Indian Institute of Astrophysics
  • Bengaluru, Karnataka, India
Recent publications
We present the results obtained from our campaign to characterize the intra-night optical variability properties of blazars detected by the Fermi large area telescope. This involves R-band monitoring observations of a sample of 18 blazars, which includes five flat spectrum radio quasars (FSRQs) and thirteen BL Lac objects (BL Lacs) covering the redshift range z=0.085-1.184. Our observations carried out using the 1.3m J. C. Bhattacharya telescope cover a total of 40 nights (∼200 h) between the period December 2016 and March 2020. We characterized variability using the power enhanced F-test. We found duty cycle (DC) variability of about 11% for FSRQs and 12% for BL Lacs. Dividing the sample into different sub-classes based on the position of the synchrotron peak in their broadband spectral energy distribution (SED), we found DC of ∼16%, ∼10% and ∼7% for low synchrotron peaked (LSP), intermediate synchrotron peaked (ISP) and high synchrotron peaked (HSP) blazars. Such high DC of variability in LSP blazars could be understood in the context of the R-band tracing the falling part (contributed by high energy electrons) of the synchrotron component of the broadband SED. Also, the R-band tracing the rising synchrotron part (produced by low energy electrons) in the case of ISP and HSP blazars, could cause lesser variability in them. Thus, the observed high DC of variability in LSP blazars relative to ISP and HSP blazars is in accordance with the leptonic model of emission from blazar jets.
  • Anjasha GangopadhyayAnjasha Gangopadhyay
  • Kuntal MisraKuntal Misra
  • Koji KawabataKoji Kawabata
  • [...]
  • Mridweeka SinghMridweeka Singh
Interacting supernovae (SNe) IIn and Ibn show narrow emission lines and have always been a mysterious and unsolved genre in SNe physics. We present a comprehensive analysis of the temporal and spectroscopic behavior of a group of interacting SNe IIn and Ibn. We choose SNe 2012ab, 2020cui, 2020rc and 2019uo as representative members of these SN sub-types to probe the nature of explosion. Our study reveals that SNe IIn are heterogeneous, bright depicting multi-staged temporal evolution while SNe Ibn are moreover homogeneous, comparatively fainter than SNe IIn and short lived, but limited in a sample to firmly constrain the homogeneity. The spectroscopic features display a great diversity in H\(\alpha \) and He profiles for both SNe IIn and Ibn. The representative SN Ibn also show flash ionization signatures of CIII and NIII. Modeling of H\(\alpha \) reveals that SNe IIn have, in general, an asymmetric CSM which interacts with SN ejecta resulting in diversity in H\(\alpha \) profiles.
  • K UjjwalK Ujjwal
  • Sreeja S KarthaSreeja S Kartha
  • Smitha SubramanianSmitha Subramanian
  • [...]
  • Blesson MathewBlesson Mathew
Secular and environmental effects play a significant role in regulating the star formation rate and hence the evolution of the galaxies. Since UV flux is a direct tracer of the star formation in galaxies, the UltraViolet Imaging Telescope (UVIT) onboard ASTROSAT enables us to characterize the star forming regions in a galaxy with its remarkable spatial resolution. In this study, we focus on the secular evolution of NGC 628, a spiral galaxy in the local universe. We exploit the resolution of UVIT to resolve up to ∼ 63 pc in NGC 628 for identification and characterization of the star forming regions. We identify 300 star forming regions in the UVIT FUV image of NGC 628 using ProFound and the identified regions are characterized using Starburst99 models. The age and mass distribution of the star forming regions across the galaxy supports the inside-out growth of the disk. We find that there is no significant difference in the star formation properties between the two arms of NGC 628. We also quantify the azimuthal offset of the star forming regions of different ages. Since we do not find an age gradient, we suggest that the spiral density waves might not be the possible formation scenario of the spiral arms of NGC 628. The headlight cloud present in the disk of the galaxy is found to be having the highest star formation rate density (0.23M⊙yr−1kpc−2) compared to other star forming regions on spiral arms and the rest of the galaxy.
Using time-sequence vector magnetic field and coronal observations from Solar Dynamics Observatory, we report the observations of the magnetic field evolution and coronal activity in four emerging active regions (ARs). The ARs emerge with leading polarity being the same as for the majority of ARs in a hemisphere of solar cycle 24. After emergence, the magnetic polarities separate each other without building a sheared polarity inversion line. In all four ARs, the magnetic fields are driven by foot point motions such that the sign of the helicity injection (dH/dt) in the first half of the evolution is changed to the opposite sign in the later part of the observation time. This successive injection of opposite helicity is also consistent with the sign of mean force-free twist parameter (αav). Further, the EUV light curves of the ARs in 94Å and GOES X-ray flux reveal flaring activity below C-class magnitude. Importantly, the white-light coronagraph images in conjunction with the AR images in AIA 94 Å delineate the absence of associated CMEs with the studied ARs. These observations imply that the ARs with successive injection of opposite sign magnetic helicity are not favorable to twisted flux rope formation with excess coronal helicity, and therefore are unable to launch CMEs, according to recent reports. This study provides the characteristics of helicity flux evolution in the ARs referring to the conservative property of magnetic helicity and more such studies would help to quantify the eruptive capability of a given AR.
