX$ FUV data.

astro-ph.HE↗

arXiv2019

SN 2017gmr: An energetic Type II-P supernova with asymmetries

We present high-cadence ultraviolet (UV), optical, and near-infrared (NIR) data on the luminous Type II-P supernova SN 2017gmr from hours after discovery through the first 180 days. SN 2017gmr does not show signs of narrow, high-ionization emission lines in the early optical spectra, yet the optical lightcurve evolution suggests that an extra energy source from circumstellar medium (CSM) interaction must be present for at least 2 days after explosion. Modeling of the early lightcurve indicates a ~500R$_{\odot}$ progenitor radius, consistent with a rather compact red supergiant, and late-time luminosities indicate up to 0.130 $\pm$ 0.026 M$_{\odot}$ of $^{56}$Ni are present, if the lightcurve is solely powered by radioactive decay, although the $^{56}$Ni mass may be lower if CSM interaction contributes to the post-plateau luminosity. Prominent multi-peaked emission lines of H$α$ and [O I] emerge after day 154, as a result of either an asymmetric explosion or asymmetries in the CSM. The lack of narrow lines within the first two days of explosion in the likely presence of CSM interaction may be an example of close, dense, asymmetric CSM that is quickly enveloped by the spherical supernova ejecta.

astro-ph.HE↗

arXiv2019

Low redshift observational constraints on tachyon models of dark energy

The background evolution of an accelerated, dark energy dominated universe is aptly described by non-canonical tachyon scalar field models. The accelerated expansion of the universe is determined by the choice of a suitable scalar field potential; in the case of a tachyon field, a 'runaway' potential. In the absence of a fundamental theory, dark energy properties are studied in a phenomenological approach. This includes determining the model parameters using observations and to probe the allowed deviation from the cosmological constant model. In this paper, we present constraints on tachyon scalar field parameters from low redshift data for two different scalar field potentials. These scalar field potentials have been crucial in tachyon dark energy studies. The datasets considered in this paper include the supernova type Ia data, independent measurements of the Hubble parameter and the Baryon Acoustic Oscillation data. In this paper, we present constraints on tachyon field parameters using these observations and their combination. A combination of the datasets indicates that those model parameters are preferred which emulate the cosmological constant model. The initial value of the scalar field, in the unit of the Hubble constant, is bounded from below and does not require fine-tuning at larger values.

gr-qc↗

arXiv2019

SN 2016B a.k.a ASASSN-16ab: a transitional type II supernova

We present photometry, polarimetry and spectroscopy of the Type II supernova ASASSN-16ab/SN 2016B in PGC 037392. The photometric and spectroscopic follow-up commenced about two weeks after shock breakout and continued until nearly six months. The light curve of SN 2016B exhibits intermediate properties between those of Type IIP and IIL. The early decline is steep (1.68 $\pm$ 0.10 mag 100 d$^{-1}$), followed by a shallower plateau phase (0.47 $\pm$ 0.24 mag 100 d$^{-1}$). The optically thick phase lasts for 118 d, similar to Type IIP. The $^{56}$Ni mass estimated from the radioactive tail of the bolometric light curve is 0.082 $\pm$ 0.019 M$_\odot$. High velocity component contributing to the absorption trough of H$α$ and H$β$ in the photospheric spectra are identified from the spectral modelling from about 57 - 97 d after the outburst, suggesting a possible SN ejecta and circumstellar material interaction. Such high velocity features are common in the spectra of Type IIL supernovae. By modelling the true bolometric light curve of SN 2016B, we estimated a total ejected mass of $\sim$15 M$_\odot$, kinetic energy of $\sim$1.4 foe and an initial radius of $\sim$400 R$_\odot$.

astro-ph.HE↗

arXiv2019

Intra- and inter-orbital correlated electron spin dynamics in $\rm Sr_2 Ir O_4$: spin-wave gap and spin-orbit exciton

Transformation of Coulomb interaction terms to the pseudo-orbital basis constituted by $J=1/2$ and $3/2$ states arising from spin-orbit coupling provides a versatile tool. This formalism is applied to investigate magnetic anisotropy effects on low-energy spin-wave excitations as well as high-energy spin-orbit exciton modes in $\rm Sr_2 Ir O_4$. The Hund's coupling term explictly yields easy-plane anisotropy, resulting in gapless (in-plane) and gapped (out-of-plane) modes, in agreement with recent resonant inelastic x-ray scattering (RIXS) measurements. The collective mode of inter-orbital, spin-flip, particle-hole excitations with appropriate interaction strengths and renormalized spin-orbit gap yields two well-defined propagating spin-orbit exciton modes, with energy scale and dispersion in good agreement with RIXS studies.

cond-mat.str-el↗

arXiv2018

The Fast, Luminous Ultraviolet Transient AT2018cow: Extreme Supernova, or Disruption of a Star by an Intermediate-Mass Black Hole?

