arXiv subjects

S. Muneer

Publications and source records attributed to S. Muneer.

At least 19 recordsLinked to original sources

arXiv2026

Multi-Band Optical Variability Characteristics of the $γ$-ray emitting blazar 1ES 0647+250 during its 2020 flare

We present a multi-band optical variability study of the $γ$-ray emitting high-synchrotron-peaked blazar 1ES~0647+250 during its enhanced optical activity in 2020. The analysis utilizes 107 epochs of high-precision $B$, $V$, and $R$-band photometry obtained with the 75 cm telescope at the Vainu Bappu Observatory, combined with high-cadence $z_g$ and $z_r$ survey data from the Zwicky Transient Facility. The combined light curves reveal coherent variability across all optical filters, with the source reaching a peak $R$-band magnitude of 15.74 mag. We calculated the variability amplitude, finding the highest intrinsic variability in the B- band. Spectral analysis using color--magnitude diagrams reveals a statistically significant ``Bluer-When-Brighter'' trend, supported by a Pearson correlation coefficient of $r = 0.68$ ($p < 0.001$) for the (V$-$R) index. Furthermore, the discrete correlation function analysis indicates a near-zero time lag between the $V$ and $R$ bands, suggesting simultaneous emission within the observational cadence. The observed spectral hardening is consistent with synchrotron-dominated optical emission, likely associated with enhanced particle acceleration within the jet during the active phase of 1ES~0647+250.

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arXiv2026

Variable stars in the field of the Galactic globular cluster M71

M71 is a nearby, metal-rich globular cluster at low Galactic latitude, where field contamination and spatially variable extinction complicate colour-magnitude diagrams (CMDs) and the identification of cluster member variable stars. Our aims are (i) to construct a homogeneous census of variable stars in M71 by refining their periods and classifications and identifying new candidates, and (ii) to derive a decontaminated, differentially dereddened CMD to constrain its physical properties. We obtained Johnson-Kron-Cousins $VI$ time-series CCD photometry and reduced it using difference image analysis. Cluster membership was established from \textit{Gaia}~DR3 proper motions, and a differential-reddening correction was applied across the field of view. The resulting CMD, cleaned of field stars, was compared with tailored isochrones to estimate age ($12.9^{+0.9}_{-0.8}$ Gyr), metallicity ([Fe/H] =$-0.88^{+0.13}_{-0.15}$), mean reddening ($E(B-V)$ = $0.21 \pm 0.02$), and distance modulus ($(m-M)_{0}$ = $13.01 \pm 0.06$). Variable stars were identified using two complementary approaches: a periodogram-free string-length scan refined with phase dispersion minimisation, and a robust inter-site screening based on median statistics combined with a generalised Lomb-Scargle significance criterion. We identified 21 variable stars not previously reported in the Catalog of Variable Stars in Globular Clusters and provided their periods, amplitudes, classifications, membership status, and light curves. This combined strategy yields a consistent picture of M71, expanding its known variable-star population and confirming parameters typical of metal-rich Galactic disk globular clusters.

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arXiv2025

Identification of Large-Scale (>100 kpc) Radio Jets in Narrow-Line Seyfert 1 Galaxies

Powerful, large-scale relativistic jets are usually associated with massive, old elliptical galaxies. This paradigm has recently been challenged by the identification of narrow-line Seyfert 1 (NLSy1) galaxies, thought to be young active galactic nuclei with low-mass black holes, harboring relativistic jets. Among them, sources hosting $>$100 kpc radio jets are extremely rare. Here, we report the discovery of large-scale, double-lobed radio structures in 33 NLSy1s with the projected linear size of at least 100 kpc from a recently published catalog of 22656 NLSy1 galaxies. These 33 include 29 confirmed double-lobed sources and 4 candidates whose radio structure requires further study. We suggest that their low black hole masses are unlikely to be due to their small angles of inclination to the line of sight. These enigmatic sources were identified by examining the radio observations taken with the Faint Images of the Radio Sky at Twenty centimeters, Very Large Array Sky Survey, Low Frequency Array, and Rapid ASKAP Continuum Survey. Among them, the NLSy1 source J1318+2626 ($z=0.62$) is found to host a radio jet with the projected linear size of 4.3 Mpc, making it the only NLSy1 galaxy hosting a Mpc-scale radio jet known as of today. We conclude that future observations of NLSy1 sources with the next generation of sensitive telescopes may reveal a much larger population harboring large-scale jets, thus providing crucial clues on their origin, propagation, and interaction with the ambient environment.

