arXiv subjects

Jayant Murthy

Publications and source records attributed to Jayant Murthy.

At least 73 records · Page 4Linked to original sources

arXiv2016

An Ultraviolet imager to study bright UV sources

We have designed and developed a compact ultraviolet imaging payload to fly on a range of possible platforms such as high altitude balloon experiments, cubesats, space missions, etc. The primary science goals are to study the bright UV sources (mag < 10) and also to look for transients in the Near UV (200 - 300 nm) domain. Our first choice is to place this instrument on a spacecraft going to the Moon as part of the Indian entry into Google lunar X-Prize competition. The major constraints for the instrument are, it should be lightweight (< 2Kg), compact (length < 50cm) and cost effective. The instrument is an 80 mm diameter Cassegrain telescope with a field of view of around half a degree designated for UV imaging. In this paper we will discuss about the various science cases that can be performed by having observations with the instrument on different platforms. We will also describe the design, development and the current state of implementation of the instrument. This includes opto-mechanical and electrical design of the instrument. We have adopted an all spherical optical design which would make the system less complex to realize and a cost effective solution compared to other telescope configuration. The structural design has been chosen in such a way that it will ensure that the instrument could withstand all the launch load vibrations. An FPGA based electronics board is used for the data acquisition, processing and CCD control. We will also briefly discuss about the hardware implementation of the detector interface and algorithms for the detector readout and data processing.

astro-ph.IM↗

arXiv2016

In-orbit Performance of UVIT on ASTROSAT

We present the in-orbit performance and the first results from the ultra-violet Imaging telescope (UVIT) on ASTROSAT. UVIT consists of two identical 38cm coaligned telescopes, one for the FUV channel (130-180nm) and the other for the NUV (200-300nm) and VIS (320-550nm) channels, with a field of view of 28 $arcmin$. The FUV and the NUV detectors are operated in the high gain photon counting mode whereas the VIS detector is operated in the low gain integration mode. The FUV and NUV channels have filters and gratings, whereas the VIS channel has filters. The ASTROSAT was launched on 28th September 2015. The performance verification of UVIT was carried out after the opening of the UVIT doors on 30th November 2015, till the end of March 2016 within the allotted time of 50 days for calibration. All the on-board systems were found to be working satisfactorily. During the PV phase, the UVIT observed several calibration sources to characterise the instrument and a few objects to demonstrate the capability of the UVIT. The resolution of the UVIT was found to be about 1.4 - 1.7 $arcsec$ in the FUV and NUV. The sensitivity in various filters were calibrated using standard stars (white dwarfs), to estimate the zero-point magnitudes as well as the flux conversion factor. The gratings were also calibrated to estimate their resolution as well as effective area. The sensitivity of the filters were found to be reduced up to 15\% with respect to the ground calibrations. The sensitivity variation is monitored on a monthly basis. UVIT is all set to roll out science results with its imaging capability with good resolution and large field of view, capability to sample the UV spectral region using different filters and capability to perform variability studies in the UV.

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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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arXiv2016

Modelling Dust Scattering in our Galaxy

I have used Monte Carlo models with multiple scattering to predict the dust scattered light from our Galaxy and have compared the predictions with data in two UV bands from the GALEX spacecraft. I find that 90\% of the scattered light arises from less than 1000 stars with 25\% from the 10 brightest. About half of the diffuse radiation originates within 200 pc of the Sun with a maximum distance of 600 pc. Multiple scattering is important at any optical depth with 30\% of the flux being multiply scattered even at zero reddening. I find that the global distribution of the scattered light is insensitive to the dust distribution with grains of $0.3 < a < 0.5$ and $g < 0.6$. There is an offset between the model and the data of 100 and 200 ph cm$^{-2}$ s$^{-1}$ sr$^{-1}$ Å$^{-1}$ in the FUV and NUV, respectively, at the poles rising to 200 --- 400 ph cm$^{-2}$ s$^{-1}$ sr$^{-1}$ Å$^{-1}$ at lower latitudes. The Monte Carlo code and the models of diffuse radiation for different values of the optical constants are available for download.

astro-ph.GA↗

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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arXiv2015

Diffuse Radiation from the Aquila Rift

We present an analysis of the diffuse ultraviolet (UV) background in a low latitude region near the Aquila Rift based on observations made by the Galaxy Evolution Explorer (GALEX). The UV background is at a level of about 2000 ph cm^-2 s^-1 sr^-1 Å^-1 with no correlation with either the Galactic latitude or the 100 micron infrared (IR) emission. Rather, the UV emission falls off with distance from the bright B2 star HIP 88149, which is in the centre of the field. We have used a Monte Carlo model to derive an albedo of 0.6 - 0.7 in the UV with a phase function asymmetry factor (g) of 0.2 - 0.4. The value for the albedo is dependent on the dust distribution while g is determined by the extent of the halo.

