arXiv subjects

S. Ambily

Publications and source records attributed to S. Ambily.

14 recordsLinked to original sources

arXiv2023

CUTE reveals escaping metals in the upper atmosphere of the ultra-hot Jupiter WASP-189b

Ultraviolet observations of Ultra-hot Jupiters (UHJs), exoplanets with temperatures over 2000\,K, provide us with an opportunity to investigate if and how atmospheric escape shapes their upper atmosphere. Near-ultraviolet transit spectroscopy offers a unique tool to study this process owing to the presence of strong metal lines and a bright photospheric continuum as the light source against which the absorbing gas is observed. WASP-189b is one of the hottest planets discovered to date, with a day-side temperature of about 3400\,K orbiting a bright A-type star. We present the first near-ultraviolet observations of WASP-189b, acquired with the Colorado Ultraviolet Transit Experiment ($CUTE$). $CUTE$ is a 6U NASA-funded ultraviolet spectroscopy mission, dedicated to monitoring short-period transiting planets. WASP-189b was one of the $CUTE$ early science targets and was observed during three consecutive transits in March 2022. We present an analysis of the $CUTE$ observations and results demonstrating near-ultraviolet (2500--3300~Å) broadband transit depth ($1.08^{+0.08}_{-0.08}\%$) of about twice the visual transit depth indicating that the planet has an extended, hot upper atmosphere with a temperature of about 15000\,K and a moderate mass loss rate of about \SI{4e8}{\kg\per\second}. We observe absorption by Mg{\sc ii} lines ($R_p/R_s$ of $0.212^{+0.038}_{-0.061}$) beyond the Roche lobe at $>$4$σ$ significance in the transmission spectrum at a resolution of 10~Å, while at lower resolution (100~Å), we observe a quasi-continuous absorption signal consistent with a "forest" of low-ionization metal absorption dominated by Fe{\sc ii}. The results suggest an upper atmospheric temperature ($\sim15000$\,K), higher than that predicted by current state-of-the-art hydrodynamic models.

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arXiv2022

Impact of MgII interstellar medium absorption on near-ultraviolet exoplanet transit measurements

Ultraviolet (UV) transmission spectroscopy probes atmospheric escape, which has a significant impact on planetary atmospheric evolution. If unaccounted for, interstellar medium absorption (ISM) at the position of specific UV lines might bias transit depth measurements, and thus potentially affect the (non-)detection of features in transmission spectra. Ultimately, this is connected to the so called ``resolution-linked bias'' (RLB) effect. We present a parametric study quantifying the impact of unresolved or unconsidered ISM absorption in transit depth measurements at the position of the MgII h&k resonance lines (i.e. 2802.705 Å and 2795.528 Å respectively) in the near-ultraviolet spectral range. We consider main-sequence stars of different spectral types and vary the shape and amount of chromospheric emission, ISM absorption, and planetary absorption, as well as their relative velocities. We also evaluate the role played by integration bin and spectral resolution. We present an open-source tool enabling one to quantify the impact of unresolved or unconsidered MgII ISM absorption in transit depth measurements. We further apply this tool to a few already or soon to be observed systems. On average, we find that ignoring ISM absorption leads to biases in the MgII transit depth measurements comparable to the uncertainties obtained from the observations published to date. However, considering the bias induced by ISM absorption might become necessary when analysing observations obtained with the next generation space telescopes with UV coverage (e.g. LUVOIR, HABEX), which will provide transmission spectra with significantly smaller uncertainties compared to what obtained with current facilities (e.g. HST).

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arXiv2022

The Autonomous Data Reduction Pipeline for the CUTE Mission

The Colorado Ultraviolet Transit Experiment (CUTE) is a 6U NASA CubeSat carrying on-board a low-resolution, near-ultraviolet (2479-3306 A) spectrograph. It has a Cassegrain telescope with a rectangular primary to maximize the collecting area, given the shape of the satellite bus, and an aberration correcting grating to improve the image quality, and thus spectral resolution. CUTE, launched on the 27th of September 2021 to a Low Earth Orbit, is designed to monitor transiting extra-solar planets orbiting bright, nearby stars to improve our understanding of planet atmospheric escape and star-planet interaction processes. We present here the CUTE autONomous daTa ReductiOn pipeLine (CONTROL), developed for reducing CUTE data. The pipeline has been structured with a modular approach, which also considers scalability and adaptability to other missions carrying on-board a long-slit spectrograph. The CUTE data simulator has been used to generate synthetic observations used for developing and testing the pipeline functionalities. The pipeline has been tested and updated employing ight data obtained during commissioning and initial science operations of the mission.

