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Sandeep Kumar Acharya

Publications and source records attributed to Sandeep Kumar Acharya.

At least 19 recordsLinked to original sources
arXiv2026

The role of the galaxy stellar mass function in determining the cosmological distribution of astrophysical transients with applications to fast radio bursts and merging binary black holes

The cosmological distribution and formation rate of compact astrophysical objects such as fast radio bursts (FRBs) are typically assumed to be proportional to a linear combination of cosmological star formation rate and stellar mass. In the literature, a template for star formation rate, which is just a function of redshift, is typically used. In this work, we point out the importance of galaxy stellar mass function which captures the host galaxy information of observed FRBs as well as the redshift evolution of galaxy stellar mass. Using this information and taking the stellar mass distribution of a sample of localized FRBs at face value, we find that FRB formation efficiency per stellar mass may have to be more efficient (by a factor of $\approx 3$) than previously calculated, in order to reproduce the observed volumetric rate of FRBs at $z=0$. We show that cosmological population studies of FRBs have to include host galaxy information along with its redshift evolution in order to obtain unbiased results. This consideration is also applicable to other transients, e.g. gamma-ray bursts and merging binary black hole events. We show that our approach may open up the possibility to distinguish between different scenarios of merging binary black holes formation with a detection of few thousand gravitational wave events.

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arXiv2026

Probing inflationary features with galaxy ultraviolet luminosity function observables

We use the galaxy ultraviolet luminosity function measurements at $z=4-8$ to constrain modification to standard inflationary power spectrum. These observables are sensitive to the matter power spectrum which itself depends on inflationary initial conditions. We consider a specific model where a bump feature is introduced to the standard power law inflation spectrum. We find that the galaxy luminosity observables can probe such modifications at wavenumbers $0.5\lesssim k \lesssim 50$ Mpc$^{-1}$. We obtain upper limits on the amplitude of bump-like features at the mentioned wavenumbers. We obtain constraints which are similar to previous constraints on these models using measurements of optical depth of reionization. However, the galaxy luminosity functions are a more direct probe for these type of models and, therefore, can complement indirect constraints coming from measurements of IGM properties.

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arXiv2025

Utilizing localized fast radio bursts to constrain their progenitors and the expansion history of the Universe

Fast radio bursts (FRBs) are increasingly being used for cosmological applications such as measuring the Hubble constant and baryon abundance. The increasing number of localized FRBs and precise measurement of dispersion measure (DM) make them a suitable probe for such an approach. We use a sample of 110 localized FRBs as well as a small sub-sample of 24 FRBs with scattering timescale measurements or limits. We infer the Hubble constant ($H_0$) and the DM distribution of the host galaxies simultaneously by fitting our model to the FRB DM measurements. With current data, our results are in agreement with both high and low redshift measurements of $H_0$, obtained using Cosmic Microwave Background (CMB) and Type Ia supernovae data respectively. We project that with about 200 localized FRBs, we would be in a position to distinguish between the two scenarios at 4$σ$ confidence. In addition, the host DM is expected to be related to star formation in the host galaxy and the stellar age of the progenitors. We show that young progenitors with an age of less than 1 Myr are consistent with our inferred distribution of host DM at 95 percent confidence. These young sources may be associated with long scatter broadening times and large DM from their source environments. Indeed, we find that scatter broadening times of FRBs are inconsistent with the Milky Way ISM, but at the same time, do not appear to be strongly correlated with the FRBs' redshift or with the SFR or stellar mass of their host galaxies. This suggests that scattering is dominated by the immediate environment of the sources.

