Stella Koch Ocker

We quantify pulsar timing array (PTA) sensitivity to anisotropy in the gravitational-wave (GW) background using the cross-correlation-based Fisher information matrix in the pixel and spherical harmonic bases. We use a set of simulations to empirically determine scaling relations of a PTA’s sensitivity to anisotropy with the number of pulsars N _psr in the array, the error δt on the times of arrival, the frequency f _GW of the GWs, and the angular scale ΔΩ of the anisotropy. The sensitivity scales approximately as N psr 0.8 , δt ^−0.08 , and ΔΩ ^1.6 − ΔΩ ^2.1 (depending on the ranges of ℓ and m under consideration). In addition, we use realistic simulations to project the NANOGrav PTA sensitivity to a 30 yr baseline and quantify the growth in sensitivity at several time slices. Except at the lowest frequencies, we find negligible effect on sensitivity through increasing the observation duration only. Finally, we introduce a multi-resolution pixel basis motivated by the large dependence of the sensitivity on sky location, and demonstrate the operation of the basis through a set of injections and recoveries.

In the published article, we presented inferences about the population of supermassive black hole (SMBH) binaries emitting at nanohertz gravitational-wave (GW) frequencies based on the 15 yr dataset from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Here, we report two bugs in the astrophysical analysis software used for the published article. While parameter posteriors of our inference can be seen to have slight differences on close inspection, none of the conclusions from the original publication have changed, including the finding that the measured GW background is dominated by the most massive, high-mass-ratio SMBH binaries. Our corrected results show slightly less evidence for environmental interactions with SMBH binaries and indicate that the population of SMBH binaries contributing to the GW background occurs at slightly higher mass ratios and redshifts than indicated in the published article. Correcting these errors does not change the central conclusion that astrophysically motivated models of SMBH binary populations are able to reproduce both the amplitude and shape of the observed low-frequency GW spectrum. These errors did not affect any other analyses of the NANOGrav 15 yr dataset, including the evidence for a GW background presented in another of our published articles.

The NANOGrav 15 yr (NG15) data set provides evidence for a gravitational-wave background (GWB) signal at nHz frequencies, which is expected to originate either from a cosmic population of inspiraling supermassive black hole binaries or new particle physics in the early Universe. A firm identification of the source of the NG15 signal requires an accurate reconstruction of its frequency spectrum. In this Letter, we provide such a spectral characterization of the NG15 signal based on a piecewise power-law (PPL) ansatz that strikes a balance between existing alternatives in the literature. Our PPL reconstruction is more flexible than the standard constant power-law model, which describes the GWB spectrum in terms of only two parameters: an amplitude A and a spectral index γ . Concurrently, it better approximates physically realistic GWB spectra—especially those of cosmological origin—than the free spectral model, since the latter allows for arbitrary variations in the GWB amplitude from one frequency bin to the next. Our PPL reconstruction of the NG15 signal relies on individual PPL models with a fixed number of internal nodes (i.e., constant power law, broken power law, doubly broken power law, etc.), which are ultimately combined in a Bayesian model average. The data products resulting from our analysis provide the basis for fast refits of spectral GWB models.

Galactic radio transients (GRTs) are mysterious short-lived (∼days to months) radio transients that are quiet at all other wavelengths. Until now, roughly half a dozen such sources have been reported, predominantly towards the Galactic center. However, no unifying properties have been identified among these, leaving their nature, emission mechanism, and even classification poorly understood. Due to the lack of periodic and uniform radio observations over wide areas of the Galactic plane until now, the sample size of such transients remained limited. Here, we use radio observations from the Australian SKA Pathfinder’s Variables and Slow Transients survey to discover six new radio transients along the Galactic plane that resemble GRTs. Detailed investigation of archival data suggests that these sources may be divided into two classes: sources that exhibit sporadic, pulse-like (minutes) radio emission, and sources that exhibit long-term (weeks) flaring-type radio emission. For the short-time variable sources, we draw similarities between optically bright long-period radio transients and our sample to propose wide-orbit (∼days) white dwarf binaries as underlying sources. For sources that show long-term outbursts, we draw comparisons between dust-obscured outbursts from WD binaries and our sample. These results could imply that the ongoing wide-field radio surveys are uncovering radio emission from subpopulations of WD binaries that were previously unexplored.