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arXiv2025

Spectroscopic Reverberation Mapping for SARM: The Case of Mrk 1048 and Mrk 618

Robust extragalactic distance measurements are crucial for resolving the persistent discrepancy in the value of the Hubble constant (H$_0$)). Active Galactic Nuclei (AGNs), through their compact and variable broad-line regions (BLRs), enable the determination of geometric distances when reverberation mapping (RM) is combined with spectroastrometry(SA). We report results from a spectroscopic RM campaign (October 2022 to March 2023) targeting two narrow-line Seyfert 1 galaxies, Mrk 1048 and Mrk 618, using 3.6-m DOT and 2-m HCT. High-cadence spectro-photometric monitoring was carried out using onboard instruments such as ADFOSC, HFOSC, and TANSPEC, resulting in well-sampled continuum and emission line light curves. The observed fractional variability ($F_{\mathrm{var}}$) ranged from 4% to 14% across the $g$-band, H$β$, and H$α$ light curves. The time lags were measured using the interpolated cross-correlation function (ICCF), PyI$^{2}$CCF, and \textsc{JAVELIN} methods. In the rest frame, the ICCF analysis yields H$β$ lags of $10.5^{+2.6}_{-4.2}$ days for Mrk 1048 and $10.2^{+3.4}_{-2.9}$ days for Mrk 618, while the corresponding H$α$ lags are $18.7^{+5.3}_{-5.4}$ and $14.4^{+4.6}_{-10.5}$ days, respectively. The emission-line widths, measured from the rms spectra using $σ_{\mathrm{line}}$, give virial black hole mass estimates of $6.3^{+2.0}_{-2.1} \times 10^7\,M_\odot$ for Mrk 1048 and $1.2^{+0.4}_{-0.6} \times 10^7\,M_\odot$ for Mrk 618. These results will serve as a basis for absolute geometric distance calibration when combined with VLTI/GRAVITY spectro-astrometric measurements, thereby contributing to the development of AGNs as standardizable cosmological probes.

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arXiv2025

Discovery of Changing-Look behavior in AGN NGC 3822: A long-term multiwavelength study

We present a comprehensive long-term multi-wavelength study of the active galactic nucleus (AGN) NGC 3822, based on 17 years (2008 to 2025) of X-ray, ultraviolet (UV), and optical observations.The dataset includes observations from Swift, XMM-Newton, and NuSTAR, the Very Large Telescope, and the Himalayan Chandra Telescope. Our multiwavelength light curve analysis reveals flux variations across X-ray to optical/UV bands, with an increased variability amplitude at shorter wavelengths. X-ray spectral analysis indicates the presence of intrinsic absorption during the 2016 and 2022 observations; however, this absorption disappeared before and after these epochs. The presence and absence of the absorber are attributed to clouds moving in and out of the line of sight. During the long-term monitoring period, the bolometric luminosity of the source varies between ($1.32-17)\times10^{43}$ erg s$^{-1}$. Optical spectroscopic monitoring reveals changing-look (CL) behaviour in NGC~3822, characterized by the appearance and disappearance of broad emission lines (BELs). These CL transitions are associated with changes in the Eddington ratio rather than changes in the obscuration. The BELs appear only when the Eddington ratio is relatively high ($\sim 3.8\times10^{-3}$) and disappear when it drops to a lower value ($\sim 0.9\times10^{-3}$).