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arXiv2014

A Raspberry Pi-Based Attitude Sensor

We have developed a lightweight low-cost attitude sensor, based on a Raspberry Pi, built with readily available commercial components. It can be used in experiments where weight and power are constrained, such as in high- altitude lightweight balloon flights. This attitude sensor will be used as a major building block in a closed-loop control system with driver motors to stabilize and point cameras and telescopes for astronomical observations from a balloon-borne payload.

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arXiv2014

The Mystery of the Cosmic Diffuse Ultraviolet Background Radiation

The diffuse cosmic background radiation in the GALEX far ultraviolet (FUV, 1300 Å - 1700 Å) is deduced to originate only partially in the dust-scattered radiation of FUV-emitting stars: the source of a substantial fraction of the FUV background radiation remains a mystery. The radiation is remarkably uniform at both far northern and far southern Galactic latitudes, and it increases toward lower Galactic latitudes at all Galactic longitudes. We examine speculation that it might be due to interaction of the dark matter with the nuclei of the interstellar medium but we are unable to point to a plausible mechanism for an effective interaction. We also explore the possibility that we are seeing radiation from bright FUV-emitting stars scattering from a "second population" of interstellar grains---grains that are small compared with FUV wavelengths. Such grains are known to exist (Draine 2011) and they scatter with very high albedo, with an isotropic scattering pattern. However, comparison with the observed distribution (deduced from their $100\ μ$m emission) of grains at high Galactic latitudes shows no correlation between the grains' location and the observed FUV emission. Our modeling of the FUV scattering by small grains also shows that there must be remarkably few such "smaller" grains at high Galactic latitudes, both North and South; this likely means simply that there is very little interstellar dust of any kind at the Galactic poles, in agreement with Perry & Johnston (1982). We also review our limited knowledge of the cosmic diffuse background at ultraviolet wavelengths shortward of Lyman $α$---it could be that our "second component" of the diffuse far-ultraviolet background persists shortward of the Lyman limit, and is the cause of the re-ionization of the Universe (Kollmeier et al. 2014).

astro-ph.GA↗

arXiv2014

Prospect for UV observations from the Moon

Space astronomy in the last 40 years has largely been done from spacecraft in low Earth orbit (LEO) for which the technology is proven and delivery mechanisms are readily available. However, new opportunities are arising with the surge in commercial aerospace missions. We describe here one such possibility: deploying a small instrument on the Moon. This can be accomplished by flying onboard the Indian entry to the Google Lunar X PRIZE competition, Team Indus mission, which is expected to deliver a nearly 30 kgs of payloads to the Moon, with a rover as its primary payload. We propose to mount a wide-field far-UV (130--180 nm) imaging telescope as a payload on the Team Indus lander. Our baseline operation is a fixed zenith pointing but with the option of a mechanism to allow observations of different attitudes. Pointing towards intermediate ecliptic latitude (50 deg or above) ensures that the Sun is at least 40 deg off the line of sight at all times. In this position, the telescope can cover higher galactic latitudes as well as parts of Galactic plane. The scientific objectives of such a prospective are delineated and discussed.

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arXiv2014

Nightfall: Can Kalgash Exist

We investigate the imaginary world of Kalgash, a planetary system based on the novel "Nightfall" (Asimov & Silverberg, 1991). The system consists of a planet, a moon and an astonishing six suns. The six stars cause the wider universe to be invisible to the inhabitants of the planet. The author explores the consequences of an eclipse and the resulting darkness which the Kalgash people experience for the first time. Our task is to verify if this system is feasible, from the duration of the eclipse, the "invisibility" of the universe to the complex orbital dynamics.

physics.pop-ph↗

arXiv2014

GALEX Diffuse Observations of the Sky: The Data

I present tabulations of the diffuse observations made by the GALEX spacecraft in two UV bands (FUV: 1539 A and NUV: 2316 A) from the (almost) final data release of the GALEX spacecraft (GR6/GR7). This data release includes all the FUV observations and the majority of the NUV observations. I discuss overall trends in the data but the primary purpose is to make the data available to the public. These data files described in this paper are hosted by the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Insitutute from whence they may be downloaded. For ease of use, I have also created maps of the diffuse radiation in both bands over the entire observed sky at 6' resolution.

astro-ph.GA↗

arXiv2014

The Diffuse Ultraviolet Foreground

Ultraviolet observations from low Earth orbit (LEO) have to deal with a foreground comprised of airglow and zodiacal light which depend on the look direction and on the date and time of the observation. We have used all-sky observations from the GALEX spacecraft to find that the airglow may be divided into a baseline dependent on the sun angle and a component dependent only on the time from local midnight. The zodiacal light is observable only in the near ultraviolet band (2321 Å) of GALEX and is proportional to the zodiacal light in the visible but with a color of 0.65 indicating that the dust grains are less reflective in the UV.

