arXiv subjects

Dipanjan Mitra

Publications and source records attributed to Dipanjan Mitra.

At least 19 recordsLinked to original sources

arXiv2026

Rubin J122659.4+090236: An Extremely Low Surface Brightness Galaxy Candidate Discovered in the Rubin LSST Early Data Preview 2

We report the serendipitous discovery of an exceptionally low surface brightness galaxy (LSBG) candidate, Rubin J122659.4+090236, in Rubin Observatory imaging of the interacting NGC 4410 system, identified in the Cosmic Treasure Chest public release. 2D Sérsic modelling of the Rubin g, r, and i images reveals a nearly round system with a shallow profile (n ~ 0.4), an effective radius of R_e ~ 6'', and central surface brightnesses of $μ_{0,g}=27.52\pm0.04$, $μ_{0,r}=27.62\pm0.07$, and $μ_{0,i}=27.04\pm0.08$ mag arcsec$^{-2}$. EAZY photo-z fitting favours an intermediate-z solution at z~0.3, while a low-redshift solution at z~0.028, consistent with the NGC 4410 system, is also permitted by a restricted EAZY fit over 0<z<0.1 without imposing a redshift prior. These alternatives imply substantially different physical interpretations, ranging from a diffuse dwarf-like system to an exceptionally extended background LSBG. This discovery demonstrates Rubin's sensitivity to extremely diffuse galaxies and highlights the potential of the LSST survey to uncover large samples of such elusive systems across wide areas, enabling systematic studies of the LSBG population and its role in galaxy evolution.

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arXiv2026

Multi-band Structural Analysis of KiDS-selected Low Surface Brightness Galaxies with Hyper Suprime-Cam Imaging

We present a homogeneous multi-band structural analysis of 205 KiDS-selected low surface brightness galaxy (LSBG) candidates using deep Hyper Suprime-Cam (HSC) $G$, $R$, and $I$-band imaging. Structural parameters were derived using single-component Sérsic modeling with GALFIT. The sample is dominated by diffuse systems with low Sérsic indices, with the distributions consistently peaking near $n\approx0.7$ across all bands. The estimated $B$-band central surface brightness distribution has a median value of $\tildeμ_{0,B}=24.55$ mag arcsec$^{-2}$, indicating that the galaxies lie firmly within the low surface brightness regime. The catalog is strongly dominated by red systems, comprising 178 red LSBGs (87.3$\%$) and 27 blue LSBGs (12.7$\%$). Despite this color bimodality, the red and blue subsamples show similar structural properties, with no statistically significant differences in Sérsic index, effective radius, axis ratio, or surface brightness distributions. The absence of a correlation between color and axis ratio further suggests that dust reddening is unlikely to be the primary driver of the red colors. Overall, the sample provides a well-characterized structural reference set of LSBGs in the HSC footprint and confirms that the KiDS selected candidates are predominantly genuine low surface brightness galaxies.

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arXiv2026

UnReal-B : Real-Space DFT Solver for Matter in Extreme Magnetic Fields

As new observational technologies reveal increasingly detailed properties of neutron star surfaces, the demand for accessible and extensible theoretical modeling tools continues to grow. We present UnReal-B, a real-space Density Functional Theory solver for one-dimensional chains of matter in extreme magnetic fields $B \approx 10^{12} - 10^{15},\mathrm{G}$. By employing the adiabatic approximation, UnReal-B, provides a streamlined numerical framework for calculating the electronic structure of strongly magnetized condensed matter. The solver is benchmarked against published results for several astrophysically relevant elements, demonstrating excellent agreement while maintaining a comparatively simple and transparent implementation. Released as open-source software, UnReal-B facilitates reproducible and community-driven investigations of neutron-star surface matter and provides a foundation for future developments motivated by emerging observational constraints.

