arXiv subjects

Vikram Ravi

Publications and source records attributed to Vikram Ravi.

At least 19 recordsLinked to original sources

arXiv2026

Detection and excision of radio-frequency interference in radio arrays

The detection and excision of radio-frequency interference (RFI) is usually approached by asking how best to identify RFI within a given data product. I address a prior question: which data product is a given RFI emitter is most detectable in? I develop a model for the suppression of RFI in visibilities and tied-array beams by the antenna primary beam, near-field wavefront curvature, bandwidth smearing, and fringe tracking. The comparison between antenna-based and array-based detection reduces to a single quantity related to the mean per-baseline RFI coherence. Cross-correlation detection is preferred only above a coherence threshold of $(N-1)^{-1/2}$, where $N$ is the number of antennas. I evaluate this model for four array configurations (the Very Large Array A and D configurations, and the fixed MeerKAT and Deep Synoptic Array / DSA configurations) and five representative RFI emitters spanning distances of 5\,km to medium-Earth orbit. Compact, filled, arrays favor the cross-correlations for stationary terrestrial emitters, whereas extended sparse arrays do not. RFI emitters with significant motion relative to radio arrays, in particular aircraft and low- and medium-Earth-orbit satellites, favor the sum of antenna autocorrelations everywhere by one to seven orders of magnitude, because their coherence is destroyed by rapid fringe rotation. For RFI below the single-antenna noise, no phase-unknown cross-correlation statistic is more sensitive than the autocorrelation sum. Tied-array beams are the least sensitive data product of all: the array factor supplies no suppression of terrestrial RFI, so a beam sees an emitter as a single antenna does. I describe a real-time, GPU-based implementation of autocorrelation-based excision for the 110-antenna Deep Synoptic Array, and outline the design implications for upcoming arrays.

astro-ph.IM↗

arXiv2026

Optimizing Prompt Radio Counterpart Searches for Gravitational-Wave Events with the Deep Synoptic Array

Prompt millisecond-duration coherent radio emission is proposed as a counterpart to neutron-star-containing compact mergers, produced before, during, or shortly after coalescence. Its detection would probe the plasma environment, magnetic fields, remnant evolution, and emission physics of these mergers. At GHz frequencies, dispersive delays can range from fractions of a second to several seconds, requiring the telescope to be on source before or soon after coalescence. Negative- or low-latency alerts, rapid response, and optimized pointing under uncertain, evolving localizations are therefore essential. We present a framework for prompt gravitational-wave follow-up with the upcoming Deep Synoptic Array (DSA), a sensitive, fast-slewing, wide-field GHz radio array. We simulate low-latency localization sequences and embed them in a follow-up environment with elevation, field-of-view, and slew constraints. We compare a reinforcement-learning pointing policy with a baseline that slews toward the highest-probability accessible region at each step. This peak-tracking rule is a strong, interpretable baseline for initial deployment, while the learned policy provides a complementary strategy for a subset of broad, multimodal, or arc-shaped localizations. At a random merger time, 37% of events are observable above $20^\circ$ elevation; among these, $\sim90\%$ of BNS events are successfully reached and tracked using zero-noise localization maps, decreasing to $\sim75\%$ with Gaussian-noise maps. We validate the optimized pointing trajectories on prototype DSA antennas. DSA follow-up can reach a fluence sensitivity of $\sim20$ mJy ms over a representative 700-1025 MHz sub-band, probing millisecond bursts with luminosities of approximately $10^{38}$ to $10^{39}$ erg s$^{-1}$, at or below the lower end of several coherent-emission predictions for neutron-star mergers.