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arXiv2025

Persistent radio sources associated with fast radio bursts: Implications from magnetar progenitors

The rare association of three persistent radio sources (confirmed PRS1 and PRS2, candidate PRS3) with repeating fast radio bursts (FRB 20121102A, 20190520B, 20201124A) offers a unique probe into their magneto-ionic environments. PRSs are attributed to synchrotron emission from relativistic charged particles of magnetar wind nebula (MWN) powered by spin-down magnetohydrodynamic wind or internal magnetic field decay. Using a multizone hydrodynamic model, we track MWN evolution to constrain magnetar progenitor properties. For PRS1 and PRS2, we find an equipartition radius $R_\mathrm{eq} \sim 0.1 $ pc that is consistent with the radio scintillation estimates ($> 0.03$ pc) and radio imaging limits ($<0.7$ pc). This compact size favors low expansion speeds and large initial spin periods, $P_\mathrm{i} \gtrsim 10$ ms, ruling out millisecond magnetar progenitors. Given $P_\mathrm{i} \gtrsim 10$ ms, a current size of $\sim 0.1$ pc, a supernova kinetic energy $E_\mathrm{ SN} \sim 10^{50}-10^{51}$ erg and an ejecta mass $M \sim 3-10 \; M_{\odot}$, the PRS age is $t \sim 10-10^{2}$ yr. PRSs with $t>20$ years require an internal field ($B_\mathrm{int} \sim 10^{16}-10^{16.5}\;$G) with a decay timescale $t_\mathrm{d} \sim 10-10^{2.5}$ yr. The slowest field decay ($t_\mathrm{d,max} \sim 500$ yr) favors sub-energetic supernovae ($E_\mathrm{SN} \sim 10^{50}$ erg) with massive ejecta ($M \gtrsim 10\; M_{\odot}$) and low ionization fraction ($\sim 3\% $). For the sub-energetic scenario for the confirmed PRSs, we predict a cooling break at $100-150$ gigahertz at $20-40 \; μ\text{Jy}$ and self-absorption near 200 megahertz at $180\; μ\text{Jy}$. For PRS3, a rotation-powered MWN is viable only if $t \sim 10$ yr; an inverted spectrum beyond 150 gigahertz would rule out this scenario.

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arXiv2025

Redshift dependence of FRB host dispersion measures across cosmic epochs

We constrain the redshift dependence of (rest frame) host galaxy dispersion measures of localized Fast Radio Bursts (FRBs) by assuming it to vary as a simple power law ($\propto (1+z)^α$). We simultaneously fit $α$ as well as the host dispersion measure to the data of FRBs with known redshifts. We find that \color{black} $α$ between 0 to 1 is preferred depending upon our modelling choices. Current data can constrain $|α|\lesssim 2$ at a 68 percent confidence interval \color{black}. Such constraints have implications for our understanding of galaxy formation and can be used to inform galaxy and large scale simulations.

astro-ph.CO↗
arXiv2025

Features and prospects for kilonova remnant detection with current and future surveys

We study the observable spectral and temporal properties of kilonova remnants (KNRs) analytically, and point out quantitative differences with respect to supernova remnants. We provide detection prospects of KNRs in the context of ongoing radio surveys. We find that there is a good chance to expect tens of these objects in future surveys with a flux threshold of $\sim 0.1$ mJy. Kilonova remnants from a postulated population of long-lived supermassive neutron star remnants of neutron star mergers are even more likely to be detected, as they are extremely bright and peak earlier. For an ongoing survey with a threshold of $\sim$ 1 mJy, we expect to find tens to hundreds of such objects if they are a significant fraction of the total kilonova (KN) population. Considering that there are no such promising KN candidates in presently ongoing surveys, we constrain the fraction of these extreme KN to be no more than 30 percent of the overall KN population. This constraint depends sensitively on the details of ejecta mass and external density distribution.

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arXiv2024

Sunyaev-Zeldovich signatures from non-thermal, relativistic electrons using CMB maps

Relativistically energetic, non-thermal population of electrons can give rise to unique CMB spectral distortion signatures which can be significantly different from thermal Sunyaev-Zeldovich signal or $y$-distortion. These signatures depend upon the spectrum of non-thermal electrons, therefore, a detection can inform us about the existence and abundance of non-thermal electrons in our universe. Using public CMB maps and data, we derive upper limits on non-thermal $y$-parameter for a relativistic, power-law electron distribution. With future CMB experiments, we may be in a position to detect or put significantly tighter constraints on these signals which can affect our understanding of non-thermal electron distributions in our universe.

