arXiv subjects

Indrani Pal

Publications and source records attributed to Indrani Pal.

18 recordsLinked to original sources

arXiv2026

X-ray Absorption Variability in NGC 1142: Another Constraint on the Nature of the Torus/Broad-Line Region in Active Galactic Nuclei

The physics, geometry, and kinematics of the gas and dust surrounding the central engine of active galactic nuclei (AGN) are not well understood. In some sources, this material obscures the coronal X-ray emission, allowing its line-of-sight column density ($N_H$) to be measured. Many sources display $N_H$ variability across multiple epochs. Studying this variability can provide information about the distribution and dynamics of the obscuring material. Particularly, previous works have shown that the probability of observing $N_H$ variability increases with the number of observations available. Additionally, the observed magnitudes of the $N_H$ variability depend on the timescale between observations. In this work, we present the $N_H$ variability analysis of NGC 1142, which has a total of nine X-ray observations over $\sim20$ years, including five from a NuSTAR+XMM-Newton monitoring campaign performed in 2024-2025 and reported here for the first time. We use the physically-motivated torus models UXCLUMPY, XSKIRTor, and RXTorusD to model the spectra. Our results confirm the previously observed trends, and we compare these trends to simulated obscuration curves assuming different locations and distributions of obscuring clouds.

astro-ph.GA↗

arXiv2026

The hidden variability of the torus in local Active Galactic Nuclei: 20 years of Chandra, XMM-Newton, and NuSTAR observations

X-ray absorption variability in active galactic nuclei (AGN) provides key constraints on the structure and dynamics of the circumnuclear obscuring medium, the so-called torus. A fraction of nearby AGN, however, have been classified as non-variable in line-of-sight (LoS) column density based on limited temporal coverage. We present the first systematic study of a sample of 11 local (z $\leq$ 0.1) obscured (N$_{\rm H} \geq 10^{22}$ cm$^{-2}$) AGN, initially classified as non-variable. The sample is selected from the Swift-BAT 100-month catalog, and comprises 60 observations from Chandra, XMM-Newton, and NuSTAR, spanning timescales from days to nearly two decades. We simultaneously model all available spectra for each source adopting physically motivated torus models: X-skirtor, RXTorusD, and UXCLUMPY. This approach allows us to derive the global properties of the obscurer while tracking possible epoch-to-epoch variations in the LoS column density and intrinsic X-ray emission. We find that the original non-variable classification (based on only two X-ray observations) is frequently not robust: clear N$_{\rm H,LoS}$ variability is detected in half of the sample, whereas 7 out of 10 AGN require intrinsic flux variability, with the rest showing flux-N$_{\rm H,LoS}$ degeneracies. We also find that the probability of identifying absorption variability increases with the number of observations, and the largest column density changes preferentially occur on long timescales, consistent with absorption by extended, structured clouds on torus scales. These findings support a clumpy and dynamic obscuring medium as a common feature of nearby AGN and highlight the importance of long-term X-ray monitoring for accurately characterizing AGN obscuration.

astro-ph.GA↗

arXiv2026

Compton-thick AGN in the NuSTAR Era. XI. Analyzing 11 CT-AGN Candidates Selected with Machine Learning

This work discusses the broadband X-ray spectral analysis of 11 candidate heavily-obscured active galactic nuclei (AGN) selected based on their infrared and X-ray properties by a recently published machine learning algorithm. This paper is part of a larger work to identify and characterize all AGN in the local universe (z < 0.1) with the largest line-of-sight (los) column densities (NH), the so-called Compton-thick (CT-, NH,los >= 1024 cm-2) AGN. We modeled the X-ray spectra using two physically- motivated models, UXClumpy and RXTorusD. Of the 11 AGN in our sample, we found three to be obscured with 22.7 < LogNH,los <= 23.0, five have 23.0 < LogNH,los <= 23.25, and three have 23.4 < LogNH,los <= 23.9, according to UXClumpy. Meanwhile, according to RXTorusD, we found three AGN to be obscured with 22.7 < LogNH,los <= 23.0, four with 23.0 < LogNH,los <= 23.4, and four with 23.85 < LogNH,los <= 23.96. Additionally, this work served as a comparison between UXClumpy and RXTorusD. We found broad agreement between the two, with 8/11 sources agreeing on the value of the photon index Gamma, while only 5/11 sources agreeing on the NH,los value within the 90% confidence level.