astro-ph.HE↗

arXiv2026

Measuring the magnetization profiles of galaxy groups and clusters

Magnetic fields are vital in determining the physical conditions of baryons in massive halos. They are, however, notoriously difficult to observe. This impacts problems from our understanding of galaxy formation, to the observed deficit in the amplitude of thermal Sunyaev-Zel'dovich (tSZ) power spectrum that affects cosmological inferences. Although Faraday rotation measures (RMs) offer a direct probe of magnetic fields, it weights magnetic fields by the free-electron density. Several probes of free electrons, including fast radio burst (FRB) dispersion measures (DMs) and kinematic SZ (kSZ) effect, are now maturing into precision tools. We combine RM$^2$ stacks from SPICE-RACS around DESI galaxy groups and ROSAT clusters with optical depth profiles constrained jointly by the FRB DM-redshift relation and the kSZ effect, and obtain 95% upper limits on the rms line-of-sight magnetic field of $<3.9$-$11.6~μ$G ($<2.3$-$6.4~μ$G) in groups (clusters) for a 10~kpc coherence length. Combining these with thermal pressure profiles inferred from tSZ-shear cross-correlations, we obtain 95% lower limits on the plasma $β$ of $>0.31$ ($>11$) in groups (clusters). Although current constraints are only limits, we forecast a first detection with larger datasets from the Deep Synoptic Array (DSA). We present the first forecast of the DSA RM grid density ($\sim365$ RMs deg$^{-2}$), which when combined with 10$^4$ FRBs would measure the field in groups to $3$-$13\%$ precision, compared to current constraints of $38$-$75\%$. Although the interpretation of current measurements remains challenging owing to depolarization along turbulent halo sightlines and the unknown coherence length, larger samples and DSA/Square Kilometer Array observations may resolve these uncertainties by measuring polarization-fraction profiles and the RM structure function using extended radio sources, respectively.

astro-ph.GA↗

arXiv2026

The Radio-FIR Correlation in the Context of Deep Radio Source Counts

Increasingly deep, confusion-limited radio surveys have pushed direct radio source-count measurements down to tens of $μ$Jy at 1.4 GHz. Confusion-noise P(D) analyses extend the statistical counts to below $1\,\mathrm{μJy}$. Radio source counts have allowed for constraints on the radio-derived star formation rate density (SFRD) history through models of the backwards evolution of the local radio luminosity function, using the radio-FIR correlation, $q \propto \log(L_{\mathrm{FIR}}/L_{1.4})$, to convert radio luminosities to FIR luminosities and hence star-formation rates. Recent deep radio source counts from MeerKAT suggest a potential tension in the SFRD history between radio and UV/IR measurements at $1\lesssim z\lesssim 2$. This corresponds to a $\gtrsim 3σ$ discrepancy between the predicted and measured source counts near the star forming galaxy source count $S^2n(S)$ peak of ${\sim}30\,\mathrm{μJy}$ under both a pure luminosity (PLE) and combined luminosity and density evolution (LADE). We consider what the requirement of agreement between radio source counts and the observed UV/IR SFRD indicates about the redshift evolution of the radio-FIR correlation and its intrinsic scatter out to $z=3$. We introduce a radio-luminosity based parameterization to $q_{\mathrm{FIR}}(z)$ based on changing thermal radio fractions alone that agrees with the observed stellar-mass dependent $q_{\mathrm{FIR}}(z)$ better than a non-evolving or decreasing $q_{\mathrm{FIR}}(z)$. Despite this, we find that a decreasing $q_{\mathrm{FIR}}(z)$ at fixed radio luminosity provides better agreement between source counts and the observed SFRD, while a $q_{\mathrm{FIR}}(z)$ that breaks down due to cosmic ray losses requires an intrinsic scatter up to $σ_q\approx 0.3\,\mathrm{dex}$.