This paper is dedicated to the memory of Jean Rösch, a great figure in astronomy in the years 1947–1981 who designed, among several innovative devices, a 15-cm spectro-coronagraph. This instrument was installed at Pic du Midi observatory (south-west France), was in use during the mid-60s, fully dedicated to the observation from the ground of the coronal highly ionized iron lines, which was a true challenge at that time. This program is here reconsidered in the context of the time, at Pic du Midi observatory, which has been the cradle of routine visual coronal observations initiated by Bernard Lyot. We take advantage of this review to underline that the goals and objectives of this ground-based coronal program are taken over since 2008, by an Indian team from Bangalore (Indian Institute of Astrophysics), through a space mission (ADITYA-L1 or Sun in Sanskrit), showing a-posteriori the very innovative aspects developed with the help of this 15-cm spectro-coronagraph and thanks to the skills of J. Rösch’s collaborators.
Galactic black hole candidate MAXI J1910-057/Swift J1910.2-0546 was simultaneously discovered by MAXI/GSC and Swift/BAT satellites during its first outburst in 2012. We study the detailed spectral and temporal properties of the source in a broad energy range using archival data from Swift/XRT, MAXI/GSC, and Swift/BAT satellites/instruments. Low frequency quasi periodic oscillations are observed during the outburst. The combined 1-50 keV spectra are analyzed using the transonic flow solution based Two Component Advective Flow (TCAF) model. Based on the variations of soft and hard X-ray fluxes, their hardness ratios and the variations of the spectral model fitted parameters, we find that the source has evolved through six spectral states. We interpret this spectral state evolution to be a result of the release of the leftover matter from the pile-up radius due to a sudden rise of viscosity causing a rebrightening. We show a possible configuration of the evolution of accretion flow during the outburst. From the spectral analysis with TCAF model, we estimate the probable mass of the black hole to lie in the range 6.31 M⊙ to 13.65 M⊙, and the source distance is estimated to be 1.9-8.3 kpc from transition luminosity considerations.
Observing the ultraviolet (UV) sky for time-varyiable 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∘) 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–Chrétien (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.
Parker Solar Probe observations show ubiquitous magnetic-field reversals closer to the Sun, often referred to as “switchbacks”. The switchbacks have been observed before in the solar wind near 1 AU and beyond, but their occurrence was historically rare. PSP measurements below ∼ 0.2 AU show that switchbacks are, however, the most prominent structures in the “young” solar wind. In this work, we analyze remote-sensing observations of a small equatorial coronal hole to which PSP was connected during the perihelion of Encounter 1. We investigate whether some of the switchbacks captured during the encounter were of coronal origin by correlating common switchback in situ signatures with remote observations of their expected coronal footpoint. We find strong evidence that timescales present in the corona are relevant to the outflowing, switchback-filled solar wind, as illustrated by strong linear correlation. We also determine that spatial analysis of the observed region is optimal, as the implied average solar-wind speed more closely matches that observed by PSP at the time. We observe that hemispherical structures are strongly correlated with the radial proton velocity and the mass flux in the solar wind. The above findings suggest that a subpopulation of the switchbacks are seeded at the corona and travel into interplanetary space.
The diagnostic capabilities of spectral lines in the far-ultraviolet (FUV) and extreme-ultraviolet (EUV) wavelength ranges are explored in terms of their Hanle and Zeeman sensitivity to probe the vector magnetic fields in the solar corona. The temperature range covered is log(10T)=5.5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$_{10}(T)=5.5$\end{document} – 6.3. The circular-polarization signal due to longitudinal Zeeman effect is estimated for spectral lines in the wavelength range of 500 to 1600 Å. The Stokes V/I\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$V/I$\end{document} signal for a FUV line is found to be of the order of 10⁻⁴ for a longitudinal field strength of 10 gauss, which further reduces to 10⁻⁵ for wavelengths below 1200 Å. Due to such low signals, the present study aims to find combinations of spectral lines having different Hanle sensitivity but with identical peak formation temperature to probe the coronal magnetic-field vector. The combination of Hanle-sensitive lines is better suited because the Hanle signals are stronger by at least an order of magnitude compared to Zeeman signals. The linear polarization signals due to the Hanle effect from at least two spectral lines are required to derive information on the full vector. It is found from this study that there is always a pair of Hanle-sensitive lines for a given temperature range suitable for probing the coronal vector magnetic field and they are located in close proximity to each other in terms of their wavelength.
