arXiv subjects

Subhajit Sarkar

Publications and source records attributed to Subhajit Sarkar.

46 records · Page 3Linked to original sources

arXiv2019

JexoSim: A time domain simulator of exoplanet transit spectroscopy with JWST

The James Webb Space Telescope (JWST) will perform exoplanet transit spectroscopy in the coming decade promising transformative science results. All four instruments on board can be used for this technique which reconstructs the atmospheric transmission or emission spectrum of an exoplanet from wavelength-dependent light curve measurements. Astrophysical and instrumental noise and systematics can affect the precision and accuracy of the final spectrum, and hence, the atmospheric properties derived from the spectrum. Correlated noise and time-dependent systematics that can bias the measured signal must be accounted for in the final uncertainties. However quantifying these effects can be difficult with real data or simple analytic tools. Existing publicly-available simulators for JWST do not adequately simulate complex time domain processes on exoplanetary transit observations. We report JexoSim, a dedicated time domain simulator for JWST including all four instruments for exoplanet transit spectroscopy. JexoSim models both the astrophysics and the instrument, generating 2-D images in simulated time akin to a real observation. JexoSim can capture correlated noise and systematic biases on the light curve giving it great versatility. Potential applications of JexoSim include performance testing of JWST instruments, assessing science return, and testing data reduction pipelines. We describe JexoSim, validate it against other simulators, and present examples of its utility.

astro-ph.EP↗

arXiv2019

Observing exoplanets in the near-infrared from a high altitude balloon platform

Although there exists a large sample of known exoplanets, little spectroscopic data exists that can be used to study their global atmospheric properties. This deficiency can be addressed by performing phase-resolved spectroscopy -- continuous spectroscopic observations of a planet's entire orbit about its host star -- of transiting exoplanets. Planets with characteristics suitable for atmospheric characterization have orbits of several days, thus phase curve observations are highly resource intensive, especially for shared use facilities. In this work, we show that an infrared spectrograph operating from a high altitude balloon platform can perform phase-resolved spectroscopy of hot Jupiter-type exoplanets with performance comparable to a space-based telescope. Using the EXoplanet Climate Infrared TElescope (EXCITE) experiment as an example, we quantify the impact of the most important systematic effects that we expect to encounter from a balloon platform. We show an instrument like EXCITE will have the stability and sensitivity to significantly advance our understanding of exoplanet atmospheres. Such an instrument will both complement and serve as a critical bridge between current and future space-based near infrared spectroscopic instruments.

astro-ph.IM↗

arXiv2018

Fermi surface instabilities of symmetry-breaking and topological types on the surface of a three-dimensional topological insulator

The emergence of the Pomeranchuk instability (PI) in a Helical Fermi liquid (HFL) residing on the surface of a three-dimensional topological insulator (3D TI) is addressed at the mean-field level. An expression for the PI condition is derived in terms of a few microscopic parameters in each angular momentum channel corresponding to a central interaction between the helical electrons. It is found that because of the presence of strong spin-orbit coupling (SOC) the Landau parameter, $\bar{F}_{l}$ corresponding to a particular angular momentum channel $l$ depends not only on the electron-electron interaction in the same channel but also interactions in $(l+1)$ and $(l-1)$ channels. The formalism automatically excludes the $l=1$ PI in the HFL where the Galilean invariance is broken because of the presence of strong SOC. It is also found that the competing PIs can only be avoided until the appearance of $l=2$ PI. In this case, the corresponding nematic instability can even be achieved in the $l=1$ angular momentum channel of interaction between the electrons. The range of interaction between the electrons plays a pivotal role in bringing out the PIs. This is established by analysing a few realistic profiles of the interaction. Another class of instability, involving a change in the topology of the Fermi surface without breaking the rotational symmetry, is found which competes with the PIs. Quantum phase transition originating from this instability is quite similar to the Lifshitz transition but is driven by electron-electron interaction. Possible connections of this instability with experiments are also described briefly.

