Searches for Prompt Low-Frequency Radio Counterparts to Gravitational Wave Event S250206dm with the OVRO-LWA Time Machine

TL;DR

This study targets prompt, low-frequency radio emission from the gravitational-wave event S250206dm using the OVRO-LWA Time Machine, a ring-buffer system enabling retrospective millisecond-scale searches across a wide field of view. An offline beamforming and GPU-accelerated dedispersion pipeline examined 69–86 MHz data within the 50% GW localization, covering DMs from 100 to 400 pc cm$^{-3}$ and a 1.3 ms time resolution, ultimately finding no detection above a 7σ fluence threshold of ~150 Jy ms. A Bayesian framework combining LVK sky/distance priors, dispersion-measure models, and the OVRO-LWA data yields a 95% upper limit on the isotropic-equivalent radio luminosity of L$_{95}$ = 4.0 × 10$^{41}$ erg s$^{-1}$ for the searched region, beginning to constrain the brightest coherent-emission scenarios (e.g., jet–ISM shocks, post-merger magnetars, and blitzar-like events) at the event distance of ~373 Mpc. The work demonstrates a robust methodology for integrating prompt low-frequency radio limits into population inferences and highlights the need for multiple well-covered events to obtain strong ensemble constraints, with future facilities anticipated to significantly improve sensitivity and sky coverage. This framework can be re-expressed for other instruments to enable cross-comparison of limits across different frequency ranges and detection capabilities.

Abstract

We report on a search for prompt, low-frequency radio emission from the gravitational-wave (GW) merger S250206dm using the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). Early alerts favored a neutron-star-containing merger, making this a compelling target. Motivated by theoretical predictions of coherent radio bursts from mergers involving a neutron star, we utilized the OVRO-LWA Time Machine system to analyze voltage data recorded around the time of the event. The Time Machine is a two-stage voltage buffer and processing pipeline that continuously buffers raw data from all antennas across the array's nearly full-hemisphere instantaneous field of view, enabling retrospective beamforming, dedispersion, and fast-transient candidate identification. For this event, we analyzed a 30-minute interval beginning 3.5 minutes after the merger, which included two minutes of pre-alert data recovered by the ring buffer. We searched the 50% localization probability region with millisecond time resolution in the 69-86 MHz frequency band. No radio counterpart was detected above a 7-sigma fluence detection threshold of ~150 Jy ms. Using Bayesian analysis, we place a 95% confidence upper limit on the source luminosity of L95 = 4 x 10^41 erg s^-1. These constraints start to probe the bright end of the coherent-emission parameter space predicted by jet-ISM shock processes, magnetar and blitzar-like mechanisms, and recent simulation-based scenarios for neutron-star-containing mergers. This study presents the first sensitive, large-area, millisecond-timescale search for prompt low-frequency radio emission from a GW merger with the OVRO-LWA, establishing a framework in which about ten additional events will yield stringent population-level constraints.

Figures (6)

• Figure 1: All-sky OVRO-LWA map generated from online cross-correlation data for a single 10 s integration following the merger, averaged over the 55--86 MHz band. The GW localization probability is overlaid, with the white contours marking the 50% and 90% credible regions. The Sun is visible near $\mathrm{RA}\approx321^{\circ},\ \mathrm{Dec}\approx-15^{\circ}$.
• Figure 2: Normalized dynamic spectra (waterfall plots) from two offline-formed OVRO-LWA beams, centered at J2000 $\mathrm{RA}=30.87^{\circ},\ \mathrm{Dec}=+51.98^{\circ}$ (top) and $\mathrm{RA}=338.70^{\circ},\ \mathrm{Dec}=+12.17^{\circ}$ (bottom). For visualization, the raw data ($>2\times10^{4}$ channels and $>10^{6}$ time samples) were averaged in time and frequency. Vertical guide lines mark key times: the merger epoch (solid), the first and second preliminary GW alerts (dashed, dash--dotted), and the start of the buffer (dotted). To enhance faint, time-variable structure, a time-median bandpass was subtracted from each integration; the result was then min--max normalized using the 0.5th and 99.5th percentiles and clipped to $[0,1]$. Visible features include: thin, persistent horizontal lines at fixed frequencies that are consistent with narrowband terrestrial carriers such as legacy VHF-low TV channels; brief band-wide brightenings that appear as near-vertical stripes in both beams, likely due to broadband impulsive RFI or occasional instrumental gain changes; and brief, few-channel brightenings consistent with meteor or aircraft scatter of narrowband signals. Slow background variations differ between beams, likely reflecting sky brightness, sidelobe pickup from bright sources, and ionospheric effects.
• Figure 3: Left: Signal-to-noise ratio (SNR) plotted over dispersion measure DM and time since the start of the observation (in minutes), for the same beam shown in the top panel of \ref{['fig:waterfall']}. Right: Similar plot for the beam with an injected signal. Pixels with $\mathrm{SNR} > 7$ are overlaid using a semi-transparent colormap with slight dilation and blurring to enhance visual contrast. The shared color bar indicates SNR values for both panels, and the inset highlights the peak detection region. The slight negative bowl around the injected signal is purely from the visualization baseline subtraction and is not astrophysical.
• Figure 4: Complementary cumulative SNR distribution of surviving triggers with $\mathrm{SNR}\ge5$. The blue curve is the empirical survival probability with a 95% binomial band (shaded). The dotted purple curve is a conditional Gaussian fit with effective width $\sigma_{\rm eff}\approx0.95$ that accounts for the SNR cut and mild correlations. The vertical dashed line marks the adopted $7\sigma$ detection threshold; no triggers exceed this level.
• Figure 5: Comparison of the posterior distributions for an injected detection (upper triangle, purple color) versus a non-detection (lower triangle, blue color). Diagonal panels show the marginal histograms of the non-detection posterior (blue lines) overlaid with the 16–84% credible intervals and medians of the detection posterior (purple spans and lines). Off-diagonal panels illustrate the joint posteriors via kernel density estimates for each case. From left to right (and top to bottom), the parameters are: RA, Dec, Distance, $\mathrm{DM}_{\mathrm{Gal}}$, $\mathrm{DM}_{\mathrm{IGM}}$, $\mathrm{DM}_{\mathrm{host}}$ and Flux density.