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arXiv2026

Tracing the Host-Galaxy Environments of FRBs with a New Sample of Localized Sources Detected with the DSA-110

We characterize the source environments of a sample of fast radio bursts (FRBs) detected with the Deep Synoptic Array (DSA-110). We present new analyses of a sample of 24 FRBs which have been localized to host galaxies with spectroscopic redshifts. Four of these bursts have been confidently associated with galaxies near or beyond redshift 1, and several are published here for the first time. Combining these new bursts with previously analyzed DSA-110 sources, we use a sample of 43 DSA-110 FRBs with known redshifts to measure host-galaxy contributions to dispersion measure (DM) and Faraday rotation measure (RM). We find characteristically large and significantly correlated host-galaxy contributions to DM and RM, suggesting that most FRBs reside in overdense, magnetized regions within their host ISM. Additionally, we identify a subset of FRBs with detectable scattering, and show that their scattering timescales and inferred host DM values are broadly consistent with an empirical relation established for Galactic pulsars. Combined with two-screen modeling of some bursts, our results indicate that density fluctuations within the host-galaxy ISM are the dominant source of extragalactic scattering. Comparisons of FRB host-galaxy properties with pulsars and simulations of young and old progenitor populations support a scenario in which FRBs preferentially form and remain associated with HII regions throughout their emission lifetime, favoring magnetars formed in core-collapse supernovae as the sources of FRBs.

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arXiv2026

The DSA/Chronoscope fast radio burst survey: forecasts and science overview

Fast radio bursts (FRBs) are bright extragalactic transients with several mysteries surrounding their origins. Large FRB samples enable accurate measurements of the cosmic matter distribution, in particular on scales $\lesssim 10$ Mpc. These measurements will impact cosmological inference and our understanding of astrophysical feedback, from the circumgalactic medium to cluster scales. Here we forecast the expected yields, redshifts, and host galaxies of FRBs as observed by the Deep Synoptic Array (DSA), and describe the key science cases enabled by the large FRB sample. The DSA will be an interferometer consisting of 1650$\times$6.15 m antennas, operating between 0.7--2 GHz, to be located in Nevada, USA. The Chronoscope backend on the DSA, hereafter DSA/Chronoscope, is designed to search for FRBs across the field of view in real time, enabling the storage of full-polarization voltage data. Extrapolating from existing FRB surveys, we expect roughly $10^4$ FRB detections per year in each of three search sub-bands. Combining across sub-bands, the survey could produce $\sim$ 10$^5$ FRBs over the nominal 5-year DSA survey, assuming Euclidean source counts and a baseline compute backend that can search $6\times10^6$ beams at 1 ms sampling. The well-characterized DSA synthesized beam and deep simultaneous reference images will enable localization precisions of $\lesssim$ 250 milliarcseconds. Key science cases include the use of FRB propagation effects in probing cosmic baryons, and studies of the FRB phenomenon using FRB host galaxies and their local environment, as well as multiwavelength counterparts.

astro-ph.HE↗

arXiv2026

VLASS Discovery of a Luminous Galactic Radio Transient Evolving on Decade Timescales

We present a multiwavelength analysis of the radio transient VT J1906+0849, discovered as a 70 mJy source in Epoch 1 of the Very Large Array Sky Survey (VLASS), 21 yr after an NRAO VLA Sky Survey (NVSS) non-detection. Radio observations reveal the source was first detected in 2005, peaking at $\gtrsim200$ mJy in 2014, then declining until a late 2025 rebrightening. The transient sits at a Galactic latitude of $\approx0.74^\circ$ and the properties of its optical-infrared counterpart support a Galactic origin. At $d\gtrsim15$ kpc, the extreme radio luminosity is likely powered by sustained accretion onto a compact object. However, a Swift-XRT non-detection shows it is X-ray faint relative to the Galactic X-ray binary population, and the radio emission is distinct from X-ray binaries in its temporal and spectral behavior. Broadband radio spectra suggest synchrotron self-absorption, but size constraints from equipartition and very long baseline interferometry show little to no expansion in the radio-emitting region over 5+ yr, despite significant spectral evolution. Near-infrared spectroscopy reveals a single broad emission line with a stable centroid but variable width and luminosity. We attribute this feature to blueshifted Br$γ$ tracing a persistent asymmetric $\approx2000$ km s$^{-1}$ outflow. These properties are unlike any previously identified Galactic radio source. One possible interpretation is that VT J1906+0849 is a young analog of the microquasar SS 433, with a dense disk wind confining a continuously powered synchrotron outflow. This jet-wind interaction explains the compact, slowly expanding radio source and may contribute to the absence of bright X-ray emission.