Corona is the outermost layer of the Sun, which extends to several thousand kilometers from the visible photosphere. It is made up of very tenuous plasma but is very hot. The sudden increase of temperature in the coronal layer from the underlying chromosphere and photosphere makes it very interesting. The average temperature of a corona is measured about 2 MK, and it may show a large variation in temperature with respect to different surface features. Studying the variation of the temperature of the full-disk corona and of individual feature’s temperature along with the solar cycle will be interesting and important to understand the Physics of the Sun. Although an attempt has been made earlier to measure the temperature of coronal XBPs for a short period with high cadence of observations in localized regions, but the variation in temperature of the full-disk image and of individual features over the solar cycle is not measured yet. Since Hinode/XRT allows capturing images of the Sun in 8 different filters, it has got a unique feature that, using 2 different filters, the temperature map of the Sun can be generated. In order to study the temperature variations, we have used Al mesh and Ti poly filters of full-disk composite, level-2, X-ray images of the Sun obtained from Hin- ode/XRT. We developed a sophisticated python algorithm to segment different coronal features (ARs, CHs, BGs, & XBPs), derived the integrated intensity of all the features in both the filters, and generated the temperature maps of the corona using the filter ratio method. Because of the XRT straylight issue and unavailability of a good pair of images we have restricted our analysis for the period of 4 years (February 01, 2008 - May 08, 2012, covering the starting part of the solar cycle 24). In this paper, the first analysis in using direct energy values of the coronal features from segmented solar disk and its relation to solar activity is presented. We have discussed the variations of the temperature of a full-disk corona, and of all the features (ARs, CHs, BGs & XBPs).We found from the time series plots of the average temperature of the full-disk and of all the features show temperature fluctuations and they vary in phase with the sunspot numbers (solar activity). Although the temperature of all the features varies but the mean temperature estimated for the whole observed period of the full-disk is around 1.39 ± 0.28 MK and active regions (ARs) will be around 1.98 ± 0.44 MK, whereas BGs, CHs & XBPs are 1.37 ± 0.26 MK, 1.34 ± 0.33 MK, and 1.52± 0.34 MK respectively. In addition, we found that the mean temperature contribution estimated of the background regions (BG) will be around 91 %, whereas ARs, CHs, & XBPs are 5 %, 2 % and 2 % respectively to the average coronal temperature of the full-disk for the period: 2008-2012. The temperature values and their variations of all the features suggest that the features show a high variability in their temperature and that the heating rate of the emission features may be highly variable on solar cycle timescales. It is clear from the analysis that the filter ratio method can be directly used for temperature analysis of coronal features and to study their surface temperature variability as a function of solar magnetic activity.
Spectroscopic properties of magnetic dipolar and quadrupolar transitions (M1, M2) of He atom encapsulated under neutral and charged C60 fullerene cage have been estimated here with a view to understand the effect of cage confinement on the ground state energy (E), ionization potential (IP), excitation energies (ΔE) and transition probabilities (TP) of the low lying transitions 1s2:1Se→1sns:3Se1 and 1s2:1Se→1snp: 3Po2 (n = 2,3,4,5). The methodology adopted is the time dependent coupled Hartree-Fock (TDCHF)theory within a variational scheme which incorporates partial correlation effects. The effect of encapsulation has been introduced through an effective potential in the one particle potential part due to the fullerene cage. Interesting pattern of the relevant properties of the confined He atom has been observed depending upon overall charge of the fullerene cage.
We study flux and spectral variability of the high energy peaked TeV blazar PG 1553 + 113 on diverse timescales using the data collected from 2005 to 2019 which also includes the intensive intra-night monitoring of the target. Additionally, we recorded the brightest flare of the blazar PG 1553 + 113 during April 2019 when the source attained an R-band magnitude of 13.2. Analyzing the spectral evolution of the source during the flare gave a clockwise spectral hysteresis loop and a time lag with V-band variations leading to the R-band ones. Various statistical tests, fitting procedures and cross-correlation techniques are applied to search for periodicity and examine the color-magnitude relationship. We find a median period of (2.21 ± 0.04) years along with the secondary period of about 210 days. Finally, we briefly discuss various physical mechanisms which are capable of explaining our findings.
