arXiv subjects

Jayant Murthy

Publications and source records attributed to Jayant Murthy.

At least 19 recordsLinked to original sources

arXiv2026

The Cosmic Ultraviolet Background at the Galactic Poles

We have used archival GALEX data to separate the cosmic ultraviolet background at the Galactic Poles into two components: the dust scattered light and an offset. We have modeled the dust-scattered light using a single- scattering model finding 1 sigma limits of 0.54 -- 0.71 for the albedo (a) and 0.74 -- 0.83 for the phase function asymmetry factor (g) at 1530 {\AA} and 0.66 -- 0.73 for a and 0.71 -- 0.77 for g at 2360 {\AA}, that is, the grains are moderately reflective and highly forward-scattering. The offsets are 277 -- 284 photon units at 1530 {\AA} and 513 -- 520 photon units at 2360 {\AA}. We have estimated other Galactic and extragalactic contributors to the offset finding that 161 +- 18 photon units is unaccounted for at 1530 {\AA} and 335 +- 38 at 2360 {\AA}. The offsets are constant over these regions with variances of 20 -- 30 photon units.

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arXiv2026

Observations of High Galactic Latitude Line and Continuum Emission (912 - 1600 \AA) with New Horizons

We present observations of the cosmic ultraviolet background (CUVB) from 912 - 1600 A using the Stem aperture of the Alice spectrograph on the New Horizons spacecraft at 56 AU from the Sun, providing a spectral resolution of 9 A for diffuse sources. We detect emission lines of CIII (977 A) and CIV (1548/1551 A) at the 3 sigma level with strengths of 4200 +/- 1500 and 4100 +/- 1200 ph cm(-2) s(-1) sr(-1), respectively, and a marginal detection of OVI (1032/1038 A) at 1400 +/- 1300 ph cm(-2) s(-1) sr(-1). We report a 3 sigma detection of an emission line at 1135 A, which we have identified with the N I resonance triplet. Although this line had earlier been observed in FUSE and SPEAR data, it had been attributed to airglow or instrumental effects. We confirm, for the first time, that it must originate in the Galaxy. The dust-scattered continuum is dominated by a small number (N < 100) of O9 - B2 stars and shows the deep absorption feature near 1000 A seen in the stellar spectrum. Our models suggest an albedo of a < 0.5 over most of the spectrum (950 -- 1550 A) for the dust grains with the phase function asymmetry of g < 0.6. We find an offset, comprising the extragalactic background light and halo contributors, consistent with our earlier results from the Box, including the decline in the offset near the Lyman limit. We confirm that much of the emission must be from an unidentified component of the CUVB.

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arXiv2025

Near Ultraviolet Transient Explorer (NUTEx): A CubeSat-Based NUV Imaging Payload for Transient Sky Surveys

The Near Ultraviolet Transient Explorer (NUTEx) is a CubeSat-based near-ultraviolet (NUV) imaging payload designed for transient sky surveys and is currently under development. CubeSats are compact and cost-effective satellite platforms that have emerged as versatile tools for scientific exploration and technology demonstrations in space. NUTEx is an imaging telescope operating in the 200-300 nm wavelength range, intended for deployment on a micro-satellite bus. The optical system is based on a Ritchey Chretien (RC) telescope configuration, featuring a 146 mm primary mirror. The detector is a photon-counting microchannel plate (MCP) device with a solar-blind photocathode, paired with an in-house developed readout unit. The instrument has a wide field of view (FoV) of 4 deg, a peak effective area of approximately 18 sq cm at 260 nm, and can reach a sensitivity of 21 AB magnitude (SNR = 5) in a 1200 second exposure. The primary scientific objective of NUTEx is to monitor the night sky for transient phenomena, such as supernova remnants, flaring M-dwarf stars, and other short-timescale events. The payload is currently scheduled for launch in Q2 2026. This paper presents the NUTEx instrument design, outlines its scientific goals and capabilities, and provides an overview of the electronics and mechanical subsystems, including structural analysis.