astro-ph.HE↗

arXiv2026

A new measurement of the FRB DM-galaxy cross correlation and a first joint analysis with the kinematic SZ effect

Understanding the distribution of cosmic baryons is crucial for advancing cosmology and galaxy formation. Localized Fast Radio Bursts (FRBs) are a promising new probe of the diffuse gas, but measurements thus far have been limited by small sample size. In this work, we measure the FRB dispersion measure (DM)-galaxy cross correlation with 130 localized FRBs, including new spectroscopic host redshifts from the DSA-110, and the DESI Legacy Survey Bright Galaxy Sample. We detect this signal at the highest significance to date ($6.5σ$) in both configuration and harmonic space. By comparing to simulations and a halo model, we demonstrate that this statistic can constrain the strength of baryonic feedback. Our measurements disfavor a no-feedback scenario in which gas traces the underlying dark matter at $\sim\,9σ$. In addition to small scale feedback, this represents one of the first direct measurements of the clustering of electrons on scales larger than a few Mpc. We compare our measurement to recent kinematic Sunyaev-Zeldovich effect (kSZ) studies, which show good agreement despite the differences in observational methods and sample selection. Finally, we make a first attempt at breaking the kSZ optical depth degeneracy with FRBs, jointly measuring the growth rate of large-scale structure $fσ_8$. With future samples, FRB two-point statistics will provide precision constraints on the distribution of cosmic baryons and complement other probes.

astro-ph.CO↗

arXiv2026

A spectacular multi-wavelength transient associated with an off-axis relativistic jet

Many of the most luminous extragalactic transients originate from the accretion of material onto a black hole (BH) via core-collapse, stellar mergers, or the tidal disruption of a star. Some produce energetic multi-wavelength emission, displaying short, blue optical flares and bright, long-lived radio afterglows. In rare cases, these have also launched powerful relativistic jets almost exclusively detected on-axis via high-energy emission. Here we report AT 2019ijn, a radio transient discovered in the Very Large Array Sky Survey, associated with a powerful relativistic jet viewed off-axis and accompanied by a luminous ($νL_{ν, opt} \sim 10^{44}$ erg/s) optical transient. Originating from a star-forming dwarf galaxy at z = 0.273, AT 2019ijn's optical flare exhibited a fast rise ($t_{rise, rest} \sim 7$ d) and shallow decline ($t_{dec, rest} > 38$ d), followed a year later by an energetic ($E \sim 2\times10^{52}$ erg) radio counterpart persisting for $>6$ years. These combined properties are unprecedented and preclude known classes of stellar explosion powered by core-collapse. Instead, the implied accretion onto a BH and associated off-axis jet invokes an exotic BH-stellar merger or a jetted tidal disruption event (TDE) by a $10^4 - 10^6 M_\odot$ black hole. Favoring a TDE, this work determines the jet-launching occurrence, with the low redshift allowing constraints on the late-time evolution in contrast to prior events. Upcoming radio sky surveys will offer an unprecedented new window for further discovery of AT 2019ijn-like off-axis relativistic transients.