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arXiv2022

The Near Ultraviolet Transient Surveyor (NUTS): An ultraviolet telescope to observe variable sources

Observing the ultraviolet (UV) sky for time-variable phenomena is one of the many exciting science goals that can be achieved by a relatively small aperture telescope in space. The Near Ultraviolet Transient Surveyor (NUTS) is a wide-field ($3^\circ$) imager with a photon-counting detector in the near-UV (NUV, 200-300 nm), to be flown on an upcoming small satellite mission. It has a Ritchey-Chretien (RC) telescope design with correction optics to enable wide-field observations while minimizing optical aberrations. We have used an intensified CMOS detector with a solar blind photocathode, to be operated in photon-counting mode. The main science goal of the instrument is the observation of transient sources in the UV, including flare stars, supernovae, and active galactic nuclei. NUTS's aperture size and effective area enable observation of relatively unexplored, brighter parts of the UV sky which are usually not accessible to larger missions. We have designed, fabricated, and assembled the instrument, and the final calibrations and environmental tests are being carried out. In this paper, we provide the scientific motivation and technical overview of the instrument and describe the assembly and calibration steps.

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arXiv2020

Ca II H&K stellar activity parameter: a proxy for stellar Extreme Ultraviolet Fluxes

Atmospheric escape is an important factor shaping the exoplanet population and hence drives our understanding of planet formation. Atmospheric escape from giant planets is driven primarily by the stellar X-ray and extreme-ultraviolet (EUV) radiation. Furthermore, EUV and longer wavelength UV radiation power disequilibrium chemistry in the middle and upper atmosphere. Our understanding of atmospheric escape and chemistry, therefore, depends on our knowledge of the stellar UV fluxes. While the far-ultraviolet fluxes can be observed for some stars, most of the EUV range is unobservable due to the lack of a space telescope with EUV capabilities and, for the more distant stars, to interstellar medium absorption. Thus, it becomes essential to have indirect means for inferring EUV fluxes from features observable at other wavelengths. We present here analytic functions for predicting the EUV emission of F-, G-, K-, and M-type stars from the log $R'_{HK}$ activity parameter that is commonly obtained from ground-based optical observations of the Ca II H&K lines. The scaling relations are based on a collection of about 100 nearby stars with published log $R'_{HK}$ and EUV flux values, where the latter are either direct measurements or inferences from high-quality far-ultraviolet (FUV) spectra. The scaling relations presented here return EUV flux values with an accuracy of about three, which is slightly lower than that of other similar methods based on FUV or X-ray measurements.

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arXiv2019

Prospect for UV observations from the Moon. III. Assembly and ground calibration of Lunar Ultraviolet Cosmic Imager (LUCI)

The Lunar Ultraviolet Cosmic Imager (LUCI) is a near-ultraviolet (NUV) telescope with all-spherical mirrors, designed and built to fly as a scientific payload on a lunar mission with Team Indus - the original Indian entry to the Google Lunar X-Prize. Observations from the Moon provide a unique opportunity of a stable platform with an unobstructed view of the space at all wavelengths due to the absence of atmosphere and ionosphere. LUCI is an 80 mm aperture telescope, with a field of view of 27.6'x 20.4' and a spatial resolution of 5'', will scan the sky in the NUV (200-320 nm) domain to look for transient sources. We describe here the assembly, alignment, and calibration of the complete instrument. LUCI is now in storage in a class 1000 clean room and will be delivered to our flight partner in readiness for flight.

