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Manami Roy

Publications and source records attributed to Manami Roy.

At least 19 recordsLinked to original sources
arXiv2026

Thermal Structure and Chemical Enrichment of the North and South Polar Spurs: Supersolar N/O and Ne/O in the X-ray Plasma

The North Polar Spur (NPS) is a prominent diffuse X-ray feature whose origin has remained uncertain for decades. Using a uniform analysis of archival \textit{Suzaku} and \textit{XMM--Newton} data with new \textit{Chandra} observations, we constrain its thermal and chemical properties. The NPS emission is fully absorbed by the neutral interstellar medium, demonstrating that the plasma lies beyond the Galactic disk and is not a local supernova remnant or nearby superbubble. The spectra require a two-temperature model with a warm--hot component ($kT \approx 0.2$ keV) and a hotter component ($kT = 0.4$--$0.5$ keV), with emission measures of $(41.8 \pm 4.9) \times 10^{-3}$ and $(12.9 \pm 2.2) \times 10^{-3} \mathrm{cm^{-6}~pc}$, respectively. A key result is the detection of super-solar abundance ratios in the warm--hot phase, with N/O $= 3.6 \pm 0.3$ and Ne/O $= 1.9 \pm 0.1$ solar. A Suzaku observation of the outer South Polar Spur (SPS) shows similar absorption, temperatures, and enhanced abundances (N/O $= 2.9 \pm 0.4$, Ne/O $= 1.6 \pm 0.2$), though with lower emission measures. The similar super-solar abundance ratios suggest a common enrichment history. These properties are consistent with those measured along other sightlines through the X-ray--bright shells of the Galactic bubbles. Together, these results support that the NPS and SPS trace opposite limbs of the Galactic bubbles. The chemical properties suggest a strong contribution from stellar feedback in shaping the Galactic bubbles.

astro-ph.HE↗
arXiv2026

Not Where You Left Them: Displaced $\gamma$-Rays and X-Rays Reveal the Cosmic Ray Scattering Rate

Modern X-ray and $\gamma$-ray instruments are revealing a growing class of Galactic non-thermal sources whose emission centroids are measurably offset from the nearest plausible sites of cosmic ray (CR) acceleration. Such "displaced" sources are seen in keV X-rays and TeV-PeV $\gamma$-rays but not in GeV $\gamma$-rays, have hard spectra, and are not associated with gas clumps, suggesting a leptonic origin. We develop a general framework for understanding displacement, whereby relativistic CR electrons (CRe) injected into the interstellar medium (ISM) with a strongly anisotropic pitch-angle distribution propagate a finite distance from their acceleration site before scattering processes isotropise their directions sufficiently for the emission to become visible. We use CR transport simulations to investigate under what circumstances displacement is likely, finding that it requires an initial pitch angle distribution $\lesssim 45^\circ$ wide, a line of sight broadly edge-on to the magnetic field, and that the source be measured in a waveband where emission is dominated by CRe for which the radiative-loss and pitch-angle scattering timescales are comparable. For typical Galactic conditions the latter condition is satisfied only for CRe energies $\gtrsim$ 10 TeV, explaining why displaced sources appear at X-ray and TeV but not GeV energies. We further show that, when displacement is detected, it allows a direct inference of the CRe pitch-angle scattering rate.

astro-ph.HE↗
arXiv2026

Weibel Instability-Driven Seed Magnetic Fields during Reionization

Cosmological reionization was a highly out-of-equilibrium event that affected every parcel of the intergalactic medium, making it a candidate for astrophysical generation of intergalactic magnetic fields. During reionization, the first stars and galaxies ionized the surrounding, largely neutral, medium in ever-expanding envelopes. Photoionization from sources on one side of the front, combined with the quadrupolar angular dependence of the photoionization cross section, leads to an anisotropic electron velocity distribution. We investigate instabilities in these reionization fronts as a mechanism to generate seed magnetic fields. The Weibel instability has the potential to create a magnetic field from these anisotropies. We calculate the magnitude of the isotropic and anisotropic distribution within a simulated reionization front. We find that the fractional anisotropy can grow to $6\times 10^{-3}$ toward the middle of the ionization front. We show that the linear growth timescale of the Weibel instability is fast compared to the crossing time of the ionization front ($\sim 2\times 10^5$ seconds). We briefly speculate on the possible non-linear evolution of the instability and the implications for cosmological magnetogenesis.