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arXiv2026

Systematic Polarization Errors from Parallactic-Angle Dependent Leakage in Pseudo-Circular Feeds

Wideband radio interferometers increasingly rely on analog quadrature hybrids to synthesize circular polarization from linear feeds. These systems are typically calibrated under the assumption that instrumental polarization leakage can be represented as a static complex offset, independent of parallactic angle. In this work, we demonstrate that this assumption breaks down in the presence of realistic hybrid imperfections. We show that amplitude and phase errors in the hybrid $\mathbf{H}(ν)$ introduce a non-commutative interaction with parallactic rotation $\mathbf{R}(χ)$, such that [$\mathbf{H}(ν)\mathbf{R}(χ) \neq \mathbf{R}(χ)\mathbf{H}(ν)$] leading to a time-dependent effective leakage term that rotates in the Stokes $(Q,U)$ plane. This effect causes systematic distortions in polarization angle and introduces frequency-dependent biases that can mimic or corrupt Faraday rotation measurements. We derive a first-order analytic model for this leakage and demonstrate that it manifests as a deterministic, geometrically modulated error proportional to total intensity. To mitigate this effect, we introduce a Static Offset Pre-correction (SOP) method that operates in the antenna frame, inverting the hybrid response prior to parallactic de-rotation. Unlike conventional calibration approaches, SOP removes the non-commutative error in the Jones domain, preventing its projection into the sky frame. Our results show that hybrid-induced leakage is not merely a calibration artifact but a fundamental systematic error that must be addressed to achieve high-fidelity wideband polarimetry.

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arXiv2026

Born to be recycled: a comprehensive population synthesis of the Galactic millisecond pulsars

Millisecond pulsars (MSPs) are the oldest but fastest pulsars known to date. In the 1980s, to explain how these pulsars could be formed, a new hypothesis was formulated: the recycling of pulsars, i.e the fact that a pulsar could accrete matter from a companion and been spun up. In this paper, we developed a population synthesis algorithm for pulsars which belong to a binary, in order to check whether most of the observed recycled pulsars were formed via an accretion mechanism and derive statistics about their properties, that are difficult to obtain through observations. We also make predictions for future surveys. Toward the presented objectives, we use the code Stellar EVolution for N-body (SEVN) to take into account all the binary processes and our own code to evolve each pulsar self-consistently by taking into account the secular evolution of a force-free magnetosphere, the magnetic field decay, gravitational braking and spatial evolution. Each pulsar is born in binary with a main sequence companion, and evolve to present time. The radio and $γ$-ray emission locations were modeled by the polar cap geometry and striped wind model, respectively. Our simulations seem to reproduce well the population of radio and $γ$-ray pulsars observed in the selected surveys. We also found that there should be less than $220$ unidentified pulsars in the Fourth Fermi-LAT catalogue of $γ$-ray sources (4FGL). High values of the viewing angle $ζ$ seem to be needed to be able to observe the recycled pulsars, and it also seems difficult to observe recycled pulsars with an aligned rotation axis and magnetic axis (i.e., $χ\leq 10$°). We find that only a small fraction, approximately $\sim 7.6\times10^{-3}$ %, of oxygen-neon white dwarfs (ONeWDs) in binary systems appear to contribute to the population of mildly recycled pulsars through accretion-induced collapse.

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arXiv2025

Characterizing the Nature of Periodic Amplitude Modulation in Pulsars

In recent years periodic amplitude modulation has emerged as a unique emission feature in the single pulse sequence of pulsars alongside periodic nulling and subpulse drifting. Despite ample evidence for the uniqueness of this phenomenon, the periodic modulation in several pulsars are often confused with subpulse drifting, primarily due to lack of clear characterisation of the emission features from a representative sample of pulsars. In this work we present a detailed analysis of the single pulse behaviour from seventeen pulsars exhibiting periodic amplitude modulation, six of them being new detections. The pulsar switches between different intensity states as a result of periodic amplitude modulation and we propose a novel statistical scheme to identify these emission states. The periodic modulation can be divided into three broad categories, phase stationary modulation, modulations with phase shift and intermittent periodic modulations. The phase stationary behaviour is seen when the emission intensity across a major part of the pulse window changes periodically. The phase shifts are associated with intensity changes at specific locations within the emission window in a periodic manner; while in some pulsars the periodic modulations become more prominent only at specific intervals resulting in intermittent behaviour.