astro-ph.GA↗

arXiv2025

Unveiling Obscured Accretion in the Local Universe

Heavily obscured Active Galactic Nuclei (AGN), especially Compton-thick sources with line-of-sight column density ($N_{\rm H,los}$) $>$ 10$^{24}$ cm$^{-2}$, are critical to understanding supermassive black hole (SMBH) growth and the origin of the Cosmic X-ray Background (CXB). However, their observed fraction remains significantly below model predictions, due to strong absorption bias, even in the hard X-ray (i.e., above 10 keV) band. We analyze a sample of 26 nearby ($z < 0.1$) AGN from the Swift-BAT 150-month catalog, selected via mid-IR to X-ray diagnostics and observed with NuSTAR and soft X-ray telescopes (Xmm-Newton, Chandra, or Swift-xrt). Using self-consistent torus models (MyTorus, Borus02, and UXCLUMPY), we aim to constrain $N_{\rm H,los}$, the average torus column density, and other geometrical parameters of the obscuring medium. A comparative analysis among the three torus models showed that while estimates of $N_{\rm{H,los}}$ were generally in agreement, Borus02 tended to classify a slightly larger number of sources as Compton-thick AGN (CT-AGN). Building on this comparison, we benchmark two prediction schemes -- a mid-IR/X-ray relation and a machine-learning model -- against our broadband best-fit $N_{\rm H,los}$ measurements to assess which approach more effectively bridges the gap between predicted and measured obscuration, finding that while the former works effectively in the heavily obscured region (log$\rm{N_H} \gtrsim$ 23.5 $\rm{cm^{-2}}$), the latter provides improved accuracy, particularly for Compton-thin to moderately thick regimes (log$\rm{N_H} \lesssim$ 23.5 $\rm{cm^{-2}}$).

astro-ph.HE↗

arXiv2025

A multi-wavelength view of the multi-messenger sources NGC 1068 and PKS 1502+1061

Multi-messenger astronomy offers a powerful approach to studying high-energy radiative processes in astrophysical sources. A notable example was seen in 2017, when the IceCube Neutrino Observatory detected a high-energy neutrino event that was found to coincide with a gamma-ray flare from a blazar. Since then, numerous multi-messenger studies combining neutrino and photon data have been conducted, yet the origin of neutrinos from active galactic nuclei (AGN) remains uncertain. In this work, we present the results of an X-ray observing program targeting two AGNs, NGC 1068 and PKS 1502+106. The multi-wavelength dataset includes new observations from NICER and NuSTAR from the observing proposal along with gamma-ray data collected using Fermi-LAT, and one archival observation from Chandra. Additionally, we derive the neutrino fluxes for both AGNs using ten years of IceCube data and neutrino spectra predicted by theoretical models. These results demonstrate the value of combining multi-messenger data in building and constraining theoretical models. They also highlight the importance of testing model predictions against observational data to refine measurements of both the neutrino flux and spectral shape.