astro-ph.CO↗
arXiv2025

To Survive or to Shatter: The Impact of Cosmic Rays on the Fate of Stripped Cold Clouds

Does cosmic ray (CR) pressure matter for the circumgalactic medium (CGM)? Despite growing interest, this remains a debated question, complicated by limited observational constraints and differing implementations of CR physics in simulations. While prior studies suggest that CRs influence the thermal and dynamical state of the CGM, their role in shaping cold gas structures remains underexplored. This paper investigates how CRs affect ram-pressure stripped cold gas clouds originating from satellite galaxies in a Milky Way-like halo. Using high-resolution simulations with varying CR energy densities, we find that CRs can significantly modify the size and survival of stripped clouds. Specifically, CR pressure puffs up the cold clouds, increasing their surface area and enabling more efficient mixing-layer cooling, allowing them to grow in mass. This enhanced growth results in higher cold gas inflow rates into the central galaxy, leading to an increase in the star formation rate compared to the no-CR case at a later time. Moreover, CRs can boost the total cold gas mass in the CGM by up to a factor of four. These effects are most pronounced in simulations where the CR energy density is in equipartition with the thermal gas. Our results demonstrate that CRs can play a critical role in regulating the cold phase of the CGM contributed by satellites and therefore their ability to feed galaxies.

astro-ph.GA↗
arXiv2025

Detection of 'super-virial' gas in the Circumgalactic medium of the Milky Way towards PKS 2155-304

We present the first simultaneous detection of four distinct highly ionized $z=0$ absorbing phases using Chandra and XMM-Newton grating spectra toward the blazar PKS 2155$\hbox{-}$304. We detect the MgXII K$\alpha$ absorption line for the first time in the circumgalactic medium (CGM) of the Milky Way. Along with MgXII K$\alpha$, we detect SiXIV K$\alpha$ absorption, which are the tell-tale signatures of the hot 'super-virial' gas in the CGM. Both from the model-independent calculations and hybrid-ionization modeling, we infer four phases at distinct temperatures, hot 'super-virial' ($5.4^{+1.9}_{-0.8} \times 10^7$ K), warm-hot 'virial' ($1.8^{+0.3}_{-0.2} \times 10^6$ K), warm 'sub-virial' ($2.2\pm 0.5 \times 10^5$ K), and cool phase ($<1.7 \times 10^5$ K). The warm-hot and hot phases are $\alpha$-enhanced, and [C/O] and [Ne/O] are super-solar in the warm-hot phase, while [Mg/O] and [Si/O] are super-solar in the hot phase. The low-ionization lines are blue-shifted (v$_{\rm los} \approx -100$ km s$^{-1}$), whereas the high-ionization lines are red-shifted. It suggests a scenario of infalling sub-virial, quasi-static virial, and outflowing super-virial phases along this sightline. Earlier studies on individual sightlines were confined to the Northern Hemisphere. Our sightline is located in the Southern hemisphere, demonstrating that hot super-virial gas is also present at Southern Galactic latitudes as well. This confirms a more widespread distribution of the super-virial gas across both hemispheres.

astro-ph.GA↗
arXiv2025

Where is the Supervirial Gas? III. Insights from X-ray Shadow Observations and a revised Model for the Soft Diffuse X-ray Background