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arXiv2025

Investigating the physical properties of dusty star-forming galaxies at z>=1.5 in the GOODS-South field using JWST

We investigated how well the physical properties of progenitors of present-day massive spheroidal galaxies (proto-spheroids) can be constrained by the JWST Advanced Deep Extragalactic Survey (JADES) in the GOODS-South field, which benefits from extensive photometric and spectroscopic data, including those from the Hubble, Spitzer and Herschel. We adopted a physical model for the evolution of proto-spheroidal galaxies, which form the bulk of dusty star-forming galaxies (DSFGs) at z>=1.5 and confirmed its consistency with recent mid-infrared high-z galaxy luminosity functions. Using the model and the JADES survey strategy, we simulated a sample of proto-spheroids over 87.5 arcmin^2, matching the JADES/GOODS-S survey area. Photometric redshifts estimated from simulated JWST photometry showed >=95% accuracy and were used in SED fitting with CIGALE. We demonstrated that JWST will provide reliable stellar mass estimates up to 0.1 dex for the majority of proto-spheroids at z>=1.5 and can detect low-mass systems during cosmic noon that were inaccessible in the pre-JWST era. Focusing on the active star-forming phase of the proto-spheroid evolution, we defined a sub-sample flux limited at 250 micron (DSFG sample) and derived SFR, dust luminosity and dust mass complementing the JWST photometry with that from Spitzer/MIPS and Herschel. We also constructed a JWST-selected DSFG catalog from ASTRODEEP data using NIRCam colour criteria and demonstrated strong consistency between the observed and simulated DSFG populations.

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arXiv2025

Polarization Properties of Energetic Pulsars at Meterwavelengths

Polarization behaviour shows a transition in the pulsar population, where energetic sources with higher spin-down energy loss, $\dot{E} > 10^{34}$ erg~s$^{-1}$, often have fractional linear polarisation ($L/I$) close to 100\%, while below this range $L/I$ is usually lower than 50\%. The polarisation behaviour has been primarily studied at higher frequencies above 1 GHz, and in this work we explore the single pulse polarisation behaviour in pulsars with $\dot{E} > 5\times10^{33}$ erg~s$^{-1}$ at a lower frequency range of 300-750 MHz. The polarisation behaviour can be divided into two categories, the first with $L/I>$ 70\% where the polarisation position angle (PPA) follows a single track, and a second group with $L/I <$ 70\% and scattered PPA behaviour with or without orthogonal modes. However, there are some single pulses in the first category that also have lower $L/I$ and exhibit the presence of two polarisation modes along orthogonal tracks. The radio emission in pulsars arises due to coherent curvature radiation (CCR) from charge bunches, which develops due to non-linear instabilities in the pulsar plasma forming charge separated envelope solitons. The CCR excites orthogonally polarised X and O modes oriented perpendicular and parallel to the magnetic field line planes, that detach in the plasma and propagate independently. The O-mode is seven times stronger than the X-mode but gets damped in the medium. We show that incoherent mixing of the X and O modes with different levels of damping can reproduce the observed polarisation features in the energetic pulsar population.

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arXiv2025

Pulsar Coherent Radio Emission from Solitons : Average Emission Properties

Observations have established that coherent radio emission from pulsars arise at few hundred kilometers above stellar surface. Recent polarization studies have further demonstrated that plasma instabilities are necessary for charge bunching that gives rise to coherent emission. The formation of charged solitons in the electron-positron plasma is the only known bunching mechanism that can be realised at these heights. More than five decades of observations have revealed a number of emission features that should emerge from any valid radio emission mechanism. We have carried out numerical calculations to find the features of average emission from curvature radiation due to charged solitons. The characteristic curvature radiation spectrum has been updated from the well known one-dimensional dependence into a general two-dimensional form, and contribution from each soliton along observer's line of sight (LOS) has been added to reproduce the pulsar emission. The outflowing plasma is formed by sparking discharges above the stellar surface that are located within concentric rings resembling the core-cone emission beam, and uniform distribution of solitons along any LOS has been assumed. The observed effects of radius to frequency mapping, where the lower frequency emission originates from higher altitudes, is seen in this setup. The power law spectrum and relative steepening of the core spectra with respect to the cones also emerges. The estimated polarization position angle reflects the geometrical configuration of pulsars as expected. These studies demonstrate the efficacy of coherent curvature radiation from charged solitons to reproduce the average observational features of pulsars.