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arXiv2025

The Detection of Teraelectronvolt Radiation from a Flat Spectrum Radio Quasar

The very high-energy (VHE; $>$100 GeV) radiation carries the signatures of the matter-energy interaction in some of the most extreme astrophysical environments. Considering broad emission line blazars, i.e., flat spectrum radio quasars (FSRQs), the dense photon fields surrounding the relativistic jet can prohibit the particle population from accelerating to very high energies and producing VHE radiation. They can also possibly make the environment opaque for the VHE $γ$ rays due to $γγ$ pair production, thus explaining the paucity of VHE-detected FSRQs and non-detection of TeV radiation ($>$1 TeV) from them. Here we report, for the first time, a $>$7$σ$ detection of an FSRQ, S5 1027+74 ($z=0.123$), in the VHE band, including the first ever detection of TeV emission from an object of this class, using the Fermi Large Area Telescope observations. Its $γ$-ray spectrum covering the 100 MeV to 2 TeV band revealed a prominent spectral break with a flat, rising shape above $\sim$10 GeV, a feature never detected from other VHE-detected FSRQs. The radio-to-$γ$-ray spectral energy distribution of S5 1027+74 provides strong evidence of a third bump peaking at multi-TeV energies. These enigmatic findings imply that FSRQ jets can accelerate particles to extremely high energies and provide tantalizing clues about the complex radiative environment of relativistic jets.

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arXiv2025

The variable stars in the compact halo globular cluster NGC 5634 visited again

We present new time-series CCD \emph{VR} photometry of the globular cluster NGC 5634. We aim to use the known RR Lyrae stars, members of the cluster, as indicators of mean metallicity and distance. Accurate coordinates, periods and an identification chart of the variables in the field of our images are provided. A membership analysis was performed, based on $Gaia$-DR3 proper motions, for 3525 point sources within 15 arcmin from the cluster center. The membership status for each known variable was established and it was found that V10, V11 and V16 are most likely field stars. The variability of the RRab star V7, considered non-variable for a number of years, is demonstrated. Via the Fourier decomposition of the light curves of cluster member RR Lyrae, the mean metallicity and distance were calculated independently from RRab and RRc stars to find [Fe/H]$_{\rm ZW}= -1.67 \pm 0.11$ y $D=23.9 \pm 0.8$ kpc, and [Fe/H]$_{\rm ZW}= -1.69 \pm 0.22$ y $D=22.9 \pm 1.0$ kpc respectively.

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arXiv2024

Serendipitous detection of an intense X-ray flare in the weak-line T Tauri star KM Ori with SRG/eROSITA

Weak-line T Tauri stars (WTTS) exhibit X-ray flares, likely resulting from magnetic reconnection that heats the stellar plasma to very high temperatures. These flares are difficult to identify through targeted observations. Here, we report the serendipitous detection of the brightest X-ray flaring state of KM Ori in the eROSITA DR1 survey. Observations from SRG/eROSITA, Chandra X-ray Observatory, and XMM-Newton are analysed to assess the X-ray properties of KM Ori, thereby establishing its flaring state at the eROSITA epoch. The long-term (1999-2020) X-ray light curve generated for the Chandra observations confirmed that eROSITA captured the source at its highest X-ray flaring state recorded to date. Multi-instrument observations support the X-ray flaring state of the source, with time-averaged X-ray luminosity ($L_{0.2-5keV}$) reaching $\sim 1.9\times10^{32}{erg~s^{-1}}$ at the eROSITA epoch, marking it the brightest and possibly the longest flare observed to date. Such intense X-ray flares have been detected only in a few WTTS. The X-ray spectral analysis unveils the presence of multiple thermal plasma components at all epochs. The notably high luminosity ($L_{0.5-8~keV}\sim10^{32}{erg~s}^{-1}$), energy ($E_{0.5-8~keV}\sim10^{37}$erg), and the elevated emission measures of the thermal components in the eROSITA epoch indicate a superflare/megaflare state of KM Ori. Additionally, the H$α$ line equivalent width of $\sim$-5Åfrom our optical spectral analysis, combined with the lack of infrared excess in the spectral energy distribution, were used to re-confirm the WTTS (thin disk/disk-less) classification of the source. The long-duration flare of KM Ori observed by eROSITA indicates the possibility of a slow-rise top-flat flare. The detection demonstrates the potential of eROSITA to uncover such rare, transient events, thereby providing new insights into the X-ray activity of WTTS.