Shadow observations provide a powerful tool to separate foreground components of the soft diffuse X-ray background (SDXB) from the background components. Such observations have now established that the ``local'' foreground is made of the solar wind charge exchange and the local bubble, and the background emission is from the extended circumgalactic medium (CGM) of the Milky Way and from the unresolved extragalactic sources. New data and careful analyses of the SDXB led to two new discoveries in recent years: (1) excess emission near 0.5 keV that is identified as the NVII emission line, and (2) excess emission near 0.8-1.0 keV that is identified with an additional, super-virial temperature hot thermal component of the CGM. The goal of this paper is to use Suzaku shadow observations along six sightlines to determine whether either of these components is from the ``local'' sources. We eliminate the ambiguity regarding the origin of NVII emission, ruling out the local origin. We confirm that the Milky Way CGM contains nitrogen-rich plasma, with a super-solar average (N/O) of 2.6+-0.5, and suggest that nitrogen-enhanced plasma is widespread throughout the CGM. We find super-solar Ne abundance in two sighlines, also from the CGM. Similarly, we rule out the local origin of the hot thermal component and confirm that it is present beyond the shadowing clouds. Furthermore, we provide a revised model of the soft diffuse X-ray background, which is crucial for extragalactic astronomy.

astro-ph.GA↗
arXiv2025

Local data of elliptic curves under quadratic twist

Let $K$ be the field of fractions of a complete discrete valuation ring with a perfect residue field. In this article, we investigate how the Tamagawa number of $E/K$ changes under quadratic twist. To accomplish this, we introduce the notion of a strongly-minimal model for an elliptic curve $E/K$, which is a minimal Weierstrass model satisfying certain conditions that lead one to easily infer the local data of $E/K$. Our main results provide explicit conditions on the Weierstrass coefficients of a strongly-minimal model of $E/K$ to determine the local data of a quadratic twist $E^{d}/K$. We note that when the residue field has characteristic $2$, we only consider the special case $K=\mathbb{Q}_{2}$. In this setting, we also determine the minimal discriminant valuation and conductor exponent of $E$ and $E^d$ from further conditions on the coefficients of a strongly-minimal model for $E$.

math.NT↗
arXiv2024

Where is the Super-virial Gas? II: Insight from the Survey of Galactic Sightlines

Recent observations have revealed a super-virial temperature gas phase at log(T/K) $\sim7$ in the Milky Way, challenging existing galaxy-formation models. This hot gas phase was discovered toward extragalactic absorption sightlines and blank-sky emission fields, both at high galactic latitudes. The location of this hot component is unknown; is it in the extended circumgalactic medium (CGM) or in the interstellar medium (ISM) instead? We analyzed X-ray spectra from Chandra's High-Energy Transmission Grating (HETG) observations of 27 Galactic X-ray binaries (XRBs) to investigate whether the hot gas component is present in the ISM. We searched for absorption lines of SXVI K$\alpha$, SiXIV K$\alpha$, and NeX K$\alpha$, which are the tell-tale signatures of the hot gas and which have been detected toward extragalactic sightlines. Of the 27 targets, these lines were detected in the spectra of only 7, with two sources displaying broad line features likely intrinsic to the XRB systems. Additionally, most of the detected lines are time-variable, reinforcing their likely association with the XRBs. Our results suggest that the super-virial temperature gas is not a widespread component of the ISM but may instead be located in extraplanar regions or the extended CGM, in line with some recent simulation results.

astro-ph.GA↗
arXiv2024

Where is the Super-Virial Gas? The Supply from hot inflows

In an effort to understand the presence of super-virial gas detected in the Milky Way, we present our findings from isolated galaxy simulations of Milky Way-like systems using GIZMO with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. It unveils the presence of a significant super-virial temperature ( $T>6\times10^6$K) gas component within 20 kpc from the galactic center. This super-virial gas has a mass of $1-2\times10^7$ ${\rm M_\odot}$ and is found close to the disk, where typical gas densities are $0.004-0.01 \, \rm cm^{-3}$. We find that some of the virial gas ($T\sim10^6$K) forms a rotating hot inflow, where gravitational energy is converted to heat mainly via compressive heating. This process causes gas infalling close to the rotation axis to reach super-virial temperatures just before cooling and joining the disk. Stellar feedback heating accounts for less than 1% of the super-virial gas, indicating its minimal influence despite expectations. Even in scenarios with no stellar feedback effects considered, abundant super-virial gas persists, highlighting the dominance of alternative heating mechanisms. We also show that cosmic rays do not have a significant effect on heating the gas to a super-virial temperature. Our study illuminates the intricate dynamics of hot virial and super-virial gas surrounding Milky Way-like galaxies, emphasizing the prominent role of infall-driven and compressive heating processes in shaping thermal evolution.