astro-ph.HE↗

arXiv2026

A Decade to Map the Diffuse Universe: FRB-QSO Pairs with HST/COS Spectroscopy

Jointly analyzing the sightlines of arcsecond-localized fast radio bursts (FRBs) and UV-bright quasars (QSOs) nearby in projection has the potential to provide strong constraints on the phases, mass distributions, and magnetic structure of the diffuse universe. Each probe supplies what the other cannot: FRBs provide integrated electron columns (DM), line-of-sight magnetic field estimates (RM), and scattering constraints ($τ_{\rm scatt}$) that are independent of gas phase; QSOs provide the redshift- and phase-resolved column densities needed to interpret them. Today, there are only $\sim100$ arcsecond-localized FRBs at $z < 1$, making statistical FRB-QSO pair surveys impossible. By 2035, there will be $\sim10^{5}$. Using the most recent FRB localization forecasts and UV-bright QSO catalogs, we estimate that next-generation interferometers will yield thousands of FRB--QSO pairs at angular separations $θ< 10'$, including $\sim100$ pairs at $θ< 1'$, over a common 20,000\,deg$^2$ footprint by 2035. We outline the science enabled by this sample: constraints on CGM ionization fractions and baryon masses; observational constraints on the role of magnetic fields and turbulence in the CGM and cosmic web; sightline-by-sightline partitioning of the cosmic DM budget; and three-dimensional mapping of the multiphase Milky Way and M31 halos. Together, these measurements directly address the physics of feedback, non-thermal pressure support, and energy balance in the diffuse gas that regulates galaxy growth. HST/COS is the only instrument that can carry out this program, and the 2030s are the only decade in which to do it before Habitable Worlds Observatory (HWO) defines the next era of diffuse universe science.

astro-ph.GA↗

arXiv2026

Detecting the Axion-Photon Conversion Background

The potential to detect axion dark matter through astrophysical processes has shown high promise in recent years. We therefore expand on previous work studying the axion-to-photon conversion efficacy of neutron stars and the interstellar medium (ISM) in this endeavor, respectively. For neutron stars (NS), we examine the possibility of a background signal emanating from all NS magnetospheres in the galaxy. Using a heuristic Galactic model, we find a significant background signal emanating from such magnetospheres in the Milky Way. This signal, while weak in absolute power ($\gtrsim 1$ mJy sr$^{-1}$ from the Galactic Center, at 2 GHz), can be detected through new statistical techniques with current instrumentation like the Atacama Large Millimeter Array (ALMA) at high radio frequencies (200 - 950 GHz). These techniques make use of higher order statistics like spectrally-limited ($\sim 300$ km s$^{-1}$) increases in confusion noise levels and kurtoses of survey images, and also show promise for general population estimation techniques. For the ISM, we consider Primakoff processes between free electrons and axions, and derive typical signal strengths of $10^{-15}$ Jy sr$^{-1}$ $\cdot$ $m_a$/eV, with a local, cosmological upper bound of $10^{-8}$ Jy sr$^{-1}$ $\cdot$ $m_a$/eV. Hence, we find that any diffuse axion signal from the ISM and other, large-scale, astrophysical plasmas to be too weak to be detected with modern technologies. We therefore find that the best avenue towards detecting a potential quantum chromodynamics (QCD) axion dark-matter particle is through the radio imaging of large swaths of the Galactic Center and other regions where we expect large numbers of pulsars and neutron stars.