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arXiv2024

Long term CCD photometry of the distant cluster NGC 2419: the CMD revisited

Employing \emph{VI} images of NGC 2419 acquired over 17 years, light curves for most of the known variables in the field of the cluster are produced. A cluster membership analysis for about 3100 stars in the cluster field with proper motions from $Gaia$-DR3, revealed the presence of member stars as far as 140 pc from the cluster center and enabled the construction of a cleaner CMD free of field stars. It was found that RRab and RRc stars share the inter-order region in the instability strip, which is unusual for OoII clusters. Theoretical considerations confirm that Pop II cepheids are descendants of extreme ZAHB blue tail stars with very thin envelopes of about 10\% of the total mass. Member RR Lyrae stars were employed to calculate independent estimates of the mean cluster metallicity and distance; we found [Fe/H]$_{\rm UV}= -1.90 \pm 0.27$ and $D=86.3 \pm 5.0$ kpc from the RRab and [Fe/H]$_{\rm UV}= -1.88 \pm 0.30$ and $D=83.1 \pm 8.1$ kpc from the RRc light curves.

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arXiv2024

Insights on the Optical and Infrared Nature of MAXI J0709-159: Implications for High-Mass X-ray Binaries

In our previous study (Bhattacharyya et al., 2022), HD~54786, the optical counterpart of the MAXI J0709-159 system, was identified to be an evolved star, departing from the main sequence, based on comparisons with non-X-ray binary systems. In this paper, using color-magnitude diagram (CMD) analysis for High-Mass X-ray Binaries (HMXBs) and statistical t-tests, we found evidence supporting HD 54786's potential membership in both Be/X-ray binaries (BeXRBs) and supergaint X-ray binaries (SgXBs) populations of HMXBs. Hence, our study points towards dual optical characteristics of HD~54786, as an X-ray binary star and also belonging to a distinct evolutionary phase from BeXRB towards SgXB. Our further analysis suggests that MAXI J0709-159, associated with HD 54786, exhibits low-level activity during the current epoch and possesses a limited amount of circumstellar material. Although similarities with the previously studied BeXRB system LSI +61$^{\circ}$ 235 (Coe et al., 1994) are noted, continued monitoring and data collection are essential to fully comprehend the complexities of MAXI J0709-159 and its evolutionary trajectory within the realm of HMXBs.

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arXiv2023

Star formation in the dwarf Seyfert galaxy NGC 4395: Evidence for both AGN and SNe feedback?