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arXiv2025

SAMPLE -- Stratospheric Altitude Microbiology Probe for Life Existence -- A Method of Collection of Stratospheric Samples Using Balloon-Borne Payload System

The Earth possesses many environmental extremes that mimic conditions on extraterrestrial worlds. The stratosphere at 30-40 km altitude closely resembles the surface of Mars in terms of pressure, temperature, and radiation levels (UV, proton, and Galactic cosmic rays). While microbial life in the troposphere is well documented, the true upper limit of Earth's biosphere remains unclear. The stratosphere offers a promising environment to explore microbial survival in such extreme conditions. Despite its significance to astrobiology, this region remains largely unexplored due to difficulties in access and avoiding contamination. To address this, we have developed SAMPLE (Stratospheric Altitude Microbiology Probe for Life Existence), a balloon-borne payload designed to collect dust samples from the stratosphere and return them in conditions suitable for lab analysis. The entire system is novel and designed in-house, with weight- and stress-optimized components. The main payload includes three pre-sterilized sampling trays and a controller that determines altitude and governs tray operation. One tray will remain closed during flight (airborne control) and another on the ground (cleanroom control) to monitor contamination. Additional systems include environmental sensors, GPS trackers, cameras, and a Flight Termination Unit (FTU) to end the mission once sampling is complete. A parachute ensures the safe recovery of the payload. Upon retrieving the payload, the sampling trays (including controls) will be sent to a suitable laboratory where the samples will be examined for the presence and nature of collected material.

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arXiv2025

Modeling the Cosmic Ultraviolet Background at the North Galactic Pole

I explore models of the dust-scattered component of the Cosmic Ultraviolet Background (CUVB) at the North Galactic Pole (NGP) in order to develop a framework for calculating the dust-scattered light as a function of the optical depths. As expected, I find that the dust-scattered emission scales linearly with reddening up to $E(B-V) \approx 0.1$\ mag and derive a parametric model for this dependence. I have applied these models to fit the far-ultraviolet (1350--1800 \AA) observations from the \textit{Galaxy Evolution Explorer (GALEX)} finding that the optical constants of the interstellar dust grains -- albedo ($a$) and phase function asymmetry factor ($g$) -- are consistent with predictions from the Astrodust model ($a = 0.33$, $g = 0.68$). I detect an isotropic offset of $267 \pm 7$ ph cm$^{-2}$ s$^{-1}$ sr$^{-1}$ \AA$^{-1}$, half of which remains unaccounted for by known Galactic or extragalactic sources. I will now extend my analysis to wider sky regions with the goal of generating high-resolution extinction maps.

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arXiv2025

Excess Ultraviolet Emission at High Galactic Latitudes: A New Horizons View

We present new observations of the cosmic ultraviolet background (CUVB) at high Galactic latitudes ($|b| > 40^{\circ}$), made using the Alice UV spectrograph on board the New Horizons spacecraft. These observations were taken at about 57 AU from the Sun, outside much of the foreground emission affecting previous missions, and allowed a new determination of the spectrum of the CUVB between 912 -- 1100~\AA\ and 1400 -- 1800~\AA. We found a linear correlation between the CUVB and the Planck E(B~-~V) with offsets at zero-reddening of $221 \pm 11$ photon units at 1000~\AA\ and $264 \pm 24$ \photu\ at 1500~\AA\ ($4.4 \pm 0.2$ nW m$^{-2}$ sr$^{-1}$ at 1000~\AA\ and $5.3 \pm 0.5$ nW m$^{-2}$ sr$^{-1}$ at 1500~\AA). The former is the first firm detection of the offset in the range 912 -- 1100 \AA\ while the latter result confirms previous results from \galex, showing that there is little emission from the Solar System from 1400 -- 1800 \AA. About half of the offset may be explained by known sources (the integrated light of unresolved galaxies, unresolved stars, emission from ionized gas, and two-photon emission from warm hydrogen in the halo) with the source of the remaining emission as yet unidentified. There is no detectable emission below the Lyman limit with an upper limit of $3.2 \pm 3.0$ photon units.