astro-ph.HE↗

arXiv2025

A systematic search for AGN obscuration variability in the Chandra archive

The nature of the obscuring material in active galactic nuclei (AGN) can be studied by measuring changes in the line-of-sight column density, $N_{\rm H,los}$, over time. This can be accomplished by monitoring AGN over long periods of time and at all timescales. However, this can only be done for a few selected objects as it is resource intensive. Therefore, the best option currently is to focus on population statistics based on the available archival data. In this work, we study 79 Seyfert 1 and Seyfert 2 galaxies from the Milliquas catalog to estimate a lower limit on the fraction of sources in the local $(z<0.1)$ universe that display spectral variability among observations. We find that 43 sources $(54\pm11\%)$, show indications of $N_{\rm H,los}$ variability at 90% confidence level. Interestingly, we also find that the variable fraction is similar for both Seyfert 1 $(f_{\rm Sy1}\sim61^{+13}_{-15}\%)$ and Seyfert 2 $(f_{\rm Sy2}\sim47\pm15\%)$ galaxies. The slightly higher $f_{\rm Sy1}$ fraction could be due to either a physical difference in the obscurers or the higher data quality in the Sy1 population. We also search for potential dependencies on the timescale between variable and non-variable observation pairs within a given source. In agreement with previous studies, we find evidence that more variability occurs on longer timescales than on shorter timescales. We present the 43 variable sources as a promising sample for future $N_{\rm H}$ variability studies.

astro-ph.GA↗

arXiv2025

Contemporaneous X-ray and Optical Polarization of EHSP Blazar H 1426+428

We present the first contemporaneous X-ray and optical polarimetric measurement of the extremely high synchrotron peaked (EHSP) blazar H 1426+428. The X-ray polarimetric observations were undertaken using the Imaging X-ray Polarimetry Explorer (\textit{IXPE}) on 2024 May 27, and 2024 July 5. The \textit{IXPE} pointings were accompanied by contemporaneous optical observations of the Observatorio de Sierra Nevada, Calar Alto Observatory and the Perkins Telescope Observatory. While we observed the X-ray degree of polarization to be $>20\%$, the polarization in the optical band was found to be only $1-3\%$. This trend has been observed in several HSP blazars with available optical and X-ray polarimetric data and is typically explained in terms of energy stratification downstream of a shock. However, we observed a significant difference between the optical and X-ray polarization angles, a feature that has been observed in certain HSP blazars, such as Mrk 421, but remains a relatively rare or underreported phenomenon. We discuss possible scenarios for these findings within the framework of a partially turbulent jet model.

astro-ph.HE↗

arXiv2025

Chandra Follow-up Observations of Swift-BAT-selected AGNs III

The cosmic X-ray background (CXB) is dominated by the obscured and unobscured coronal light of active galactic nuclei (AGN). At energies below 10 keV, the CXB can be well explained by models taking into account the known AGN and the observed distribution of their obscuring, line-of-sight column densities, $N_{\rm H,l.o.s}$. However, at energies around the Compton reflection hump ($\sim30$ keV), the models fall short of the data. This suggests the existence of a population of as yet undetected Compton-thick AGN ($N_{\rm H,l.o.s}>1.5\times10^{24}$ cm$^{-2}$) whose X-ray spectra are dominated by the light that has been reprocessed by the obscuring material. In this work, we continue the effort to find and catalog all local ($z<0.05$) Compton-thick (CT) AGN. To this end, we obtained soft X-ray data with Chandra for six local BAT detected sources lacking ROSAT (0.1-2.4 keV) counterparts, indicating potential obscuration. We fit their spectra with Bayesian and least squares methods using two different models, borus02 and UXCLUMPY. We compare the results of the different models and methods and find that the $N_{\rm H,l.o.s}$ is consistently measured in each case. Three of the sources also were observed with XMM-Newton allowing the opportunity to search for variability in soft X-ray flux or $N_{\rm H,l.o.s}$. From this sample, we find one strong CT candidate (NGC 5759) and one weaker CT candidate (CGCG 1822.3+2053). Furthermore, we find tentative evidence of $N_{\rm H,l.o.s}$ variability in 2MASX J17253053-4510279, which has $N_{\rm H,l.o.s}<10^{22}$ cm$^{-2}$.