We present a detailed multi-wavelength study of star formation in the dwarf galaxy NGC 4395 which hosts an active galactic nucleus (AGN). From our observations with the Ultra-Violet Imaging Telescope, we have compiled a catalogue of 284 star forming (SF) regions, out of which we could detect 120 SF regions in H$α$ observations. Across the entire galaxy, we found the extinction corrected star formation rate (SFR) in the far ultra-violet (FUV) to range from 2.0 $\times$ 10$^{-5}$ M$_\odot$yr$^{-1}$ to 1.5 $\times$ 10$^{-2}$ M$_\odot$yr$^{-1}$ with a median of 3.0 $\times$ 10$^{-4}$ M$_\odot$yr$^{-1}$ and age to lie in the range of $\sim$ 1 to 98 Myr with a median of 14 Myr. In H$α$ we found the SFR to range from 7.2 $\times$ 10$^{-6}$ M$_\odot$yr$^{-1}$ to 2.7 $\times$ 10$^{-2}$ M$_\odot$yr$^{-1}$ with a median of 1.7 $\times$ 10$^{-4}$ M$_\odot$yr$^{-1}$ and age to lie between 3 to 6 Myr with a median of 5 Myr. The stellar ages derived from H$α$ show a gradual decline with galactocentric distance. We found three SF regions close to the center of NGC~4395 with high SFR both from H$α$ and UV which could be attributed to feedback effects from the AGN. We also found six other SF regions in one of the spiral arms having higher SFR. These are very close to supernovae remnants which could have enhanced the SFR locally. We obtained a specific SFR (SFR per unit mass) for the whole galaxy 4.64 $\times$ 10$^{-10}$ yr$^{-1}$.

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arXiv2022

Detailed analysis of the variable star population in the globular cluster NGC 7006

A five year CCD photometric \emph{VI} time-series of NGC 7006 is employed to perform a detailed analysis of the known population of variable stars. In the process we have corrected inconsistent classifications, sky coordinates and found ten new cluster member variables. An independent reddening estimate with a value $E(B-V)=0.08\pm0.05$ is made. Using Fourier decompositions of RR Lyrae light curves and well established calibrations, the cluster mean metallicity and distance [Fe/H]$_{\rm ZW}= -1.53\pm0.15$ and $41.2\pm1.4$ kpc are estimated based on an extended sample of cluster member RRab stars. Using the $Gaia$-DR3 data we performed an extensive membership analysis which leads to a clean Colour-Magnitude diagram, and hence to the identification of variables that are likely field stars, and to considerations on the variables distribution in the Horizontal Branch (HB). A double mode RR Lyrae and three CW star are discussed. The origin of CW stars from precursors in the blue tail of the HB with very thin ($\sim 0.06 \pm 0.01 M_{\odot}$) envelopes is argued. Our models indicate that the Main Sequence predecessor of RR Lyrae stars had a mass of 0.82-0.85 $M_{\odot}$ and lost about 25-35\% of its mass during the red giant branch events before settling in the HB some 12-13.5 Gyrs later.

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arXiv2022

RR Lyrae stars in the globular cluster Palomar 2

A CCD VI imaging time-series over 11-year is employed to explore the light curves of stars in the field of Palomar 2. We discovered 20 RRab and 1 RRc variables. A revision of Gaia-DR3 data enabled us to identify 10 more variables and confirm the RRab nature of 6 of them and one RGB. The cluster membership is discussed and 18 variables are most likely cluster members. The Fourier light curve decomposition for the 11 best quality light curves of cluster member stars leads to independent estimates of the cluster distance 27.2 +- 1.8 kpc and [Fe/H]ZW=-1.39 +- 0.55. We confirm the cluster as of the Oo I type.

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arXiv2022

Intra-night optical variability monitoring of $γ$-ray emitting blazars

We present the results obtained from our campaign to characterize the intra-night-optical variability properties of blazars detected by the {\it Fermi} Large Area Telescope. This involves R-band monitoring observations of a sample of 18 blazars, that includes five flat-spectrum radio quasars (FSRQs) and thirteen BL Lac objects (BL Lacs) covering the redshift range z = 0.085$-$1.184. Our observations, carried out using the 1.3 m J.C. Bhattacharya Telescope cover a total of 40 nights ($\sim$200 hrs) between the period 2016 December and 2020 March. We characterized variability using the power enhanced $F-$test. We found a duty cycle (DC) of the 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 $\sim$16\%, $\sim$10\% and $\sim$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.