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arXiv2024

A sky survey of ultraviolet sources observed through AstroSat's UVIT: A point source catalog

The Ultra Violet Imaging Telescope (UVIT) onboard India's first dedicated multiwavelength satellite \textit{AstroSat} observed a significant fraction of the sky in the ultraviolet with a spatial resolution of 1.4\arcsec. We present a catalog of the point sources observed by UVIT in the far ultraviolet (FUV; 1300-1800 \AA) and near ultraviolet (NUV; 2000-3000 \AA). We carried out astrometry and photometry of 428 field pointings in the FUV and 54 field pointings in the NUV band, observed in 5 filter bands in each channel respectively, covering an area of about 63 square degrees. The final catalog contains about 102,773 sources. The limiting magnitude(AB) of the F148W band filter, that has the largest number of detections is $\sim21.3$. For the NUV channel, we find the limiting magnitude at around $\sim23$. We describe the final catalog and present the results of the statistical analysis.

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arXiv2024

Ultraviolet Extinction Sky Survey (UVESS): A mission concept for probing the interstellar medium in the Milky Way and Local Group galaxies

The 2175 {\AA} bump shows considerable variations in its strength, width, and central wavelength when observed along different sightlines in the Milky Way and other galaxies. These variations offer valuable insights into the composition, size distribution, and processing of interstellar dust grains along different sightlines. This paper introduces a mission concept called UVESS (Ultraviolet Extinction Sky Survey) aimed at exploring the composition of the interstellar medium (ISM) within both the Milky Way and nearby Local Group Galaxies by mapping the variation of UV extinction curve slopes and the 2175 {\AA} feature across a majority of the sky to gain insights into the makeup of the ISM. Recent advancements in UV instrumentation and technologies pave the way for the development of high-throughput instruments in compact form factors. In this paper, we outline mission science goals and instrument concept tailored for a small satellite-based platform dedicated to the study of UV extinction.

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arXiv2024

Comet C/2012 S1 (ISON) crossing the Jupiter orbit

We report results of intensive time-resolved imaging photometry and synoptic deep imaging of the comet C/2012 S1 (ISON) performed in February 2013. The data were obtained at the Wise Observatory in Israel (WO), at the Himalayan Chandra Telescope (HCT) in India, and at the Polaris Observatory Association in California, USA. During this period, the comet's heliocentric distance changed from 4.9 to 4.6 AU, just within the orbit of Jupiter. We analyze these early images in an attempt to determine the nuclear rotation period, assuming that at these relatively large heliocentric distances it would be possible to detect the photometric modulation of a rotating nucleus against an underdeveloped coma. Since this is not evident in our February 2013 data, with more than 400 independent photometric measurements analyzed, we can only set upper limits of 0.05 mag for periodic brightness modulations. We discuss (and discount) a possible brightening event (minor outburst) that occurred on $15-16$ February 2013. We also present deep synoptic images of the comet, obtained by combining our exposures for each night, and analyze them. We find that during the period of our observations the comet exhibited a $\sim$$30^{\prime\prime}\simeq 60000$-km tail with no substructures visible and that this appearance did not change throughout our campaign. The comet, as indicated by a single spectroscopic measurement obtained during this observation period, showed a dust coma reflecting the solar light. Our observations indicate that during February 2013, comet ISON was relatively quiet, with the dust coma presumably hiding any light modulation by a spinning nucleus.

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arXiv2024

Spectroscopic Investigation of Nebular Gas (SING): Instrument Design, Assembly and Calibration

The Spectroscopic Investigation of Nebular Gas (SING) is a near-ultraviolet (NUV) low-resolution spectrograph payload designed to operate in the NUV range, 1400 $\unicode{x212B}$ -- 2700 $\unicode{x212B}$, from a stable space platform. SING telescope has a primary aperture of 298 mm, feeding the light to the long-slit UV spectrograph. SING has a field of view (FOV) of 1$^{\circ}$, achieving a spatial resolution of 1.33 arc minute and spectral resolution of 3.7 $\unicode{x212B}$ ($R\sim600$) at the central wavelength. SING employs a micro-channel plate (MCP) with a CMOS readout-based photon-counting detector. The instrument is designed to observe diffuse sources such as nebulae, supernova remnants, and the interstellar medium (ISM) to understand their chemistry. SING was selected by the United Nations Office for Outer Space Affairs to be hosted on the Chinese Space Station. The instrument will undergo qualification tests as per the launch requirements. In this paper, we describe the hardware design, optomechanical assembly, and calibration of the instrument.

