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"Ravi, Vikram"
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The prevalence of repeating fast radio bursts
Fast radio bursts are extragalactic, sub-millisecond radio impulses of unknown origin1,2. Their dispersion measures, which quantify the observed frequency-dependent dispersive delays in terms of free-electron column densities, greatly exceed predictions from models3 of the Milky Way interstellar medium. The excess dispersions are probably accrued as fast radio bursts propagate through their host galaxies, gaseous galactic halos and the intergalactic medium4,5. Despite extensive follow-up observations of the published sample of 72 burst sources6, only two have been observed to repeat7,8, and it is unknown whether the remainder are truly one-off events. Here I show that the volumetric occurrence rate of the fast radio bursts that have not been observed to repeat thus far probably exceeds the rates of candidate cataclysmic progenitor events, and also probably exceeds the birth rates of candidate compact-object sources. This analysis is based on the high detection rate of bursts with low dispersion measures by the Canadian Hydrogen Intensity Mapping Experiment (CHIME)9. Within the existing suite of astrophysical scenarios for fast radio burst progenitors, I conclude that most observed cases must originate from sources that emit several bursts over their lifetimes.Of the 72 known fast radio burst (FRB) sources only two have been observed to emit repeat bursts. By calculating the volumetric occurrence rate of non-repeating FRBs, Vikram Ravi shows that there are not enough candidate cataclysmic progenitor events for most FRBs to be one-off phenomena, and therefore most FRBs must repeat.
Journal Article
The Origin of the Nano-Hertz Stochastic Gravitational-wave Background: The Contribution from z ≳ 1 Supermassive Black Hole Binaries
The nano-Hertz gravitational wave background (GWB) is a key probe of supermassive black hole (SMBH) formation and evolution if the background arises predominantly from SMBH binaries (SMBHBs). The GWB amplitude, which is typically quantified as the characteristic strain, Ayr at a frequency 1 yr−1, encodes significant astrophysical information about the SMBHB population, including the SMBHB mass and redshift distributions. Recent results from a number of pulsar timing arrays have identified a common-spectrum noise process, correlated between pulsars, that is consistent with a loud GWB signal with Ayr ∼ 2 × 10−15, which is higher than most predictions Ayr ≲ 10−15. These predictions usually assume theoretically motivated but highly uncertain prescriptions for SMBH seeding and evolution. Recent observations, largely by the James Webb Space Telescope, have uncovered a population of obscured, overmassive, accreting black holes in the early Universe that may suggest that the black hole mass density and net accretion were larger at high redshifts than previously thought. In this work, we use two simple, flexible models of SMBH evolution to explore the possible range of GWB amplitudes, given observational constraints. In particular, we explore enhanced contributions to the GWB from high redshift (z ≳ 1) SMBHBs. We find that the GWB amplitude may be higher than fiducial predictions by as much as ∼1 dex if much of the SMBH mass density was established by z ∼ 1. Beyond pulsar timing constraints, further observations of the high redshift SMBH population from the James Webb Space Telescope and the Laser Interferometer Space Antenna will be key for constraining the GWB contribution of mid-high-z SMBHBs.
Journal Article
VLASS Tidal Disruption Events with Optical Flares. II. Discovery of Two TDEs with Intermediate Width Balmer Emission Lines and Connections to the Ambiguous Extreme Coronal Line Emitters
The multiwavelength properties of radio-emitting tidal disruption events (TDEs) are poorly understood. In a previous paper, we presented the first sample of radio-selected, optically detected TDEs, which included two events (VT J1008 and VT J2012) associated with late-time (∼2 yr post-optical flare) intermediate width emission lines that are largely unprecedented from TDEs. In this paper, we investigate these two events in detail. The multiwavelength properties of these events are otherwise consistent with optically selected TDEs. They are hosted by green valley, E+A/Balmer dominated galaxies with low star formation rates and black holes masses MBH ≈ 105−6 M⊙. The optical flare shapes are fully consistent with those of optically selected TDEs, although they are slightly faint and cool at peak. The radio emission from both events is consistent with wide-angle, nonrelativistic outflows with LR(GHz) ∼ 1038 erg s−1. Balmer and helium emission lines are detected from both events with FWHM ∼700 km s−1 and asymmetric line profiles. VT J1008 additionally shows coronal line emission with a similar width. The lines from VT J2012 are redshifted by ∼700 km s−1 relative to the host galaxy. We show that these events share many characteristics in common with the ambiguous class of extreme coronal line emitters. We argue that the lines are likely associated with a radiative shock or dense, photoionized clumps of outflowing gas in the circumnuclear medium.