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arXiv2024

Soft Photon Heating: A Semi-Analytic Framework and Applications to $21$cm Cosmology

The presence of an abundant population of low frequency photons at high redshifts (such as a radio background) can source leading order effects on the evolution of the matter and spin temperatures through rapid free-free absorptions. This effect, known as soft photon heating, can have a dramatic impact on the differential brightness temperature, $ΔT_{\rm b}$, a central observable in $21$cm cosmology. Here, we introduce a semi-analytic framework to describe the dynamics of soft photon heating, providing a simplified set of evolution equations and a useful numerical scheme which can be used to study this generic effect. We also perform quasi-instantaneous and continuous soft photon injections to elucidate the different regimes in which soft photon heating is expected to impart a significant contribution to the global $21$cm signal and its fluctuations. We find that soft photon backgrounds produced after recombination with spectral index $γ> 3.0$ undergo significant free-free absorption, and therefore this heating effect cannot be neglected. The effect becomes stronger with steeper spectral index, and in some cases the injection of a synchrotron-like spectrum ($γ= 3.6$) can suppress the amplitude of $ΔT_{\rm b}$ relative to the standard model prediction, making the global $21$cm signal even more difficult to detect in these scenarios.

astro-ph.CO↗
arXiv2023

Disentangling the primordial nature of stochastic gravitational wave backgrounds with CMB spectral distortions

The recent detection of a stochastic gravitational wave background (SGWB) at nanohertz frequencies by pulsar timing arrays (PTAs) has sparked a flurry of interest. Beyond the standard interpretation that the progenitor is a network of supermassive black hole binaries, many exotic models have also been proposed, some of which can potentially offer a better fit to the data. We explore how the various connections between gravitational waves and CMB spectral distortions can be leveraged to help determine whether a SGWB was generated primordially or astrophysically. To this end, we present updated $k$-space window functions which can be used for distortion parameter estimation on enhancements to the primordial scalar power spectrum. These same enhancements can also source gravitational waves (GWs) directly at second order in perturbation theory, so-called scalar-induced GWs (SIGWs), and indirectly through the formation of primordial black holes (PBHs). We perform a mapping of scalar power spectrum constraints into limits on the GW parameter space of SIGWs for $δ$-function features. We highlight that broader features in the scalar spectrum can explain the PTA results while simultaneously producing a spectral distortion (SD) within reach of future experiments. We additionally update PBH constraints from $μ$- and $y$-type spectral distortions. Refined treatments of the distortion window functions widen existing SD constraints, and we find that a future CMB spectrometer could play a pivotal role in unraveling the origin of GWs imprinted at or below CMB anisotropy scales.

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arXiv2023

Constraining broad photon spectrum injections from exotic and astrophysical sources

We study the evolution of photon injections with a power-law type spectrum inserted at various epochs of the universe, and obtain constraints on their parameter space from multiple different cosmological probes. Our work is motivated by the realistic possibility of having extended photon spectra from astrophysical and exotic sources. Going beyond a $δ$-function like approximation, the physics becomes richer and the constraining power of cosmological probes starts to depend on the photon injection history in a complex way. As a toy model, we first consider a decaying particle scenario, and then generalize to a more model independent power law type injection in redshift. Different combinations of our parameters can be mapped to a wide variety of realistic astrophysical and exotic sources, providing useful benchmarks for study in future work.

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arXiv2023

A cosmic string solution to the radio synchrotron background

We investigate the low-frequency spectral emission from a network of superconducting cosmic string loops in hopes of explaining the observed radio synchrotron background. After considering constraints from a variety of astrophysical and cosmological measurements, we identify a best-fit solution with string tension $Gμ\simeq 6.5 \times 10^{-12}$ and current $\mathcal{I} \simeq 2.5 \times 10^6$ GeV. This model yields a convincing fit to the data and may be testable in the near future by spectral distortion (TMS, BISOU) and 21 cm experiments (HERA, SKA, REACH). We also find that soft photon heating protects us against current constraints from global $21$ cm experiments.