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arXiv2024

Development Of Raspberry Pi-based Processing Unit for UV Photon-Counting Detectors

In ultraviolet (UV) astronomical observations, photons from the sources are very few compared to the visible or infrared (IR) wavelength ranges. Detectors operating in the UV usually employ a photon-counting mode of operation. These detectors usually have an image intensifier sensitive to UV photons and a readout mechanism that employs photon counting. The development of readouts for these detectors is resource-intensive and expensive. In this paper, we describe the development of a low-cost UV photon-counting detector processing unit that employs a Raspberry Pi with its in built readout to perform the photon-counting operation. Our system can operate in both 3x3 and 5x5 window modes at 30 frames per sec (fps), where 5x5 window mode also enables the provision of detection of double events. The system can be built quickly from readily available custom-off-the-shelf (COTS) components and is thus used in inexpensive CubeSats or small satellite missions. This low-cost solution promises to broaden access to UV observations, advancing research possibilities in space-based astronomy.

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arXiv2023

Dust Scattered Radiation in the Galactic Poles

We have modeled the diffuse background at the Galactic Poles in the far-ultraviolet (FUV: 1536 \AA) and the near-ultraviolet (NUV: 2316 \AA). The background is well-fit using a single-scattering dust model with an offset representing the extragalactic light plus any other contribution to the diffuse background. We have found a dust albedo of 0.35 -- 0.40 (FUV) and 0.11 -- 0.19 in the NGP ($b > 70^{\circ}$) and 0.46 -- 0.56 (FUV) and 0.31 -- 0.33 (NUV) in the SGP ($b < 70^{\circ}$. The differences in the albedo may reflect changes in the dust-to-gas ratio over the sky or in the dust distribution. We find offsets at zero-reddening of 273 -- 286 and 553 -- 581 photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$ \AA$^{-1}$ in the FUV and NUV, respectively, in the NGP with similar values in the SGP.

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arXiv2022

Low-Cost Raspberry Pi Star Sensor for Small Satellites

We present here a low-cost Raspberry Pi (RPi)-based star sensor StarberrySense using commercial-off-the-shelf (COTS) components, developed and built for applications in small satellites and CubeSat-based missions. A star sensor is one of the essential instruments onboard a satellite for attitude determination. However, most commercially available star sensors are expensive and bulky to be used in small satellite missions. StarberrySense is a configurable system -- it can operate as an imaging camera, a centroiding camera, or as a star sensor. We describe the algorithms implemented in the sensor, its assembly and calibration. This payload was selected by a recent Announcement of Opportunity call for payloads to fly on the PS4-Orbital Platform by the Indian Space Research Organization (ISRO).

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arXiv2022

Study of UV bright sources in globular cluster NGC 4590 using Ultraviolet Imaging Telescope (UVIT) observations

We have studied ultraviolet (UV) bright sources in the Galactic globular cluster (GGC) NGC 4590 using Ultraviolet Imaging Telescope (UVIT) on-board the \mbox{{\em AstroSat}} satellite. Using UV-optical color-magnitude diagrams (CMDs), we have identified and characterized the sources of different evolutionary stages i.e., blue horizontal branch stars (BHBs), extremely blue horizontal branch stars (EHBs), blue straggler stars (BSs), variable stars, etc. We estimated effective temperature (T$_{\mathrm{eff}}$), gravity ($\log$(g)), luminosity (L$_{bol}$), and hence the radius (R) of these hot stars by fitting spectral energy distribution (SED) with the help of stellar atmosphere models. Two new far-UV (FUV) bright cluster member stars situated near the core of the cluster have been detected; one of them is an EHB star and the other one is either in its post-blue hook evolutionary phase or in white dwarf phase. The evolutionary status of all the hot stars, identified in the cluster, has been investigated by using various evolutionary models. We find the massive and younger BSs are concentrated at the center of the cluster whereas the older and less massive BSs are distributed though out the cluster. The BSs normalized radial distribution seems to be bi-modal with a minimum located at r$_{\mathrm{min}}$ = 4.3 r$_c$. We calculated A$^+$ parameter of the cluster which is obtained using cumulative normalized radial distribution of horizontal branch stars (HBs) and BSs. We measured this value up to half-mass radius of the cluster to be $+ 0.13$, which indicates that NGC 4590 is one of the youngest clusters among dynamically intermediate age GGCs with a dynamical age of $0.423\pm0.096$ Gyr.