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arXiv2018

Wide-field Ultraviolet Imager for Astronomical Transient Studies

Though the ultraviolet (UV) domain plays a vital role in the studies of astronomical transient events, the UV time-domain sky remains largely unexplored. We have designed a wide-field UV imager that can be flown on a range of available platforms, such as high-altitude balloons, CubeSats, and larger space missions. The major scientific goals are the variability of astronomical sources, detection of transients such as supernovae, novae, tidal disruption events, and characterizing AGN variability. The instrument has an 80 mm aperture with a circular field of view of 10.8 degrees, an angular resolution of around 22 arcsec, and a 240-390 nm spectral observation window. The detector for the instrument is a Microchannel Plate (MCP)-based image intensifier with both photon counting and integration capabilities. An FPGA-based detector readout mechanism and real-time data processing have been implemented. The imager is designed in such a way that its lightweight and compact nature are well fitted for the CubeSat dimensions. Here we present various design and developmental aspects of this UV wide-field transient explorer.

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arXiv2017

Near UV Imager with an MCP Based Photon Counting Detector

We are developing a compact UV Imager using light weight components, that can be flown on a small CubeSat or a balloon platform. The system has a lens-based optics that can provide an aberration-free image over a wide field of view. The backend instrument is a photon counting detector with off-the-shelf MCP, CMOS sensor and electronics. We are using a Z-stack MCP with a compact high voltage power supply and a phosphor screen anode, which is read out by a CMOS sensor and the associated electronics. The instrument can be used to observe solar system objects and detect bright transients from the upper atmosphere with the help of CubeSats or high altitude balloons. We have designed the imager to be capable of working in direct frame transfer mode as well in the photon-counting mode for single photon event detection. The identification and centroiding of each photon event are done using an FPGA-based data acquisition and real-time processing system.

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arXiv2017

Development of Data Acquisition Methods for an FPGA-Based Photon Counting Detector

MCP-based detectors are widely used in the ultraviolet (UV) region due to their low noise levels, high sensitivity and good spatial and temporal resolution. We have developed a compact near-UV (NUV) detector for high-altitude balloon and space flights, using off-the-shelf MCP, CMOS sensor, and optics. The detector is designed to be capable of working in the direct frame transfer mode as well in the photon-counting mode for single photon event detection. The identification and centroiding of each photon event are done using an FPGA-based data acquisition and real-time processing system. In this paper, we discuss various algorithms and methods used in both operating modes, as well as their implementation on the hardware.

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arXiv2017

Prospect for UV observations from the Moon. II. Instrumental Design of an Ultraviolet Imager LUCI

We present a design for a near-ultraviolet (NUV) imaging instrument which may be flown on a range of available platforms, including high-altitude balloons, nanosatellites, or space missions. Although all current UV space missions adopt a Ritchey-Chretain telescope design, this requires aspheric optics, making the optical system complex, expensive and challenging for manufacturing and alignment. An all-spherical configuration is a cost-effective and simple solution. We have aimed for a small payload which may be launched by different platforms and we have designed a compact, light-weight payload which will withstand all launch loads. No other UV payloads have been previously reported with an all-spherical optical design for imaging in the NUV domain and a weight below 2 kg. Our main science goal is focussed on bright UV sources not accessible by the more sensitive large space UV missions. Here we discuss various aspects of design and development of the complete instrument, the structural and finite-element analysis of the system performed to ensure that the payload withstands launch-load stresses and vibrations. We expect to fly this telescope -- Lunar Ultraviolet Cosmic Imager (LUCI) -- on a spacecraft to the Moon as part of the Indian entry into Google X-Prize competition. Observations from the Moon provide a unique opportunity to observe the sky from a stable platform far above the Earth's atmosphere. However, we will explore other opportunities as well, and will fly this telescope on a high-altitude balloon later this year.