Wide-field optical surveys have begun to uncover large samples of fast (t_rise < 5d), luminous (M_peak < -18), blue transients. While commonly attributed to the breakout of a supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered detailed investigation of their properties. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), the first fast-luminous optical transient to be found in real time at low redshift. Our first spectra (<2 days after discovery) are entirely featureless. A very broad absorption feature suggestive of near-relativistic velocities develops between 3-8 days, then disappears. Broad emission features of H and He develop after >10 days. The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R<10^14 cm after 1 month). This behaviour does not match that of any known supernova, although a relativistic jet within a fallback supernova could explain some of the observed features. Alternatively, the transient could originate from the disruption of a star by an intermediate-mass black hole, although this would require long-lasting emission of highly super-Eddington thermal radiation. In either case, AT2018cow suggests that the population of fast luminous transients represents a new class of astrophysical event. Intensive follow-up of this event in its late phases, and of any future events found at comparable distance, will be essential to better constrain their origins.

astro-ph.HE↗

arXiv2018

Spin waves and stability of zigzag order in the Hubbard model with spin-dependent hopping terms - Application to the honeycomb lattice compounds ${\rm Na_2 Ir O_3}$ and ${\rm α- Ru Cl_3}$

Spin waves in the zigzag ordered state on a honeycomb lattice are investigated within a Hubbard model with spin-dependent hopping terms. Roles of the emergent Kitaev, Heisenberg, Dzyaloshinskii-Moriya, and symmetric-off-diagonal spin interactions are investigated on the stability of the zigzag order. While the DM interactions are found to destabilize the zigzag order, the secondary spin-dependent hopping terms (associated with structural distortions) are shown to strongly stabilize the zigzag order and account for magnetocrystalline anisotropy, easy axis, and spin wave gap. The calculated spin wave dispersion and energy scale are in good agreement with inelastic neutron scattering measurements on $\rm α- RuCl_3$ and $\rm Na_2 Ir O_3$.

cond-mat.str-el↗

arXiv2018

ASASSN-14dq: A fast-declining type II-P Supernova in a low-luminosity host galaxy

Optical broadband (UBVRI) photometric and low-resolution spectroscopic observations of the type II-P supernova (SN) ASASSN-14dq are presented. ASASSN-14dq exploded in a low-luminosity/metallicity host galaxy UGC 11860, the signatures of which are present as weak iron lines in the photospheric phase spectra. The SN has a plateau duration of $\sim\,$90 d, with a plateau decline rate of 1.38 $\rm mag\ (100 d)^{-1}$ in V-band which is higher than most type II-P SNe. ASASSN-14dq is a luminous type II-P SN with a peak $V$-band absolute magnitude of -17.7$\,\pm\,$0.2 mag. The light curve of ASASSN-14dq indicates it to be a fast-declining type II-P SN, making it a transitional event between the type II-P and II-L SNe. The empirical relation between the steepness parameter and $\rm ^{56}Ni$ mass for type II SNe was rebuilt with the help of well-sampled light curves from the literature. A $\rm ^{56}Ni$ mass of $\sim\,$0.029 M$_{\odot}$ was estimated for ASASSN-14dq, which is slightly lower than the expected $\rm ^{56}Ni$ mass for a luminous type II-P SN. Using analytical light curve modelling, a progenitor radius of $\rm \sim3.6\times10^{13}$ cm, an ejecta mass of $\rm \sim10\ M_{\odot}$ and a total energy of $\rm \sim\,1.8\times 10^{51}$ ergs was estimated for this event. The photospheric velocity evolution of ASASSN-14dq resembles a type II-P SN, but the Balmer features (H$α$ and H$β$) show relatively slow velocity evolution. The high-velocity H$α$ feature in the plateau phase, the asymmetric H$α$ emission line profile in the nebular phase and the inferred outburst parameters indicate an interaction of the SN ejecta with the circumstellar material (CSM).

astro-ph.HE↗