astro-ph.HE↗

arXiv2023

Detection of X-ray Polarization in the High Synchrotron Peaked Blazar 1ES 1959+650

We report the measurement of X-ray polarization in the high synchrotron peaked blazar 1ES 1959+650. Of the four epochsof observations from the {\it Imaging X-ray Polarimetry Explorer}, we detected polarization in the 2$-$8 keV band on two epochs. From model-independent analysis of the observations on 28 October 2022, in the 2$-$8 keV band, we found the degree of polarization of $\Pi_X$ = 9.0 $\pm$ 1.6\% and an electric vector position angle of $\psi_X$ = 53 $\pm$ 5 deg. Similarly, from the observations on 14 August 2023, we found $\Pi_X$ and $\psi_X$ values as 12.5 $\pm$ 0.7\% and 20 $\pm$ 2 deg, respectively. These values are also in agreement with the values obtained from spectro-polarimetric analysis of the I, Q, and U spectra. The measured X-ray polarization is larger than the reported values in the optical, that ranges between 2.5$-$9\% , when observed during 2008 to 2018. Broadband spectral energy distribution constructed for the two epochs are well described by the one zone leptonic emission model with the bulk Lorentz factor ($\Gamma$) of the jet larger on 14 August 2023 compared to 28 October 2022. Our results favour shock acceleration of the particles in the jet, with the difference in $\Pi_X$ between the two epochs being influenced by change in the $\Gamma$ of the jet.

astro-ph.HE↗

arXiv2023

On the properties of X-ray corona in Seyfert 1 galaxies

We carried out a uniform and systematic analysis of a sample of 112 nearby bright Seyfert 1 type AGN, the observations of which were carried out by the Nuclear Spectroscopic Telescope Array (NuSTAR) between August 2013 and May 2022. The main goal of this analysis is to investigate the nature of the X-ray corona in Seyfert 1 galaxies. By fitting a physical model to the {\it NuSTAR} spectra, we were able to constrain the high-energy cut-off ($\rm{E_{cut}}$) for 73 sources in our sample. To estimate the temperature of the corona ($\rm{kT_{e}}$) in our sample of 112 sources, we used the Comptonization model to fit their spectra. We could constrain $\rm{kT_{e}}$ in 42 sources. We found a strong positive correlation between $\rm{E_{cut}}$ and $\rm{kT_{e}}$ with most of the sources lying above the empirical approximation of $\rm{E_{cut}}$ = 2$-$3 $\rm{kT_{e}}$. We investigated for possible correlations between various properties of the corona obtained from physical model fits to the observed spectra and between various coronal parameters and physical properties of the sources such as Eddington ratio and black hole mass. We found (a) a strong correlation between $\rm{E_{cut}}$ and the photon index and (b) a significant negative correlation between $\rm{kT_{e}}$ and the optical depth. From detailed statistical analysis of the correlation of coronal parameters with the Eddington ratio and black hole mass, we found no significant correlation. The correlations observed in this study indicate that an optically thin corona is needed to sustain a hotter corona with a steeper spectrum.

astro-ph.HE↗

arXiv2023

X-ray polarization observations of IC 4329A with IXPE: Constraining the geometry of the X-ray corona