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arXiv2022

A new photometric study of M14 (NGC 6402): An interpretation of the Horizontal Branch and beyond

We present a CCD $VI$ photometric study of the globular cluster M14. Particular attention is given to the variable stars. This allowed new classifications and cluster membership considerations. New variables are reported; 3 RRc, 18 SR and 1 SX Phe. The Fourier decomposition of RR Lyrae light curves lead to the mean cluster metallicity of [Fe/H]$_{\rm ZW}= -1.3 \pm 0.2$. Several independent methods yield a mean distance of $9.36 \pm 0.16$ kpc. A Colour-Magnitude diagram outlined by the cluster members enabled a matching with theoretical predictions of isochrones and zero-age horizontal branches, whose fitting to the observations is in good agreement with the above distance and metallicity. The Oosterhoff type of M14 is confirmed as Oo-int, and the pulsating mode distribution of RR Lyrae stars on the HB shows that the bimodal region of the instability strip is shared by RRab and RRc stars. By modelling the mass loss at the RGB after the He flash events, we were able to represent the blue tail of the HB, using a core mass of 0.48 $M_{\odot}$ and total masses of 0.52-0.55 $M_{\odot}$. A progenitor star on the MS of 0.84 $M_{\odot}$ reaches the HB in about 12.5 Gyrs, consonant with previous age determinations of the cluster. Type II Cepheids of M14 may be interpreted as products of post-HB evolution, driven by the complex processes involving the burning of the very thin low mass hydrogen and helium shells of these stars and their minuscule envelopes. No evidences were found in favor of M14 being of extragalactic origin.

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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.

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arXiv2020

The globular cluster M10: Reassessment of stellar membership, distance and age using its variable and HB stars

Time-series VI CCD photometry of the globular cluster M10 (NGC 6254) is employed to perform a detailed identification, inspection of their light curves, their classification and their cluster membership, of all the known variables reported up to 2018. The membership analysis is based on the Gaia-DR2 positions and proper motions. The metallicity of the cluster is estimated based on the sole RRc star known in the cluster. The Fourier decomposition of its light curve leads to [Fe/H]ZW=-1:59 $\pm$ 0:23 dex. The mean cluster distance, estimated by several independent methods is 5.0 $\pm$ 0.3 kpc. A multi-approach search in a region of about 10x10 arcmin2 around the cluster revealed three new variables, one SX Phe (V35) and two sinusoidal variables on the red giant branch of unclear classification (V36,V37). Modelling the HB stars is very sensitive to the stellar hydrogen shell mass, which surrounds the 0.50 M$_\odot$ helium core. To match the full stretch of the HB population, a range of total mass of 0.56 to 0.62 M$_\odot$ is required. These models support a distance of 5.35 kpc and an age of about 13 Gyrs, and hints to some individual variation of the mass loss on the uppper RGB, perhaps caused by the presence of closed magnetic field in red giants.

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arXiv2020

K-band spectra of selected post-AGB candidates

We present medium resolution (1000) K-band spectra of 12 post-AGB candidates and related stars. For several objects in our sample, these spectra were obtained for the first time. The Br$γ$ line in emission is detected in seven objects indicating the onset of photo-ionization in these objects. Four objects show the presence of He i line. We detect H2 emission line in the spectra of IRAS 06556+1623, IRAS 22023+5249, IRAS 18062+2410 and IRAS 20462+3416. H2 emission line ratio 1-0 S(1)/2-1 S(1) indicate that H2 is radiatively excited due to the UV radiation of hot post-AGB central stars. When compared with the recent observations by other investigators, the Br$γ$ and H2 emission fluxes varied in some of the objects. The hot post-AGB stars IRAS 22495+5134, IRAS 22023+5249, IRAS 18062+2410 and IRAS 20462+3416 seem to be evolving rapidly to young low excitation planetary nebula phase. The spectra of the objects presented in this paper may be useful for future observers as some of these stars show spectral variation and since the post-AGB evolution of some of these stars is relatively rapid.

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