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arXiv2024

The Galactic population of canonical pulsars II

Pulsars are highly magnetized rotating neutron stars, emitting in a broad electromagnetic energy range. Reproducing the observed pulsars population refines our understanding of their formation and evolution scenarios as well as their radiation processes and geometry. In this paper, we improve our previous population synthesis by focusing on both the radio and $γ$-ray pulsar populations, investigating the impact of the Galactic gravitational potential and of the radio emission death line. In order to elucidate the necessity of a death line, refined initial distributions of spin period and spacial position at birth were implemented, elevating the sophistication of our simulations to the most recent state-of-the-art. The motion of each individual pulsar is tracked in the Galactic potential by a fourth order symplectic integration scheme. Our pulsar population synthesis takes into account the secular evolution of the force-free magnetosphere and magnetic field decay simultaneously and self-consistently. Each pulsar is evolved from its birth up to the present time. The radio and $γ$-ray emission locations are modelled respectively by the polar cap geometry and the striped wind model. By simulating ten million pulsars we found that including a death line better reproduces the observational trend. However, when simulating one million pulsars, we obtain an even more realistic $P-\dot{P}$ diagram, whether or not a death line is included. This suggests that the ages of the detected pulsars might be overestimated, therefore questioning the real need for a death line in pulsar population studies. Kolmogorov-Smirnov tests confirm the statistical similarity between the observed and simulated $P-\dot{P}$ diagram. Additionally, simulations with increased $γ$-ray telescope sensitivities hint to a significant contribution of $γ$-ray pulsars to the GeV excess in the Galactic centre.

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arXiv2024

On the flux density spectral property of high linearly polarized signal from Pulsar J0332+5434

The polarization position angles (PPA) of time samples with high linear polarization often show two parallel tracks across the pulsar profile that follow the rotating vector model (RVM). This feature support coherent curvature radiation (CCR) as the underlying mechanism of radio emission from pulsars, where the parallel tracks of the PPA represent the orthogonal extraordinary X and ordinary O eigen modes of strongly magnetized pair plasma. However, the frequency evolution of these high linearly polarized signals remains unexplored. In this work we explore the flux density spectral nature of high linearly polarized signals by studying the emission from PSR J0332+5434 over a frequency range between 300 MHz and 750 MHz, using the Giant Metrewave Radio Telescope. The pulsar average profile comprises of a central core and a pair of conal components. We find the high linearly polarized time samples to be broadband in nature and in many cases they resemble a narrow spiky feature in the conal regions. These spiky features are localised within a narrow pulse longitude, over the entire frequency range, and their spectral shapes sometimes resemble an inverted parabolic shape. In all such cases the PPA are exclusively along one of the orthogonal RVM tracks, likely corresponding to the X-mode. The inverted spectral shape can in principle be explained if the high linearly polarized emission in these time samples are formed due to incoherent addition of CCR from a large number of charged solitons (charge bunches) exciting the X-mode.

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arXiv2024

Decoding the nature of Coherent radio emission in Pulsars I: Observational constraints

Radio observations from normal pulsars indicate that the coherent radio emission is excited by curvature radiation from charge bunches. In this review we provide a systematic description of the various observational constraints on the radio emission mechanism. We have discussed the presence of highly polarized time samples where the polarization position angle follow two orthogonal well defined tracks across the profile, that closely match the rotating vector model in an identical manner. The observations also show the presence of circular polarization, with both the right and left handed circular polarization seen across the profile. Other constraints on the emission mechanism is provided by the detailed measurements of the spectral index variation across the profile window, where the central part of the profile, corresponding to the core component, has a steeper spectrum than the surrounding cones. Finally, the detailed measurements of the subpulse drifting behaviour can be explained by considering the presence of non-dipolar field on the stellar surface and the formation of the Partially screened Gap (PSG) above the polar cap region. The PSG gives rise to a non-stationary plasma flow, that has a multi-component nature, consisting of highly energetic primary particles, secondary pair plasma and iron ions discharged from the surface, with large fragmentation resulting is dense plasma clouds and lower density inter-cloud regions. The physical properties of the outflowing plasma and the observational constraints lead us to consider coherent curvature radiation as the most viable explanation for the emission mechanism in normal pulsars, where propagation effects due to adiabatic walking and refraction are largely inconsequential.