X-ray polarimetry is a powerful tool to probe the geometry of the hot X-ray corona in active galactic nuclei (AGN). Here, we present our results on the characterisation of the X-ray polarization of the radio-quiet Seyfert-type AGN IC 4329A at a redshift of $z$ = 0.016. This is based on observations carried out by the {\it Imaging X-ray Polarimeter (IXPE)}. {\it IXPE} observed IC 4329A on January 5, 2023, for a total observing time of 458 ks. From the model-independent analysis, we found a polarization degree ($Π_{X}$) of 3.7$\pm$1.5$\%$ and a polarization position angle ($Ψ_{X}$) of 61$^{\circ}$$\pm$12$^{\circ}$ in the 2$-$8 keV energy range (at 1$σ$ confidence level). This is also in agreement with the values of $Π_{X}$ and $Ψ_{X}$ of 4.7$\pm$2.2$\%$ and 71$^{\circ}$ $\pm$14$^{\circ}$ respectively obtained from spectro-polarimetric analysis of the I, Q and U Stokes spectra in the 2$-$8 keV energy band (at the 90$\%$ confidence). The value of $Π_X$ in the 2-8 keV band obtained from the model-independent analysis is lower than the minimum detectable polarization (MDP) value of 4.5$\%$. However, $Π_X$ obtained from spectro-polarimetric analysis in the 2-8 keV band is larger than the MDP value. In the 3-5 keV band, we found $Π_X$ of 6.5 $\pm$ 1.8, which is larger than the MDP value of 5.5$\%$. The observed moderate value of $Π_{X}$ obtained from the analysis of the {\it IXPE} data in the 3$-$5 keV band argues against a spherical lamp$-$post geometry for the X-ray corona in IC 4329A; however, considering simulations, the observed polarization measurements tend to favour a conical shape geometry for the corona. This is the first time measurement of X-ray polarization in IC 4329A. Measurements of the X-ray polarization in many such radio-quiet AGN will help in constraining the geometry of the X-ray corona in AGN.

astro-ph.HE↗

arXiv2022

Search for coronal temperature variation in Seyfert galaxies

While the temperature of the X-ray corona ($\rm{kT_e}$) in active galactic nuclei (AGN) are known for many sources, its variation, if any, is limited to a handful of objects. This is in part due to the requirement of good signal-to-noise X-ray spectra covering a wide range of energies. We present here results on the X-ray spectral analysis of 18 Seyferts, having more than one epoch of observations to look for variation in $\rm{kT_e}$. The data for a total of 52 epochs on these 18 AGN were taken from observations carried out by NuSTAR in the 3$-$79 keV energy band. From phenomenological and physical model fits to the multi-epoch data on these 18 sources from {\it NuSTAR}, we could constrain the cut-off energy ($E_{cut}$) in a large fraction of the sources. Also, from Comptonized model fits, we could obtain $\rm{kT_e}$ for our sample. Of the 18 sources, at the 90 per cent confidence level, evidence for variation in $\rm{kT_e}$ was found for only one source, namely MCG+08-11-011. For this source, between two epochs, separated by about five years, we found $\rm{kT_e}$ to decrease from 57$^{+29}_{-16}$ keV to 30$^{+11}_{-7}$ keV. During the same period, the flux decreased from (12.60 to 14.02) $\times$ 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$ and the optical depth increased from 1.68 to 2.73. We thus found a positive correlation between flux and coronal temperature with a reduction of about 40 per cent in optical depth. Our observations tend to favour the vertically outflowing corona scenario for the observed variation in $\rm{kT_e}$ in MCG+08-11-011.

astro-ph.HE↗

arXiv2022

X-ray spectral and timing analysis of the Compton Thick Seyfert 2 galaxy NGC 1068

We present the timing and spectral analysis of the Compton Thick Seyfert 2 active galactic nuclei NGC 1068 observed using {\it NuSTAR} and {\it XMM-Newton}. In this work for the first time we calculated the coronal temperature ($\rm{kT_{e}}$) of the source and checked for its variation between the epochs if any. The data analysed in this work comprised of (a) eight epochs of observations with {\it NuSTAR} carried out during the period December 2012 to November 2017, and, (b) six epochs of observations with {\it XMM-Newton} carried out during July 2000 to February 2015. From timing analysis of the {\it NuSTAR} observations, we found the source not to show any variations in the soft band. However, on examination of the flux at energies beyond 20 keV, during August 2014 and August 2017 the source was brighter by about 20\% and 30\% respectively compared to the mean flux of the three 2012 {\it NuSTAR} observations as in agreement with earlier results in literature. From an analysis of {\it XMM-Newton} data we found no variation in the hard band (2 $-$ 4 keV) between epochs as well as within epochs. In the soft band (0.2 $-$ 2 keV), while the source was found to be not variable within epochs, it was found to be brighter in epoch B relative to epoch A. By fitting physical models we determined $\rm{kT_{e}}$ to range between 8.46$^{+0.39}_{-0.66}$ keV and 9.13$^{+0.63}_{-0.98}$ keV. From our analysis, we conclude that we found no variation of $\rm{kT_{e}}$ in the source.