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arXiv2013

Photometric Identification of Objects from Galaxy Evolution Explorer Survey and Sloan Digital Sky Survey

We have used GALEX and SDSS observations to extract 7 band photometric magnitudes for over 80,000 objects in the vicinity of the North Galactic Pole. Although these had been identified as stars by the SDSS pipeline, we found through fitting with model SEDs that most were, in fact, of extragalactic origin. Only about 9% of these objects turned out to be main sequence stars and about 11% were white dwarfs and red giants collectively, while galaxies and quasars contributed to the remaining 80% of the data. We have classified these objects into different spectral types (for the stars) and into different galactic types (for the galaxies). As part of our fitting procedure, we derive the distance and extinction to each object and the photometric redshift towards galaxies and quasars. This method easily allows for the addition of any number of observations to cover a more diverse range of wavelengths, as well as the addition of any number of model templates. The primary objective of this work is to eventually derive a 3-dimensional extinction map of the Milky Way galaxy.

astro-ph.GA↗

arXiv2013

O~{\small VI} absorption in the Milky Way along the Large Magellanic Cloud lines of sight

We have used \textit{Far Ultraviolet Spectroscopic Explorer (FUSE)} observations of the Large Magellanic Cloud (LMC) to determine the O {\small VI} column densities in the Milky Way (MW) towards 6 LMC lines of sight. The mean column density of O {\small VI} in the MW is found to be log N(O {\small VI})=14.257$_{-0.084}^{+0.096}$. The results confirm the patchiness of O {\small VI} absorption in the MW and the column densities are higher or comparable to the LMC.

astro-ph.GA↗

arXiv2013

Dust properties from GALEX observations of a UV halo around Spica

GALEX has detected ultraviolet halos extending as far as 5$^{\circ}$ around four bright stars (Murthy et al. (2011)). These halos are produced by scattering of starlight by dust grains in thin foreground clouds that are not physically associated with the star. Assuming a simple model consisting of a single layer of dust in front of the star, Murthy et al.(2011) have been able to model these halo intensities and constrain the value of the phase function asymmetry factor $g$ of the scattering grains in the FUV and NUV. However due to the uncertainty in the dust geometry they could not constrain the albedo. In this work we have tried to constrain the optical constants and dust geometry by modeling the UV halo of Spica. Since the halo emission is not symmetric, we have modeled the Northern and Southern parts of the halo separately. To the North of Spica, the best-fit albedo is 0.26$\pm$0.1 and $g$ is 0.58$\pm$0.11 in the FUV at the 90% confidence level. The corresponding limits on the distance and optical depth ($τ$) of the dust sheet is 3.65$\pm$1.05 pc and 0.047$\pm$0.006 respectively. However, owing to a complicated dust distribution to the South of Spica, we were unable to uniquely constrain the dust parameters in that region. Nevertheless, by assuming the optical constants of the Northern region and assuming a denser medium, we were able to constrain the distance of the dust to 9.5$\pm$1.5 pc and the corresponding $τ$ to 0.04$\pm$0.01.

astro-ph.GA↗

arXiv2013

[O~{\small VI} from the SBs of the LMC

We have presented the observations of O {\small VI} absorption at 1032 Å towards 22 sightlines in 10 superbubbles (SBs) of the Large Magellanic Cloud (LMC) using the data obtained from the {\em Far Ultraviolet Spectroscopic Explorer (FUSE)}. The estimated abundance of O {\small VI} in the SBs varies from a minimum of (1.09 $\pm$0.22)$\times10^{14}$ atoms/cm$^{2}$ in SB N206 to a maximum of (3.71$\pm$0.23)$\times10^{14}$ atoms/cm$^{2}$ in SB N70. We find about a 46% excess in the abundance of O {\small VI} in the SBs compared to the non-SB lines of sight. Even inside a SB, O {\small VI} column density (N(O {\small VI})) varies by about a factor of 2 to 2.5. These data are useful in understanding the nature of the hot gas in SBs.

astro-ph.GA↗

arXiv2013

High-Altitude Ballooning Program at the Indian Institute of Astrophysics

We have begun a program of high altitude ballooning at the Indian Institute of Astrophysics, Bangalore. Recent advances in balloons as well as in electronics have made possible scientific payloads at costs accessible to university departments. The primary purpose of this activity is to test low-cost ultraviolet (UV) payloads for eventual space flight, but to also explore phenomena occurring in the upper atmosphere, including sprites and meteorite impacts, using balloon-borne payloads. This paper discusses the results of three tethered balloon experiments carried out at the CREST campus of IIA, Hosakote and our plans for the future. We also describe the stages of payload development for these experiments.

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arXiv2013

Large Scale Extinction Maps with UVIT

The Ultraviolet Imaging Telescope (UVIT) is scheduled to be launched as a part of the ASTROSAT satellite. As part of the mission planning for the instrument we have studied the efficacy of UVIT observations for interstellar extinction measurements. We find that in the best case scenario, the UVIT can measure the reddening to an accuracy of about 0.02 magnitudes, which combined with the derived distances to the stars, will enable us to model the three-dimensional distribution of extinction in our Galaxy. The knowledge of the distribution of the ISM will then be used to study distant objects, affected by it. This work points the way to further refining the UVIT mission plan to best satisfy different science studies.

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