cond-mat.str-el↗

arXiv2018

Exoplanet Spectroscopy and Photometry with the Twinkle Space Telescope

The Twinkle space telescope has been designed for the characterisation of exoplanets and Solar System objects. Operating in a low Earth, Sun-synchronous orbit, Twinkle is equipped with a 45 cm telescope and visible (0.4 - 1um) and infrared (1.3 - 4.5um) spectrometers which can be operated simultaneously. Twinkle is a general observatory which will provide on-demand observations of a wide variety of targets within wavelength ranges that are currently not accessible using other space telescopes or accessible only to oversubscribed observatories in the short-term future. Here we explore the ability of Twinkle's spectrometers to characterise the currently-known exoplanets. We study the spectral resolution achievable by combining multiple observations for various planetary and stellar types. We also simulate spectral retrievals for some well-known planets (HD 209458 b, GJ 3470 b and 55 Cnc e). From the exoplanets known today, we find that with a single transit or eclipse, Twinkle could probe 89 planets at low spectral resolution (R < 20) as well as 12 planets at higher resolution (R > 20) in channel 1 (1.3 - 4.5um). With 10 observations, the atmospheres of 144 planets could be characterised with R < 20 and 81 at higher resolutions. Upcoming surveys will reveal thousands of new exoplanets, many of which will be located within Twinkle's field of regard. TESS in particular is predicted to discover many targets around bright stars which will be suitable for follow-up observations. We include these anticipated planets and find that the number of planets Twinkle could observe in the near infrared in a single transit or eclipse increases to 558 for R > 20 and 41 at lower resolutions. By stacking 10 transits or eclipses, there are 1185 potential targets for study at R < 20 as well as 388 planets at higher resolutions.

astro-ph.EP↗

arXiv2018

Stellar pulsation and granulation as noise sources in exoplanet transit spectroscopy in the ARIEL space mission

Stellar variability from pulsations and granulation presents a source of correlated noise that can impact the accuracy and precision of multi-band photometric transit observations of exoplanets. This can potentially cause biased measurements in the transmission or emission spectrum or underestimation of the final error bars on the spectrum. ARIEL is a future space telescope and instrument designed to perform a transit spectroscopic survey of a large sample of exoplanets. In this paper we perform simulations to assess the impact of stellar variability arising from pulsations and granulation on ARIEL observations of GJ 1214b and HD 209458b. We take into account the correlated nature of stellar noise, quantify it, and compare it to photon noise. In the range 1.95-7.8 \textmu m, stellar pulsation and granulation noise has insignificant impact compared to photon noise for both targets. In the visual range the contribution increases significantly but remains small in absolute terms and will have minimal impact on the transmission spectra of the targets studied. The impact of pulsation and granulation will be greatest for planets with low scale height atmospheres and long transit times around bright stars.

astro-ph.EP↗

arXiv2018

Delocalization of charge and current in a chiral quasiparticle wave-packet

A chiral quasi-particle wave packet (c-QPWP) is defined as a conventional superposition of chiral quasi-particle states corresponding to an interacting electron system in two dimensions (2D) in the presence of Rashba spin-orbit coupling (RSOC). I investigate its internal structure via studying the charge and the current densities within the first order perturbation in the electron-electron interaction. It is found that the c-QPWP contains a localized charge which is less than the magnitude of the bare charge and the remaining charge resides at the system boundary. The amount of charge delocalized turns out to be inversely proportional to the degenerate Fermi velocity $v_0 ( = \sqrt{α^2 + 2μ/ m})$ when RSOC (with strength $α$) is weak, and therefore externally tunable. For strong RSOC, the magnitudes of both the delocalized charge and the current further strongly depend on the direction of propagation of the wave packet. Both the charge and the current densities consist of an anisotropic $r^{-2}$ tail away from the centre of the wave packet. Possible implications of such delocalizations in real systems corresponding to 2D semiconductor heterostructure are also discussed within the context of particle injection experiments.