astro-ph.HE↗

arXiv2022

NuSTAR spectral analysis of three Seyfert galaxies: NGC 3227, NGC 5548 and MR 2251$-$178

The observed nuclear X-ray emission in the radio-quiet category of active galactic nuclei (AGN) is believed to be from a compact region, the corona situated in the vicinity of the central supermassive black holes (SMBH). The shape of the X-ray continuum, among other factors, depends on the temperature of the corona ($\rm{kT_{e}}$). The launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) has led to the determination of the high energy cut-off ($\rm{E_{cut}}$; and thereby $\rm{kT_{e}}$) in many AGN. In a handful of sources, multiple observations with NuSTAR have also revealed changes in $\rm{E_{cut}}$. In this work we aimed to investigate the variation in $\rm{kT_{e}}$ in three AGN, namely NGC 3227, NGC 5548 and MR 2251$-$178 using more than one epoch of data on a source from NuSTAR. We carried out spectral analysis of multiple epochs of data acquired using NuSTAR on the three sources including a few new observations not published so far. By fitting Comptonization model to the data we determined the temperature of the corona and also investigated changes in $\rm{kT_{e}}$ if any in these sources. In NGC 3227, we found evidence for variation in $\rm{kT_{e}}$. We found no correlation of $\rm{kT_{e}}$, photon index ($Γ$), reflection fraction ($R$) and optical depth ($τ$) with flux, while, $τ$ is found to anti-correlate with $\rm{kT_{e}}$. These could be due to more than one physical process at work in the source that causes the change in $\rm{kT_{e}}$. Conclusive evidence for the variation in $\rm{kT_{e}}$ is not found in MR 2251$-$178 and NGC 5548.

astro-ph.HE↗

arXiv2021

TXS 1206+549: a new $γ$-ray detected narrow-line Seyfert 1 galaxy at redshift 1.34?

Radio and $γ$-ray loud narrow-line Seyfert 1 galaxies (NLS1s) are unique objects to study the formation and evolution of relativistic jets, as they are believed to have high accretion rates and powered by low mass black holes contrary to that known for blazars. However, only about a dozen $γ$-ray detected NLS1s ($γ$-NLS1s) are known to date and all of them are at $z\le1$. Here, we report the identification of a new $γ$-ray emitting NLS1 TXS 1206+549 at $z=1.344$. A near-infrared spectrum taken with the Subaru telescope showed H$β$ emission line with FWHM of $1194\pm77$ km s$^{-1}$ and weak [O III] emission line but no optical Fe II lines, due to the limited wavelength coverage and poor signal-to-noise ratio. However, UV Fe II lines are present in the SDSS optical spectrum. The source is very radio-loud, unresolved, and has a flat radio spectrum. The broadband SED of the source has the typical two hump structure shown by blazars and other $γ$-NLS1s. The source exhibits strong variability at all wavelengths such as the optical, infrared, and $γ$-ray bands. All these observed characteristics show that TXS 1206+549 is the most distant $γ$-NLS1 known to date.