Distance measurements are fundamental to radio pulsars’ use as astrophysical probes of General Relativity and the interstellar medium. One of the primary methods for determining pulsar distances is H i kinematics, which leverages the radial velocities of H i absorption and emission features detected along pulsar lines-of-sight. This method necessarily assumes a model for Galactic rotation, our knowledge of which continues to evolve in both accuracy and precision. In this research note, we derive kinematic distances for 66 pulsars with archival H i radial velocity measurements using a state-of-the-art Galactic rotation curve. The results and software are provided in an online repository. Our kinematic distances differ by σ from published parallaxes for nearly all pulsars in the sample that have both types of distance measurement available. Comparison to the NE2025 Galactic electron density model shows general consistency between measured and predicted distances.

Free electrons in the Galactic interstellar medium (ISM) disperse and scatter coherent radio waves, by amounts that depend on the distance to the radio source. Models of the Galactic electron density are thus widely used to predict distances and scattering of compact radio sources (including pulsars, fast radio bursts, and long-period transients), in addition to mitigating ISM foregrounds in Galactic and extragalactic studies. We use a sample of 171 precise pulsar distances, based entirely on parallaxes and globular cluster associations, as well as scattering measurements of 568 pulsars, active galactic nuclei, and masers, to update the NE2001 Galactic electron density model. We refit the thick and thin disks and three of the spiral arms. The new parameters for these large-scale components significantly repartition free electrons between the thick disk and spiral arms, thereby correcting NE2001’s systematic underestimation of pulsar distance and scattering. Sight lines with excessive dispersion and scattering are used to identify new clumps that are added to the model, in addition to refining clumps that were already included (e.g., Cygnus, Vela, and Gum). The Galactic Center component is revised, yielding scattering time predictions that are 10 ³ times smaller than the Galactic Center in NE2001. The updated model, NE2025, provides a factor of 20× improvement in median distance prediction accuracy and 100% median improvement in scattering predictions based on dispersion measure, relative to NE2001. There is a 15× improvement in median distance prediction accuracy relative to YMW16. NE2025 is available on Github and the Python Package Interface.

We report the discovery of a slowly evolving, extragalactic radio transient, ASKAP J005512.2–255834 (hereafter ASKAP J0055–2558), identified using the Australian SKA Pathfinder in a search for orphan afterglows associated with archival gravitational-wave events. Although discovered in this context, there is no evidence that the transient is associated with any known gravitational-wave event. Nonetheless, this source exhibits a 20-fold increase in flux density over z = 0.116 ( d _L = 543 Mpc), placing its peak radio luminosity at νL _ν ∼ 10 ³⁹ erg s ⁻¹ . Analysis of its radio light curve, inferred blast-wave velocity, energetics, host-galaxy properties, and the absence of counterparts at other wavelengths suggests that ASKAP J0055–2558 is most consistent with either the late-time phase of an orphan long gamma-ray burst afterglow or a tidal disruption event involving an intermediate-mass black hole spatially offset from the galaxy nucleus. The radio discovery of either of these phenomena is extremely rare, with only a few or no confirmed examples to date.

We present the first targeted searches for continuous gravitational waves (CWs) from 114 active galactic nuclei that may host supermassive black hole binaries, using the NANOGrav 15 yr dataset. By incorporating electromagnetic priors on sky location, distance, redshift, and CW frequency, our strain and chirp-mass upper limits are typically improved by a factor of ∼2 (median 2.2) relative to all-sky limits at the same frequency. Bayesian comparisons against a model including only a Hellings–Downs-correlated background disfavors a CW signal for all targets, with a mean Bayes factor of 0.73 ± 0.32. Two targets have Bayes factors slightly above unity, but coherence tests, random-targeting experiments, and a conservative accounting of the 114-target trials factor all indicate that they are consistent with noise. We use these two candidates as worked examples to illustrate an end-to-end targeted CW search analysis and a suite of follow-up tests that future promising candidates would need to pass. We find that the electromagnetic interpretations of both candidates are ambiguous, and we update the constraints on a putative binary in 3C 66B, ruling out part of its previously allowed parameter space. Ultimately, our results demonstrate the current sensitivity of targeted pulsar timing array searches for CWs and define a road map for future multimessenger CW detections.

We describe the design and commissioning tests for the DSA-110 Not-So-Fast Radio Burst (NSFRB) search pipeline, a 1.4 GHz image-plane single-pulse search sensitive to 134 ms−160.8 s radio bursts. Extending the pulse width range of the Fast Radio Burst 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, cerberus , implemented in Python with JAX. We summarize successful commissioning sensitivity tests with continuum sources and pulsar B0329+54, estimating the 90% completeness 25 σ flux (fluence) limit to be ∼1200 mJy (∼160 Jy 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 < b < 5 . ° 7 and 141° < l < 225° (∼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° × 3° survey grid cell) maximized across the inner ∣ b ∣ < 0 . ° 25 of the surveyed region. By imposing the ∼1200 mJy 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 and 50 s 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.