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arXiv2022

Time variations in the UV background

We have found variations in the diffuse ultraviolet background on the scale about 10 days over the 10 year life of the GALEX mission. These variations are only apparent in the near-ultraviolet band of GALEX and are most likely related to variations in the zodiacal light due to brightness variations of the solar NUV flux. The variations can be as high as 200 photon units in the NUV, amounting to about 15 % of the zodiacal light or < 10 % of the total NUV diffuse radiation. There is no effect on the far-ultraviolet band of GALEX. Further work will require better observations chosen specifically for the purpose of investigating this component and will be difficult to obtain.

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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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arXiv2021

Comparing the Inner and Outer Star Forming Complexes in the Nearby Spiral Galaxies NGC 628, NGC 5457 and NGC 6946 using UVIT Observations

We present a far-UV (FUV) study of the star-forming complexes (SFCs) in three nearby galaxies using the Ultraviolet Imaging Telescope (UVIT). The galaxies are close to face-on and show significant outer disk star formation. Two of them are isolated (NGC 628, NGC 6946), and one is interacting with distant companions (NGC 5457). We compared the properties of the SFCs inside and outside the optical radius (R$_{25}$). We estimated the sizes, star formation rates (SFRs), metallicities, and the Toomre Q parameter of the SFCs. We find that the outer disk SFCs are at least ten times smaller in area than those in the inner disk. The SFR per unit area ($\Sigma_{SFR}$) in both regions have similar mean values, but the outer SFCs have a much smaller range of $\Sigma_{SFR}$. They are also metal-poor compared to the inner disk SFCs. The FUV emission is well correlated with the neutral hydrogen gas (\HI) distribution and is detected within and near several \HI~holes. Our estimation of the Q parameter in the outer disks of the two isolated galaxies suggests that their outer disks are stable (Q$>$1). However, their FUV images indicate that there is ongoing star formation in these regions. This suggests that there may be some non-luminous mass or dark matter in their outer disks, which increases the disk surface density and supports the formation of local gravitational instabilities. In the interacting galaxy, NGC 5457, the baryonic surface density is sufficient (Q$<$1) to trigger local disk instabilities in the outer disk.

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arXiv2021

Testing a Prototype 1U CubeSat on a Stratospheric Balloon Flight

High-altitude balloon experiments are becoming very popular among universities and research institutes as they can be used for testing instruments eventually intended for space, and for simple astronomical observations of Solar System objects like the Moon, comets, and asteroids, difficult to observe from the ground due to atmosphere. Further, they are one of the best platforms for atmospheric studies. In this experiment, we build a simple 1U CubeSat and, by flying it on a high-altitude balloon to an altitude of about 30 km, where the total payload weighted 4.9 kg and examine how some parameters, such as magnetic field, humidity, temperature or pressure, vary as a function of altitude. We also calibrate the magnetometer to remove the hard iron and soft iron errors. Such experiments and studies through a stratospheric balloon flights can also be used to study the performance of easily available commercial sensors in extreme conditions as well. We present the results of the first flight, which helped us study the functionality of the various sensors and electronics at low temperatures reaching about -40 degrees Celsius. Further the motion of the payload has been tracked throughout this flight. This experiment took place on 8 March 2020 from the CREST campus of the Indian Institute of Astrophysics, Bangalore. Using the results from this flight, we identify and rectify the errors to obtain better results from the subsequent flights.

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