astro-ph.HE↗

arXiv2026

Baryons in the Darkest Sites of the Universe

The pristine underdense patches of the Universe, cosmic voids, are powerful cosmological laboratories, uniquely sensitive to dark energy, modified gravity, and neutrino masses, yet their baryonic content remains uncharacterized. We present the first observational constraint on baryon underdensity in void interiors, exploiting the dispersion measures (DMs) of Fast Radio Bursts (FRBs) as tracers of the free electron column, independent of gas phase, temperature, and metallicity. By stacking 3,455 sightlines from CHIME/FRB on 1,288 SDSS BOSS voids over redshifts $0.2 < z < 0.7$, we measure a DM deficit toward void centers at $3.2σ$ significance, establishing that diffuse baryons inhabit the emptiest corners of the cosmic web at a suppressed level. The measured signal amplitude is consistent with an effective Universe model built directly from the observed galaxy underdensity in these voids, and a baryonic model calibrated to the FRB DM-redshift relation ($α_v = 1.80 \pm 0.87$). A uniform-density void model yields an electron density contrast of $δ_\mathrm{e,v} = -0.58 \pm 0.30$, implying a $\sim 60$% underdensity of baryons in void interiors relative to the cosmic mean. Jointly interpreting our FRB measurement with existing stacks of the thermal Sunyaev-Zel'dovich effect on voids further constrains the mean void gas temperature to $T_\mathrm{e} \lesssim (1.1 \pm 0.7) \times 10^6$ K, pointing to a warm-hot diffuse phase, consistent with hydrodynamical simulation predictions. With forthcoming FRB (CHORD, DSA, SKA) and galaxy (DESI, LSST, Euclid, PFS-Subaru, SPHEREx, Roman) surveys, set to expand both samples by orders of magnitude, this approach opens a new window onto tomographic baryon mapping, with direct implications for feedback models governing gas expulsion into low-density environments, and for the use of cosmic voids to extract cosmological constraints.

astro-ph.CO↗

arXiv2026

Backlighting the Cosmic Web with Fast Radio Bursts: An Anthology of Dispersion Measure Cross-Correlations with Large-Scale Structure and Baryon Tracers

Fast Radio Bursts (FRBs) probe baryons permeating the cosmic web through their dispersion measures (DMs), which encode the integrated electron density along cosmological sightlines. Using 3,455 unique FRB sources from CHIME/FRB with $\sim 15$ arcmin localizations, we present an anthology of DM correlations with tracers of large-scale structure and baryonic matter at redshifts $z \lesssim 1.5$. We measure statistically significant correlations at $2.6-5σ$ with ten probes, including galaxies ($2.8σ$), weak gravitational lensing ($2.6σ$), cosmic infrared background ($4.0σ$), cosmic microwave background (CMB) lensing ($3.3σ$), thermal Sunyaev Zel'dovich (tSZ) effect ($3.8σ$), X-ray emission tracing galaxy clusters ($5.0σ$) and superclusters ($3.3σ$), soft X-ray background (SXRB, $4.1σ$), and radio continuum emission ($3.2σ$). These measurements reveal a consistent picture in which FRB sightlines intersecting overdense environments carry systematically larger DMs. Correlations with hot-gas tracers provide additional leverage on the strength of feedback, as they are strongly weighted towards the dense, bound gas. The measured amplitude of tSZ$\times$DM and SXRB$\times$DM correlations are consistent with theoretical predictions of baryon distribution from a DM-$z$ relation-inferred model with moderate feedback at $\sim 0.5σ$ level. Weaker feedback scenario is ruled out at $\sim 3.5σ$ by the SXRB$\times$DM correlation. Taken together, these measurements constitute a quantitative multi-tracer foundation for a new era in which FRBs from next generation facilities, such as BURSTT, CHORD, DSA, and SKA, in harmony with other probes, will map the baryon content of the full extent of the cosmic web.