Journal Article
A Comprehensive Observational Study of the FRB 121102 Persistent Radio Source
FRB 121102 is the first fast radio burst to be spatially associated with a persistent radio source (QRS 121102), the nature of which remains unknown. We constrain the physical size of QRS 121102 by measuring its flux-density variability with the VLA from 12 to 26 GHz. Any such variability would likely be due to Galactic refractive scintillation and would require the source radius to be ≲1017 cm at the host-galaxy redshift. We found the radio variability to be lower than the scintillation theory predictions for such a small source, leaving open the possibility for non-AGN models for QRS 121102. In addition, we roughly estimated the mass of any potential supermassive black hole (BH) associated with QRS 121102 from the line width of the host-galaxy Hα emission using a new optical spectrum from the Keck Observatory. The line width indicates a supermassive BH mass of ≲104∼5 M ⊙, too low for the observed radio luminosity and X-ray luminosity constraints, if QRS 121102 were an AGN. Finally, some dwarf galaxies that host supermassive BHs may be the stripped cores of massive galaxies during tidal interactions with companion systems. We find no nearby galaxy at the same redshift as the QRS 121102 host from low-resolution Keck spectra or the PanSTARRS catalog. In conclusion, we find no evidence supporting the hypothesis that QRS 121102 is an AGN. We instead argue that the inferred size and flat radio spectrum favor a plerion interpretation. We urge continued broadband radio monitoring of QRS 121102 to search for long-term evolution.
Journal Article
Deep Synoptic Array Science: First FRB and Host Galaxy Catalog
Fast radio bursts (FRBs) are a powerful and mysterious new class of transients that are luminous enough to be detected at cosmological distances. By associating FRBs to host galaxies, we can measure intrinsic and environmental properties that test FRB origin models, in addition to using them as precise probes of distant cosmic gas. The Deep Synoptic Array (DSA-110) is a radio interferometer built to maximize the rate at which it can simultaneously detect and localize FRBs. Here, we present the first sample of FRBs and host galaxies discovered by the DSA-110. This sample of 11 FRBs is the largest, most uniform sample of localized FRBs to date, as it is selected based on association with host galaxies identified in optical imaging by Pan-STARRS1. These FRBs have not been observed to repeat, and their radio properties (dispersion, temporal scattering, energy) are similar to that of the known nonrepeating FRB population. Most host galaxies have ongoing star formation, as has been identified before for FRB hosts. Two hosts of the new sample are massive, quiescent galaxies. The distribution of star formation history across this host-galaxy sample shows that the delay time distribution is wide, with a power-law model that spans from ∼100 Myr to ≳2 Gyr. This requires the existence of one or more progenitor formation channels associated with old stellar populations, such as the binary evolution of compact objects.
Journal Article
Implications for Galactic Electron Density Structure from Pulsar Sightlines Intersecting H ii Regions
Recent radio surveys have revealed pulsars with dispersion and scattering delays induced by ionized gas that are larger than the rest of the observed pulsar population, in some cases with electron column densities (or dispersion measures, DMs) larger than the maximum predictions of Galactic electron density models. By cross-matching the observed pulsar population against H ii region catalogs, we show that the majority of pulsars with DM > 600 pc cm−3 and scattering delays τ(1 GHz) > 10 ms lie behind H ii regions, and that H ii region intersections may be relevant to as much as a third of the observed pulsar population. The fraction of the full pulsar population with sightlines intersecting H ii regions is likely larger. Accounting for H ii regions resolves apparent discrepancies where Galactic electron density models place high-DM pulsars beyond the Galactic disk. By comparing emission measures inferred from recombination line observations to pulsar DMs, we show that H ii regions can contribute tens to hundreds of parsecs per cubic centimeter in electron column density along a pulsar line of sight. We find that nearly all pulsars with significant excess (and deficit) scattering from the mean τ–DM relation are spatially coincident with known discrete ionized gas structures, including H ii regions. Accounting for H ii regions is critical to the interpretation of radio dispersion and scattering measurements as electron density tracers, both in the Milky Way and in other galaxies.