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arXiv2024

Euclid view of dusty star forming galaxies at z>~1.5 detected in wide area submillimetre surveys

We investigate the constraints provided by the Euclid space observatory on the physical properties of dusty star forming galaxies (DSFGs) at z>~1.5 detected in wide area sub millimetre surveys with Herschel. We adopt a physical model for the high z progenitors of spheroidal galaxies, which form the bulk of the DSFGs at z>~1.5. We improve the model by combining the output of the equations of the model with a formalism for the spectral energy distribution(SED). After optimising the SED parameters to reproduce the measured infrared luminosity function and the number counts of DSFGs, we simulated a sample of DSFGs over 100 sq deg and then applied a 5 sigma detection limit of 37 mJy at 250 microns. We estimated the redshifts from the Euclid data and then fitted the Euclid and Herschel photometry with the code CIGALE to extract the physicsl parameters. We found that 100 % of the Herschel galaxies are detected in all 4 Euclid bands above 3 sigma. For 87% of the sources the accuracy on 1+z is better than 15%. The sample comprises mostly massive log(Mstar/Msun)~10.5-12.9, highly star forming, log(SFR/Msun/yr)~1.5-4, dusty, log(Mdust/Msun)~7.5-9.9, galaxies. The measured stellar mass have a dispersion of 0.19 dex around the true value, thus showing that Euclid will provide reliable stellar mass estimates for the majority of the bright DSFGs at z>~1.5 detected by Herschel. We also explored the effect of complementing the Euclid photometry with that from Vera C. Rubin Observatory/LSST.

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arXiv2024

The Thousand-Pulsar-Array programme on MeerKAT XIV: On the high linearly polarized pulsar signals

The S-shaped swing of the linear polarization position angle (PPA) observed in many pulsars can be interpreted by the rotating vector model (RVM). However, efforts to fit the RVM for a large sample of pulsars observed with the MeerKAT telescope as a part of the Thousand-Pulsar-Array (TPA) programme, only succeeded for about half the cases. High time-resolution studies suggest that the failed cases arise due to the presence of orthogonal polarization modes, or highly disordered distribution of PPA points. One such example is PSR~J1645-0317. Recently it has been shown that the RVM can be recovered in this pulsar by using only time samples which are greater than 80% linearly polarized. In this work we test this novel approach on the brightest 249 pulsars from the TPA sample, of which 177 yield sufficient highly polarized samples to be amenable to our method. Remarkably, only 9 of these pulsars (5%) now fail to fit the RVM as opposed to 59% from the original analysis. This result favours the paradigm that the underlying mechanism is coherent curvature radiation.

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arXiv2023

Mode changing in PSR B0844-35 and PSR B1758-29 with enhanced emission at the profile centers

We have studied the single pulse emission from two pulsars, PSR B0844-35 and PSR B1758-29, over a wide frequency range of 300-750 MHz using the uGMRT. The two pulsars have relatively wide profiles with multiple components, that are a result of the line of sight traversing near center of the emission beam. In both pulsars the single pulse sequences show the presence of two distinct emission states, where the profiles become much brighter at the center with prominent core components during one of the modes, while in the other mode the single pulses show odd-even subpulse drifting with periodicity around 2$P$, $P$ being the rotation period of the pulsar. The centrally bright mode was seen for 10 percent of the observing duration in PSR B0844-35, which usually lasted for short durations of around 10 pulses, but had two longer sequences of around 100 pulses. On the contrary the centrally bright mode was dominant in PSR B1758-29 and was seen for around 60 percent of the observing duration. PSR B1758-29 also showed period amplitude modulations of 60-70$P$ in both modes. The mode changing in these two pulsars facilitates investigation of the sparking process in the inner acceleration region, dominated by non-dipolar magnetic fields. The change in the surface magnetic field configurations likely results in the emission mode change.