astro-ph.CO↗

arXiv2026

Signatures of Suppressed Matter Clustering revealed by Fast Radio Bursts

Complex astrophysical processes regulate the growth of galaxies by injecting energy and momentum into their surroundings, redistributing baryons across megaparsec scales. The clustering of matter on these scales, as measured via weak lensing and galaxy surveys, encodes critical cosmological information on the dynamical dark energy, the nature of dark matter and the sum of neutrino masses. The suppression of matter clustering due to feedback processes limits the interpretation of cosmological measurements. Multiple probes of the baryon distribution have attempted to quantify the strength of feedback via measurements of suppression in the matter power spectrum. The dispersion measures (DMs) of fast radio bursts (FRBs) have emerged as a powerful new probe of baryons, with the advantage over other probes of being unbiased with respect to density and temperature. Here, we use a sample of 109 FRBs with redshifts and DMs to directly measure the spatial fluctuations in the baryon density field, quantifying the effects of feedback on the matter power spectrum at scales of $k \sim 0.1-3$ h/Mpc, and the gas fraction in galaxy groups and clusters ($10^{13}-10^{15} M_\odot$). We use a halo-model prescription to conduct inference, and find that FRB data reduces the posterior variance at k $\sim$ 1 h/Mpc by a factor of $\sim 8$ relative to the prior. The statistical precision of inferred FRB constraints is similar to other baryon tracers, while probing a complementary redshift regime ($z \lesssim 0.3$). A comparison with several hydrodynamical simulations excludes extreme large-scale feedback scenarios at $\sim 2σ$ confidence. This work establishes FRBs as a sensitive probe of feedback-regulated structure formation. As next-generation experiments deliver orders-of-magnitude larger samples, FRBs are poised to drive the constraints on baryonic physics in the era of precision cosmology.

astro-ph.CO↗

arXiv2026

The 256-antenna Coherent All-Sky Monitor

Radio astronomy is uniquely coupled to exponential trends in computation because the optics (cross-correlation, beamforming, and imaging) and spectrometry (i.e. channelization) can now be done digitally. Inexpensive analog-to-digital converters (ADCs) can sample signals from large numbers of antennas and graphics processing units (GPUs) allow us to coherently process wide-field radio data in real time, motivating large-$N$ aperture arrays at moderate cost. We describe the 256-antenna Coherent All-Sky Monitor (CASM-256), a dense aperture array operating at 375-500\,MHz, currently being deployed at the Owens Valley Radio Observatory (OVRO) in Big Pine, California. The large field-of-view (FoV$\sim10^4$\,deg$^2$) and point-source sensitivity of CASM-256 will allow it to detect local Universe fast radio bursts (FRBs). The nearby sample is ideal for unveiling the physical origin of FRBs, measuring the baryonic content of nearby galaxy halos, and discovering prompt multi-wavelength and multi-messenger counterparts to FRBs. CASM will search for fast transients in the Milky Way such as FRB analogs, pulsar giant pulses, and the new source class known as long-period radio transients. We describe the instrument and present on-sky data from the first two dozen antennas, including an operational real-time GPU based FRB search pipeline. We emphasize the scalability of the concept and describe paths to a future CASM array with tens of thousands of antennas that could detect one million FRBs.

astro-ph.IM↗

arXiv2026

Great Walls of Cosmic Baryons in the Northern Sky

The dispersion measures (DMs) of fast radio bursts (FRBs) encode the total ionized-gas column densities along their sightlines. Most observed FRBs originate at distances where the cosmological principle applies. Thus, variations in the DM distribution of FRBs observed in different regions on the sky trace local sources of anisotropy, such as the warm ionized medium and circum-galactic medium of the Milky Way, and local large-scale structure. We present a map of extragalactic DM variations across the Northern sky using a few thousand FRBs from the second \chime{} catalog. We detect a $\gtrsim 4σ$ excess of $\sim$150 pc cm$^{-3}$ above the global mean, extended over $\sim$30$^\circ$ scales and centered near $α\approx$ $12^{\rm h}$, $δ\approx$ $55^\circ$. This excess, termed Wall 1, is robust to variations in sample selection and jackknife resampling, and cannot be explained by Galactic-disk DM-model uncertainties. The excess is likely too large to correspond to anisotropy in the Milky Way halo. The signal spatially coincides with the Ursa Major supercluster and associated large-scale structures. A secondary, more tentative Wall 2 near $α\approx 2^{\rm h}$, $δ\approx$ $45^\circ$ is spatially coincident with the Perseus-Pisces supercluster. Although the spatial coincidences suggest that the Walls may correspond to baryons in the local large-scale structure, the probability of chance coincidence is likely too high ($\sim10-20\%$) to claim confident associations. These results highlight the potential of using FRB DMs to detect baryon overdensities associated with local large-scale structure, and have important implications for near-field baryon mapping and FRB cosmology.