astro-ph.HE↗

arXiv2020

Dust Reverberation Mapping of Z229-15

We report results of the dust reverberation mapping (DRM) on the Seyfert 1 galaxy Z229-15 at z = 0.0273. Quasi-simultaneous photometric observations for a total of 48 epochs were acquired during the period 2017 July to 2018 December in B, V, J, H and Ks bands. The calculated spectral index (α) between B and V bands for each epoch was used to correct for the accretion disk (AD) component present in the infrared light curves. The observed α ranges between -0.99 and 1.03. Using cross correlation function analysis we found significant time delays between the optical V and the AD corrected J, H and Ks light curves. The lags in the rest frame of the source are 12.52 (+10.00/-9.55) days (between V and J), 15.63 (+5.05/-5.11) days (between V and H) and 20.36 (+5.82/-5.68) days (between V and Ks). Given the large error bars, these lags are consistent with each other. However, considering the lag between V and Ks bands to represent the inner edge of the dust torus, the torus in Z229-15 lies at a distance of 0.017 pc from the central ionizing continuum. This is smaller than that expected from the radius luminosity (R-L) relationship known from DRM. Using a constant α = 0.1 to account for the AD component, as is normally done in DRM, the deduced radius (0.025 pc) lies close to the expected R-L relation. However, usage of constant α in DRM is disfavoured as the α of the ionizing continuum changes with the flux of the source.

astro-ph.GA↗

arXiv2019

REMAP: Determination of the inner edge of the dust torus in AGN by measuring time delays

Active galactic nuclei (AGN) are high luminosity sources powered by accretion of matter onto SMBHs located at the centres of galaxies. The SMBH is surrounded by a broad emission line region (BLR) and a dusty torus. It is difficult to study the extent of the dusty torus as the central region of AGN is not resolvable using any conventional imaging techniques available today. Though, current IR interferometric techniques could in principle resolve the torus in nearby AGN, it is very expensive and limited to few bright and nearby AGN. A more feasible alternative to the interferometric technique to find the extent of the dusty torus in AGN is the technique of reverberation mapping (RM). REMAP (REverberation Mapping of AGN Program) is a long term photometric monitoring program being carried out using the 2 m HCT operated by the IIA, Bangalore, aimed at measuring the torus size in many AGN using the technique of RM. It involves accumulation of suitably long and well sampled light curves in the optical and near-infrared bands to measure the time delays between the light curves in different wavebands. These delays are used to determine the radius of the inner edge of the dust torus. REMAP was initiated in the year 2016 and since then about one hour of observing time once every five days (weather permitting) has been allocated at the HCT. Our initial sample carefully selected for this program consists of a total of 8 sources observable using the HCT. REMAP has resulted in the determination of the extent of the inner edge of the dusty torus in one AGN namely H0507+164. Data accumulation for the second source is completed and observations on the third source are going on. We will outline the motivation of this observational program, the observational strategy that is followed, the analysis procedures adopted for this work and the results obtained from this program till now.

astro-ph.GA↗

arXiv2016

A Study on the Expanding Universe Based on a Model of the Time Variation of its Matter Content in the Framework of Brans-Dicke Theory

A theoretical model of cosmic expansion has been formulated on an assumption of inter-conversion of matter and dark energy, in the framework of Brans-Dicke theory. An empirical scale factor has been used, which generates a signature flip of the deceleration parameter with time. To account for the non-conservation of matter, a function of time f(t) is incorporated into the equation representing the density of matter. Its value at any instant of time is proportional to the matter content of the universe. The functional form of f(t) has been determined from the field equations by using an empirical scalar field parameter expressed in terms of the scale factor. It is found to decrease with time almost monotonically, implying a conversion of matter into dark energy. Using this function f(t), the time variation of the density of matter has been determined and also the expressions regarding the proportions of matter and dark energy of the universe have been formulated. Time variation of gravitational constant, its fractional rate of change and the Brans-Dicke dimensionless parameter has been analyzed. The dependence of Brans-Dicke parameter upon the scalar field has been determined. The present study enables us to correlate the change of matter content with the change of deceleration parameter and gravitational constant without using any specific mechanism of interaction between matter and scalar field.

gr-qc↗