astro-ph.GA↗
arXiv2024

Creating a dynamic database of finite groups

A database of abstract groups has been added to the L-functions and Modular Forms Database (LMFDB), available at https://www.lmfdb.org/Groups/Abstract/. We discuss the functionality of the database and what makes it distinct from other available databases of abstract groups. We describe solutions to mathematical problems we encountered while creating the database, as well as connections between the abstract groups database and other collections of objects in the LMFDB.

math.GR↗
arXiv2024

Supercongruences arising from Ramanujan-Sato Series

Recently, the authors with Lea Beneish established a recipe for constructing Ramanujan-Sato series for $1/\pi$, and used this to construct 11 explicit examples of Ramanujan-Sato series arising from modular forms for arithmetic triangle groups of non-compact type. Here, we use work of Chisholm, Deines, Long, Nebe and the third author to prove a general $p$-adic supercongruence theorem through an explicit connection to CM hypergeometric elliptic curves that provides $p$-adic analogues of these Ramanujan-Sato series. We further use this theorem to construct explicit examples related to each of our explicit Ramanujan-Sato series examples.

math.NT↗
arXiv2024

Where is the Supervirial hot gas? I: A pilot study with sightlines to Galactic X-ray binaries

Hot, \logT\ $\sim$ 7.5, gas was recently discovered in the Milky Way in extragalactic sightlines. In order to determine its location, here we present sightlines to Galactic X-ray binaries (XRBs) passing through the Interstellar Medium (ISM). In this pilot study we investigate absorption features of \SXVI, \SiXIV, and \NeX\ in the spectra of three XRBs, namely 4U 1735-44, 4U 1820-30, and Cyg X-2, using Chandra High Energy Transmission Grating archival observations. We do not detect any of these lines. {We determine the 2$\sigma$ upper limit for the equivalent widths of the undetected absorption lines and the column densities of the corresponding ions.} We note that the 2$\sigma$ upper limits for \SXVI\ \Ka\ and \SiXIV\ \Ka\ are an order of magnitude smaller than those previously detected in the extragalactic sightlines. Our finding suggests that if any gas at \logT\ $>7$ is present in the Galactic ISM, it is unlikely to be ubiquitous. This is an important result because it implies that \SXVI, \SiXIV\ {and \NeX\ }absorption detected in extragalactic sightlines is not from the ISM, but is likely from a hot gas phase in the extraplanar region beyond the ISM or in the extended CGM.

astro-ph.GA↗
arXiv2024

Towards a classification of $p^2$-discriminant ideal twins over number fields

Isogenous elliptic curves have the same conductor but not necessarily the same minimal discriminant ideal. In this article, we explicitly classify all $p^2$-isogenous elliptic curves defined over a number field with the same minimal discriminant ideal for odd prime $p$ where $X_0(p^2)$ has genus $0$, i.e., $p = 3$ or $5$. As a consequence, we give a list of all $p^2$-isogenous discriminant (ideal) twins over $\mathbb{Q}$ for such $p$.

math.NT↗
arXiv2024

Prime isogenous discriminant ideal twins

Let $E_{1}$ and $E_{2}$ be elliptic curves defined over a number field $K$. We say that $E_{1}$ and $E_{2}$ are discriminant ideal twins if they are not $K$-isomorphic and have the same minimal discriminant ideal and conductor. Such curves are said to be discriminant twins if, for each prime $\mathfrak{p}$ of $K$, there are $\mathfrak{p}$-minimal models for $E_{1}$ and $E_{2}$ whose discriminants are equal. This article explicitly classifies all prime-isogenous discriminant (ideal) twins over $\mathbb{Q}$. We obtain this classification as a consequence of our main results, which constructively gives all $p$-isogenous discriminant ideal twins over number fields where $p\in\left\{ 2,3,5,7,13\right\} $, i.e., where $X_0(p)$ has genus $0$. In particular, we find that up to twist, there are finitely many $p$-isogenous discriminant ideal twins if and only if $K$ is $\mathbb{Q}$ or an imaginary quadratic field. In the latter case, we provide instructions for finding the finitely many pairs of $j$-invariants that result in $p$-isogenous discriminant ideal twins. We prove our results by considering the local data of parameterized $p$-isogenous elliptic curves.