astro-ph.CO↗

arXiv2026

Deep Synoptic Array Science: Searching for Long Duration Radio Transients with the DSA-110

We describe the design and commissioning tests for the DSA-110 Not-So-Fast Radio Burst (NSFRB) search pipeline, a 1.4GHz image-plane single-pulse search sensitive to 134ms-160.8s radio bursts. Extending the pulse width range of the FRB search by 3 orders of magnitude, the NSFRB search is sensitive to the recently-discovered Galactic Long Period Radio Transients (LPRTs or LPTs). The NSFRB search operates in real-time, utilizing a custom GPU-accelerated search code, \texttt{cerberus}, implemented in Python with JAX. We summarize successful commissioning sensitivity tests with continuum sources and pulsar B0329+54, estimating the 90% completeness $25σ$ flux (fluence) limit to be ~1200mJy (~160Jy ms). Future tests of recovery of longer timescale transients, e.g. CHIME J1634+44, are planned to supplement injection testing and B0329+54 observations. An offline DSA-110 NSFRB Galactic Plane Survey was conducted to search for LPRTs, covering $-3.5^\circ<b<5.7^\circ$ and $141^\circ<l<225^\circ$ (~770 square degrees) in Galactic coordinates. We estimate an upper limit Poissonian burst rate ~2 hr$^{-1}$ per square degree (~17 hr$^{-1}$} per $3^\circ\times3^\circ$ survey grid cell) maximized across the inner $|b|<0.25^\circ$ of the surveyed region. By imposing the ~1200mJy flux limit on two representative models (the magnetar plastic flow model and the White Dwarf-M Dwarf binary model), we reject with 95% confidence the presence of White Dwarf-M Dwarf binary LPRTs (beamed in a detectable direction) with periods between ~10-50s within ~95% of the surveyed region. Combined with the prevalence of LPRTs in the Galactic Plane, our results motivate further consideration of both White Dwarf-M Dwarf binary models and isolated magnetar models. We will continue to explore novel LPRT search strategies during real-time operations, such as triggered periodicity searches and additional targeted surveys.

astro-ph.HE↗

arXiv2026

Searching for Magnetar Binaries Disrupted by Core-Collapse Supernovae

Core-collapse Supernovae (CCSNe) are considered the primary magnetar formation channel, with 15 magnetars associated with supernova remnants (SNRs). A large fraction of these should occur in massive stellar binaries that are disrupted by the explosion, meaning that $\sim45\%$ of magnetars should be nearby high-velocity stars. Here we conduct a multi-wavelength search for unbound stars, magnetar binaries, and SNR shells using public optical ($uvgrizy-$bands), infrared ($J-$, $H-$, $K-$, and $K_s-$bands), and radio ($888$ MHz, $1.4$ GHz, and $3$ GHz) catalogs. We use Monte Carlo analyses of candidates to estimate the probability of association with a given magnetar based on their proximity, distance, proper motion, and magnitude. In addition to recovering a proposed magnetar binary, a proposed unbound binary, and 13 of 15 magnetar SNRs, we identify two new candidate unbound systems: an OB star from the Gaia catalog we associate with SGR J1822.3-1606, and an X-ray pulsar we associate with 3XMM J185246.6+003317. Using a Markov-Chain Monte Carlo simulation that assumes all magnetars descend from CCSNe, we constrain the fraction of magnetars with unbound companions to $5\lesssim f_u \lesssim 24\%$, which disagrees with neutron star population synthesis results. Alternate formation channels are unlikely to wholly account for the lack of unbound binaries as this would require $31\lesssim f_{nc} \lesssim 66\%$ of magnetars to descend from such channels. Our results support a high fraction ($48\lesssim f_m \lesssim 86\%$) of pre-CCSN mergers, which can amplify fossil magnetic fields to preferentially form magnetars.

astro-ph.HE↗