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arXiv2023

Meterwavelength Single-pulse Polarimetric Emission Survey. VI. Towards understanding the Phenomenon of Pulsar Polarization in Partially Screened Vacuum Gap model

We have observed 123 pulsars with periods longer than 0.1 seconds in the Meterwavelength Single-pulse Polarimetric Emission Survey. In this work a detailed study of polarization behaviour of these pulsars have been carried out. We were able to fit the rotating vector model to the polarization position angle sweeps in 68 pulsars, and in 34 pulsars the emission heights could be measured. In all cases the radio emission was constrained to arise below 10\% of the light cylinder radius. In pulsars with low spindown energy loss, $\dot{E}<10^{34}$ ergs s$^{-1}$, we found the mean fractional linear polarization of the individual times samples in single pulses to be around 0.57 (57\%) which is significantly larger than the fractional linear polarization of 0.29 (29\%) obtained from the average profiles. On the other hand the mean fractional circular polarization of the individual time samples in single pulses is around 0.08 (8\%), similar to the measurements from the average profiles. To explain the observed polarization features, we invoke the partially screened vacuum gap model of pulsars, where dense spark associated plasma clouds exist with high pair plasma multiplicity, with significant decrease of density in the regions between the clouds, that are dominated by iron ions. The coherent radio emission is excited by curvature radiation from charge bunches in these dense plasma clouds and escape as linearly polarized waves near cloud boundaries. We suggest that the circular polarization arises due to propagation of waves in the low pair multiplicity, ion dominated inter-cloud regions.

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arXiv2023

Estimating the evolution of Sparks in Partially Screened Gap of Pulsars from Subpulse Drifting

A novel scheme has been developed to show that the observed phase behaviour associated with subpulse drifting from two pulsars, J1034$-$3224 and J1720$-$2933, can be used to obtain the magnetic field configuration in the partially screened gap (PSG). The outflowing plasma along the open magnetic field line region of pulsars is generated due to spark discharges in an inner acceleration region (IAR) above the polar cap. The IAR has been modelled as a partially screened gap (PSG) with a steady supply of positively charged ions emitted from the heated polar cap surface dominated by strong non-dipolar magnetic fields. In a PSG the sparks are tightly packed and constrained to be present along the polar cap boundary. The sparks lag behind the rotation of the star during their lifetimes. As a result the sparking pattern evolves along two different directions in the clockwise and counter-clockwise manner around a stationary central spark, and can be associated with the observed phenomenon of subpulse drifting. PSR J1034$-$3224 has four prominent components and exhibit bi-drifting where alternate components show opposite sense of drifting, while PSR J1720$-$2933 has a single component profile and shows systematic coherent drift bands. We show that the differences in their drifting behaviour can be directly linked to different natures of the non-dipolar surface magnetic field configurations.

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arXiv2023

Evidence for Coherent Curvature Radiation in PSR J1645$-$0317 with Disordered Distribution of Polarization Position Angle

The diverse polarization properties in pulsars are in conflict with applying a unique emission mechanism to the population. The polarization position angle (PPA) traverse in most pulsars shows a S-shaped curve that can be interpreted using the rotating vector model (RVM) as the radio emission being directed either parallel or perpendicular to the divergent magnetic field lines and argues for a coherent curvature radiation mechanism from charge bunches in a strongly magnetized pair plasma. However, in a subset of pulsars the radio emission is significantly depolarized and the PPA shows a complex pattern which cannot be explained using RVM. We propose that even in such cases the highly polarized time samples in the single pulses should follow the RVM with possibly two parallel tracks separated by 90\degr. We have investigated PSR J1645$-$0317, with complex PPA traverse, and demonstrated for the first time that considering only the highly polarized time samples in the single pulses, the PPA distribution clearly follows the RVM. We conclude that this strongly favour the coherent curvature radiation mechanism to be universally applicable in the pulsar population.

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