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arXiv2016

A software package for evaluating the performance of a star sensor operation

We have developed a low-cost off-the-shelf component star sensor (StarSense) for use in minisatellites and CubeSats to determine the attitude of a satellite in orbit. StarSense is an imaging camera with a limiting magnitude of 6.5, which extracts information from star patterns it records in the images. The star sensor implements a centroiding algorithm to find centroids of the stars in the image, a Geometric Voting algorithm for star pattern identification, and a QUEST algorithm for attitude quaternion calculation. Here, we describe the software package to evaluate the performance of these algorithms as a star sensor single operating system. We simulate the ideal case where sky background and instrument errors are omitted, and a more realistic case where noise and camera parameters are added to the simulated images. We evaluate such performance parameters of the algorithms as attitude accuracy, calculation time, required memory, star catalog size, sky coverage, etc., and estimate the errors introduced by each algorithm. This software package is written for use in MATLAB. The testing is parametrized for different hardware parameters, such as the focal length of the imaging setup, the field of view (FOV) of the camera, angle measurement accuracy, distortion effects, etc., and therefore, can be applied to evaluate the performance of such algorithms in any star sensor. For its hardware implementation on our StarSense, we are currently porting the codes in form of functions written in C. This is done keeping in view its easy implementation on any star sensor electronics hardware.

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arXiv2016

Balloon UV Experiments for Astronomical and Atmospheric Observations

The ultraviolet (UV) window has been largely unexplored through balloons for astronomy. We discuss here the development of a compact near-UV spectrograph with fiber optics input for balloon ights. It is a modified Czerny-Turner system built using off-the-shelf components. The system is portable and scalable to different telescopes. The use of re ecting optics reduces the transmission loss in the UV. It employs an image-intensified CMOS sensor, operating in photon counting mode, as the detector of choice. A lightweight pointing system developed for stable pointing to observe astronomical sources is also discussed, together with the methods to improve its accuracy, e.g. using the in-house build star sensor and others. Our primary scientific objectives include the observation of bright Solar System objects such as visible to eye comets, Moon and planets. Studies of planets can give us valuable information about the planetary aurorae, helping to model and compare atmospheres of other planets and the Earth. The other major objective is to look at the diffuse UV atmospheric emission features (airglow lines), and at column densities of trace gases. This UV window includes several lines important to atmospheric chemistry, e.g. SO2, O3, HCHO, BrO. The spectrograph enables simultaneous measurement of various trace gases, as well as provides better accuracy at higher altitudes compared to electromechanical trace gas measurement sondes. These lines contaminate most astronomical observations but are poorly characterized. Other objectives may include sprites in the atmosphere and meteor ashes from high altitude burn-outs. Our recent experiments and observations with high-altitude balloons are discussed.

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arXiv2016

Measurements of gondola motion on a stratospheric balloon flight

Balloon experiments are an economically feasible method of conducting observations in astronomy that are not possible from the ground. The astronomical payload may include a telescope, a detector, and a pointing/stabilization system. Determining the attitude of the payload is of primary importance in such applications, to accurately point the detector/telescope to the desired direction. This is especially important in generally unstable lightweight balloon flights. However, the conditions at float altitudes, which can be reached by zero pressure balloons, could be more stable, enabling accurate pointings. We have used the Inertial Measurement Unit (IMU), placed on a stratospheric zero pressure balloon, to observe 3-axis motion of a balloon payload over a fight time of 4.5 hours, from launch to the float altitude of 31.2 km. The balloon was launched under nominal atmospheric conditions on May 8th 2016, from a Tata Institute of Fundamental Research Balloon Facility, Hyderabad.

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arXiv2015

An Overview of High-Altitude Balloon Experiments at the Indian Institute of Astrophysics

We have initiated the High-Altitude Ballooning programme at Indian Institute of Astrophysics, Bangalore, in the year 2011 with the primary purpose of developing and flying low-cost scientific payloads on a balloon-borne platform. The main aim is the observations of extended nearby objects (e.g. comets) and of diffuse sources (e.g. zodiacal light or airglow) with wide field of view (FOV) UV instruments from near space (20 to 30 km). A brief summary and the results of the tethered flights carried out at IIA CREST campus are given in Ref.~1. Here we present an overview of the nine free-flying balloon experiments conducted from March 2013 to November 2014. We describe the launch procedures, payloads, methods of tracking and recovery. Since we fall in the light balloon category --- payload weight is limited to less than 6 kg --- we use the 3-D printer to fabricate lightweight boxes and structures for our experiments. We discuss the flight and scientific data obtained from different launches and the development of the in-house lightweight sensors and controllers, as well as a fully-fledged 2-axis pointing and stabilization system, for the flights.

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