math.NT↗
arXiv2024

The quaternionic Maass Spezialschar on split $\mathrm{SO}(8)$

The classical Maass Spezialschar is a Hecke-stable subspace of the space of holomorphic Siegel modular forms of genus two and level one cut out by certain linear relations among Fourier coefficients. We define an analogous quaternionic Maass Spezialschar, which consists of the quaternionic modular forms of level one on split $\mathrm{SO}(8)$ whose Fourier coefficients satisfy certain linear relations. We characterize this space in terms of a theta lift from the space of holomorphic Siegel modular forms on $\mathrm{Sp}(4)$, and in terms of periods. We also give a conjecture for the Dirichlet series of the standard $L$-function of quaternionic modular eigenforms on $\mathrm{SO}(8)$ and verify our conjecture on the quaternionic Maass Spezialschar.

math.NT↗
arXiv2023

Seeding the CGM: How Satellites Populate the Cold Phase of Milky Way Halos

The origin of the cold phase in the CGM is a highly debated question. We investigate the contribution of satellite galaxies to the cold gas budget in the circumgalactic medium (CGM)of a Milky Way-like host galaxy. We perform controlled experiments with three different satellite mass distributions and identify several mechanisms by which satellites can add cold gas to the CGM, including ram pressure stripping and induced cooling in the mixing layer of the stripped cold gas. These two mechanisms contribute a comparable amount of cold gas to the host CGM. We find that the less massive satellites ($\leq 10^9 M_\odot$) not only lose all of their cold gas in a short period ($\sim$ 0.5-1 Gyr), but their stripped cold clouds also mix with the hot CGM gas and get heated up quickly. However, stellar feedback from these less massive satellites can hugely alter the fate of their stripped gas. Feedback speeds up the destruction of the stripped cold clouds from these satellites by making them more diffuse with more surface area. On the other hand, the more massive satellites (LMC or SMC-like $\sim 10^{10} M_\odot$) can add cold gas to the total gas budget of the host CGM for several Gyrs.

astro-ph.GA↗
arXiv2023

Beyond radial profiles: Using log-normal distributions to model the multiphase circumgalactic medium

Recent observations and simulations reveal that the circumgalactic medium (CGM) surrounding galaxies is multiphase, with the gas temperatures spanning a wide range at most radii, $\sim 10^4\ {\rm K}$ to the virial temperature ($\sim 10^6$ K for Milky Way). Traditional CGM models using simple density profiles are inadequate at reproducing observations that indicate a broad temperature range. Alternatively, a model based on probability distribution functions (PDFs) with parameters motivated by simulations can better match multi-wavelength observations. In this work, we use log-normal distributions, commonly seen in the simulations of the multiphase interstellar and circumgalactic media, to model the multiphase CGM. We generalize the isothermal background model by Faerman et al. 2017 to include more general CGM profiles. We extend the existing probabilistic models from 1D-PDFs in temperature to 2D-PDFs in density-temperature phase space and constrain its parameters using a Milky Way-like {\tt Illustris TNG50-1} halo. We generate various synthetic observables such as column densities of different ions, UV/X-ray spectra, and dispersion and emission measures. X-ray and radio (Fast Radio Burst) observations mainly constrain the hot gas properties. However, interpreting cold/warm phase diagnostics is not straightforward since these phases are patchy, with inherent variability in intercepting these clouds along arbitrary lines of sight. We provide a tabulated comparison of model predictions with observations and plan to expand this into a comprehensive compilation of models and data. Our modeling provides a simple analytic framework that is useful for describing important aspects of the multiphase CGM.

astro-ph.GA↗