astro-ph.CO↗
arXiv2023

Constraints on the spectral signatures of superconducting cosmic strings

If they exist, networks of superconducting cosmic strings are capable of injecting copious amounts of electromagnetic energy into the background over a broad range of frequencies. We study this injection both analytically, as well as numerically using the thermalization code CosmoTherm. With our refined analytic formalism, we update constraints from CMB spectral distortions by following the injection of entropy, as well as energy, on the amplitude of the $μ$-distortion, leading to a significant improvement in those limits. Furthermore, we utilize the full shape of the distorted spectrum from CosmoTherm to include constraints from non-$μ$, non-$y$ type distortions. Additionally, we use the outputs for the ionization history and global 21cm signal to derive and update constraints on string model parameters using measurements from other datasets. Analysis of CMB anisotropies provides the most stringent constraints, though with a slightly modified shape and strength when compared to previous results. Modifications of the reionization history provide new bounds in the high current domain, and we also find that the observations of the low-frequency radio background probe a small region of parameter space not explored by other datasets. We also analyze global $21$-cm constraints, and find that the inclusion of soft photon heating plays a crucial role, essentially removing any constraints in the considered parameter domain. Spectral distortion measurements from COBE/FIRAS are covered by other constraints, but our conservative forecast shows that a PIXIE-type satellite would probe important unexplored regions of parameter space.

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arXiv2023

Importance of intracluster scattering and relativistic corrections from tSZ effect with Cosmic Infrared Background

The Sunyaev-Zeldovich effect towards clusters of galaxies has become a standard probe of cosmology. It is caused by the scattering of photons from the cosmic microwave background (CMB) by the hot cluster electron gas. In a similar manner, other photon backgrounds can be scattered when passing through the cluster medium. This problem has been recently considered for the radio and the cosmic infrared background. Here we revisit the discussion of the cosmic infrared background (CIB) including several additional effects that were omitted before. We discuss the {\it intracluster} scattering of the CIB and the role of {\it relativistic} temperature corrections to the individual cluster and all-sky averaged signals. We show that the all-sky CIB distortion introduced by the scattering of the photon field was underestimated by a factor of $\simeq 1.5$ due to neglecting the intracluster scattering contribution. Energy is essentially transferred twice from the thermal electrons to the CIB. We carefully clarify the origin of various effects in the calculation of the average CIB and also scattered signals. The single-cluster CIB scattering signal also exhibits a clear redshift dependence, which can be used in cosmological analyses, as we describe both analytically and numerically. This may open a new way for cosmological studies with future CMB experiments.

astro-ph.CO↗
arXiv2023

The role of soft photon injection and heating in 21 cm cosmology

The ARCADE radio excess and EDGES measurement remain puzzling. A link between the two has been previously considered, however, in this work we highlight an important related effect that was not analyzed in detail before. By performing cosmological thermalization calculations with soft photon injection using {\tt CosmoTherm}, we show that for the 21 cm signal generation the interplay between enhanced radio spectral distortions and the associated heating can hide a significant radio excess before the reionzation era. We illustrate this effect for a simple power-law soft photon source in decaying particle scenarios. Even if simplistic, the uncovered link between CMB spectral distortions and 21 cm cosmology should apply to a much broader range of scenarios. This could significantly affect the constraints derived from existing and future 21 cm observations on the evolution of the ambient radio background. In particular, scenarios that would be ruled out by existing data without heating could become viable solutions once the heating is accounted for in the modelling. Our calculations furthermore highlight the importance of global 21 cm observations reaching into the dark ages, where various scenarios can potentially be distinguished.