cond-mat.str-el↗

arXiv2018

The ARIEL Mission Reference Sample

The ARIEL (Atmospheric Remote-sensing Exoplanet Large-survey) mission concept is one of the three M4 mission candidates selected by the European Space Agency (ESA) for a Phase A study, competing for a launch in 2026. ARIEL has been designed to study the physical and chemical properties of a large and diverse sample of exoplanets, and through those understand how planets form and evolve in our galaxy. Here we describe the assumptions made to estimate an optimal sample of exoplanets - including both the already known exoplanets and the "expected" ones yet to be discovered - observable by ARIEL and define a realistic mission scenario. To achieve the mission objectives, the sample should include gaseous and rocky planets with a range of temperatures around stars of different spectral type and metallicity. The current ARIEL design enables the observation of ~1000 planets, covering a broad range of planetary and stellar parameters, during its four year mission lifetime. This nominal list of planets is expected to evolve over the years depending on the new exoplanet discoveries.

astro-ph.EP↗

arXiv2016

Semi-phenomenological analysis of neutron scattering results for quasi-two dimensional quantum anti-ferromagnet

The available results from the inelastic neutron scattering experiment performed on the quasi-two dimensional spin $\frac{1}{2}$ anti-ferromagnetic material $La_2 Cu O_4$ have been analysed theoretically. The formalism of ours is based on a semi-classical like treatment involving a model of an ideal gas of mobile vortices and anti-vortices built on the background of the N$\acute{e}$el state, using the bipartite classical spin configuration corresponding to an XY- anisotropic Heisenberg anti-ferromagnet on a square lattice. The results for the integrated intensities for our spin $\frac{1}{2}$ model corresponding to different temperatures, show occurrence of vigorous unphysical oscillations, when convoluted with a realistic spectral window function. These results indicate failure of the conventional semi-classical theoretical model of ideal vortex/anti-vortex gas arising in the Berezinskii-Kosterlitz-Thouless theory for the low spin magnetic systems. A full fledged quantum mechanical formalism and calculations seem crucial for the understanding of topological excitations in such low spin systems. Furthermore, a severe disagreement is found to occur at finite values of energy transfer between the integrated intensities obtained theoretically from the conventional formalism and those obtained experimentally. This further suggests strongly that the full quantum treatment should also incorporate the interaction between the fragile-magnons and the topological excitations. This is quite plausible in view of the recent work establishing such a process in XXZ quantum ferromagnet on 2D lattice. The high spin XXZ quasi-two dimensional antiferromagnet like $MnPS_3$ however follows the conventional theory quite well

cond-mat.str-el↗

arXiv2015

The connection between vortex-like topological excitations and conventional excitations in quantum ferromagnetic spin systems on two dimensional lattice and their stability

We present a scheme for the construction of quantum states of vortex like topological excitations corresponding to spin- 1/2 strongly XY anisotropic nearest neighbor Heisenberg Ferromagnet on two dimensional lattice. The procedure involving Pauli spin basis states is carried out corresponding to both infinite dilute limit and finite density limit of vortex/anti-vortex. It is found that the corresponding quantum mechanical states representing charge 1 quantum vortices/ anti-vortices can be expressed as linear combinations of single magnon states, composite multi-magnon states and the ground state. Detailed calculations show that these states are quantum mechanically stable states of the Hamiltonian only when the system size exceeds certain threshold value. Our analysis indicates that the interactions between different magnon modes can very well generate these topological excitations. Possible applications of our calculations to real magnetic systems are also discussed. Magnetic measurements probing spin dynamics may be undertaken to verify the existence of the threshold size for the stability of vortices.

cond-mat.str-el↗

arXiv2012

Theoretical analysis of neutron scattering results for quasi-two dimensional ferromagnets

A theoretical study has been carried out to analyse the available results from the inelastic neutron scattering experiment performed on a quasi-two dimensional spin-1/2 ferromagnetic material $K_2CuF_4$. Our formalism is based on a conventional semi-classical like treatment involving a model of an ideal gas of vortices/anti-vortices corresponding to an anisotropic XY Heisenberg ferromagnet on a square lattice. The results for dynamical structure functions for our model corresponding to spin-1/2, show occurrence of negative values in a large range of energy transfer even encompassing the experimental range, when convoluted with a realistic spectral window function. This result indicates failure of the conventional theoretical framework to be applicable to the experimental situation corresponding to low spin systems. A full quantum formalism seems essential for treating such systems.

cond-mat.str-el↗