The photometric accuracy in the near-infrared (NIR) wavelength range (0.9–2.6 μm) is strongly affected by the variability of atmospheric transmission. The Infrared Working Group (IRWG) has recommended filters that help alleviate this issue and provide a common standard of NIR filtersets across different observatories. However, accurate implementation of these filters are yet to be available to astronomers. In the meantime, InGaAs based detectors have emerged as a viable option for small and medium telescopes. The present work explores the combination of IRWG filtersets with InGaAs detectors. A few commercially available filtersets that approximate the IRWG profile are compared. Design of more accurate IRWG filtersets suitable for the InGaAs sensitivity range is undertaken using an open-source filter design software – OpenFilters. Along with the photometric filters iZ, iJ and iH, design of a few useful narrow band filters is also presented. These filters present opportunities for small and medium telescopes for dedicated long-term observation of interesting infrared sources.
The UBVRI CCD photometric data of open star cluster NGC 1513 are obtained with the 3.6-m Indo-Belgian Devasthal optical telescope (DOT). Analyses of the GAIA EDR3 astrometric data have identified 106 possible cluster members. The mean proper motion of the cluster is estimated as μαcosδ=1.29±0.02 and μδ=-3.74±0.02 mas yr-1. Estimated values of reddening E(B-V) and distance to the NGC 1513 are 0.65±0.03 mag and 1.33±0.1 kpc, respectively. Age of 225±25 Myr is assigned to the cluster by comparing theoretical isochrones with deeply observed cluster sequences. Using observations taken with the 3.6-m DOT, values of distance and age of the galactic globular cluster NGC 4147 are estimated as 18.2±0.2 Kpc and 14±2 Gyr, respectively. The optical observations of planetary transit around white dwarf WD 1145+017 and K-band imaging of star-forming region Sharpless Sh 2-61 demonstrate observing capability of 3.6-m DOT. Optical and near-infrared observations of celestial objects and events are being carried out routinely with the 3.6-m DOT. They indicate that the performance of the telescope is at par with those of other similar telescopes located elsewhere in the world. We, therefore, state that this observing facility augurs well for multi-wavelength astronomy including the study of astrophysical jets.
An observatory class national large optical-IR telescope (NLOT), is proposed to be built and located in the country. The telescope consists of a 10–12 m segmented primary. In order to cater to a diversity of observational programs, the telescope is designed with high throughput in both the optical and IR regions (0.3–5 μm). It should perform reasonably well up to 30 μm. The telescope and instruments should have remote operations capability, allowing for the queue as well as classical scheduling and high reliability and robustness. This article provides a brief description of the science cases that drive the telescope requirements, activities related to optics design and some thoughts on the instruments.
The presence of dark matter (DM), though well established by indirect evidence, is yet to be observed directly. Various DM detection experiments running for several years have yielded no positive results. In view of these negative results, we had earlier proposed alternate models by postulating a minimum gravitational field strength (minimum curvature) and a minimum acceleration. These postulates led to the modified Newtonian dynamics and modified Newtonian gravity (MONG). The observed flat rotation curves of galaxies were also accounted for through these postulates. Here, we extend these postulates to galaxy clusters and model the dynamical velocity-distance curve for a typical cluster such as the Virgo cluster. The radial velocities of galaxies in the Virgo cluster are also obtained through this model. Observations show an inconsistency in the Hubble flow at a mean cluster distance of 17 Mpc, which is expected in regions of high matter density. This decrease in velocity is predicted by our model of modified gravity (MONG). The radial velocity versus distance relation for galaxies in the Virgo cluster obtained using MONG is in agreement with observations.
We study the relativistic, inviscid, advective accretion flow around the black holes and investigate a key feature of the accretion flow, namely the shock waves. We observe that the shock-induced accretion solutions are prevalent and such solutions are commonly obtained for a wide range of the flow parameters, such as energy (${\cal E}$) and angular momentum (λ), around the black holes of spin value 0 ≤ ak < 1. When the shock is dissipative in nature, a part of the accretion energy is released through the upper and lower surfaces of the disc at the location of the shock transition. We find that the maximum accretion energies that can be extracted at the dissipative shock ($\Delta {\cal E}^{\rm max}$) are $\sim 1\%$ and $\sim 4.4\%$ for Schwarzschild black holes (ak → 0) and Kerr black holes (ak → 1), respectively. Using $\Delta {\cal E}^{\rm max}$, we compute the loss of kinetic power (equivalently shock luminosity, Lshock) that is enabled to comply with the energy budget for generating jets/outflows from the jet base (i.e., post-shock flow). We compare Lshock with the observed core radio luminosity (LR) of black hole sources for a wide mass range spanning 10 orders of magnitude with sub-Eddington accretion rate and perceive that the present formalism seems to be potentially viable to account LR of 16 Galactic black hole X-ray binaries (BH-XRBs) and 2176 active galactic nuclei (AGNs). We further aim to address the core radio luminosity of intermediate-mass black hole (IMBH) sources and indicate that the present model formalism perhaps adequate to explain core radio emission of IMBH sources in the sub-Eddington accretion limit.
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Prof. Jayant Murthy
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