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arXiv2023

Revisiting secondary CMB distortions due to kinetic Sunyaev-Zel'dovich effect from quasar bubbles before reionization

We discuss the secondary cosmic microwave background (CMB) anisotropy due to kinetic Sunyaev-Zel'dovich (kSZ) effect from ionized bubbles around individual quasars prior to the reionization of the Universe. The bubbles create local ionization modulations which move with the large scale structure linear bulk flow and act as sources for kSZ. We improve upon previous calculations of this effect, using a halo model based description of quasar abundance, and find that the kSZ distortion power spectrum, $C_\ell$, from the bubbles to be sub-dominant to kSZ from patchy reionization driven by galaxies. However, the shape of the two $C_\ell$'s are very different with the quasar bubble $C_\ell$ having a peak at $\ell \approx 500-700$ whereas the $C_\ell$ due to patchy reionization flattening out at $\ell > 1000$ thus making it plausible to separate the two using $C_\ell$ template-fitting in a future survey like CMB-HD. Next, we look at the imprint of a single quasar bubble on the CMB and show that it can be detected in a high-resolution, ambitious effort like CMB-HD. A detection of a high redshift quasar bubble in the CMB would carry complimentary information to its detection in HI or Lyman-$α$ and a joint analysis can be used to break parameter degeneracies.

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arXiv2023

A closer look at dark photon explanations of the excess radio background

The observed excess radio background has remained a puzzle for over a decade. A recent new physics solution involves dark matter that decays into dark photons in the presence of a thermal dark photon background. The produced non-thermal dark photon spectrum then converts into standard photons around the reionization era, yielding an approximate power-law radio excess with brightness temperature $T(ν)\simeq ν^{-2.5}$ over a wide range of frequencies, $ν$. This simple power-law model comes intriguingly close to the current data, even if several ingredients are required to make it work. In this paper, we investigate some of the details of this model, showcasing the importance of individual effects. In particular, significant deviation from a power law are present at $ν\lesssim 100\,{\rm MHz}$ and $ν\gtrsim 1\,{\rm GHz}$. These effects result in improving the fit to data compared to a power-law spectrum, and may become testable in future observations. We also highlight independent signatures that can be tested with future CMB spectral distortion experiments such as {\it PIXIE}.

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arXiv2022

Can accreting primordial black holes explain the excess radio background?

The excess radio background seen at $\simeq 0.1-10\,{\rm GHz}$ has stimulated much scientific debate in the past years. Recently, it was pointed out that the soft photon emission from accreting primordial black holes may be able to explain this signal. We show that the expected ultraviolet photon emission from these accreting black holes would ionize the universe completely at $z>6$ and thus wash out the 21 cm absorption signature at $z\simeq$ 20 as well as be in tension with existing cosmic microwave background anisotropy and average spectral distortion limits. We discuss possible augmentations of the model; however, it seems that an explanation of radio excess by accreting primordial black holes is not well-justified.

astro-ph.CO↗
arXiv2022

The non-thermal secondary CMB anisotropies from a cosmic distribution of radio galaxy lobes

Current and upcoming high angular resolution and multi-frequency experiments are well poised to explore the rich landscape of secondary CMB anisotropies. In this context, we compute for the first time, the power spectrum of CMB fluctuations from a cosmological distribution of evolving lobes of giant radio galaxies. We, also, explicitly take into account the non-thermal electron distribution, which has important implications for the inference of the CMB angular power spectrum. We calculate the mean global non-thermal y-distortion, \ynt. For observationally reasonable distribution of the jet luminosities in the range of $10^{45}-10^{47}$ ergs$^{-1}$, we find \ynt to be less than $10^{-5}$, and hence not violating the COBE limit as previously claimed. Using the unique spectral dependence of the non-thermal SZ, we show that a detection of \ynt can be within reach at the level of $\gtrsim 5σ$ from a future PIXIE-like experiment provided we understand the foregrounds precisely. The total non-thermal SZ power spectrum, $C^{NT}_\ell$, from the radio lobes peaks at $\ell \sim 3000$ with an amplitude $\sim 1\%$ of thermal SZ power spectrum from galaxy clusters. A a detection of the $C^{NT}_\ell$, with a PIXIE-like sensitivity experiment, can lead to $\sim 5σ$ constraint on the mass dependence of the jet luminosity with the constraint becoming, at least, ten times better for the proposed more ambitious CMB-HD survey. This will, further, lead to the tightest constrain on the central black hole mass -to- host halo mass scaling relations.

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