Journal Article
Tidal Disruption Event Demographics with the Zwicky Transient Facility: Volumetric Rates, Luminosity Function, and Implications for the Local Black Hole Mass Function
We conduct a systematic tidal disruption event (TDE) demographics analysis using the largest sample of optically selected TDEs. A flux-limited, spectroscopically complete sample of 33 TDEs is constructed using the Zwicky Transient Facility over 3 yr (from 2018 October to 2021 September). We infer the black hole (BH) mass (M BH) with host galaxy scaling relations, showing that the sample M BH ranges from 105.1 M ⊙ to 108.2 M ⊙. We developed a survey efficiency corrected maximum volume method to infer the rates. The rest-frame g-band luminosity function can be well described by a broken power law of ϕ(Lg)∝Lg/Lbk0.3+Lg/Lbk2.6−1 , with L bk = 1043.1 erg s−1. In the BH mass regime of 105.3 ≲ (M BH/M ⊙) ≲ 107.3, the TDE mass function follows ϕ(MBH)∝MBH−0.25 , which favors a flat local BH mass function ( dnBH/dlogMBH≈constant ). We confirm the significant rate suppression at the high-mass end (M BH ≳ 107.5 M ⊙), which is consistent with theoretical predictions considering direct capture of hydrogen-burning stars by the event horizon. At a host galaxy mass of M gal ∼ 1010 M ⊙, the average optical TDE rate is ≈3.2 × 10−5 galaxy−1 yr−1. We constrain the optical TDE rate to be [3.7, 7.4, and 1.6] × 10−5 galaxy−1 yr−1 in galaxies with red, green, and blue colors.
Journal Article
A Hydrodynamical Simulations-based Model that Connects the FRB DM–Redshift Relation to Suppression of the Matter Power Spectrum via Feedback
Understanding the impact of baryonic feedback on the small-scale (k ≳ 1 h Mpc−1) matter power spectrum is a key astrophysical challenge, and essential for interpreting data from upcoming weak-lensing surveys, which require percent-level accuracy to fully harness their potential. Astrophysical probes, such as the kinematic and thermal Sunyaev–Zel’dovich effects, have been used to constrain feedback at large scales (k ≲ 5 h Mpc−1). The sightline-to-sightline variance in the fast radio bursts (FRBs) dispersion measure (DM) correlates with the strength of baryonic feedback and offers unique sensitivity at scales up to k ∼ 10 h Mpc−1. We develop a new simulation-based formalism in which we parameterize the distribution of DM at a given redshift, p(DM∣z), as a log-normal with its first two moments computed analytically in terms of cosmological parameters and the feedback-dependent electron power spectrum Pee(k, z). We find that the log-normal parameterization provides an improved description of the p(DM∣z) distribution observed in hydrodynamical simulations as compared to the standard F-parameterization. Our model robustly captures the baryonic feedback effects across a wide range of baryonic feedback prescriptions in hydrodynamical simulations, including IllustrisTNG, SIMBA, and Astrid. Leveraging simulations incorporates the redshift evolution of the DM variance by construction and facilitates the translation of constrained feedback parameters to the suppression of matter power spectrum relative to gravity-only simulations. We show that with 104 FRBs, the suppression can be constrained to percent-level precision at large scales and ∼10% precision at scales k ≳ 10 h Mpc−1 with prior-to-posterior 1σ constraint width ratio ≳20.
Journal Article
Deep Synoptic Array Science: Discovery of the Host Galaxy of FRB 20220912A
We report the detection and interferometric localization of the repeating fast radio burst (FRB) source FRB 20220912A during commissioning observations with the Deep Synoptic Array (DSA-110). Two bursts were detected from FRB 20220912A, one each on 2022 October 18 and 2022 October 25. The best-fit position is (R.A. J2000, decl. J2000) = (23:09:04.9, +48:42:25.4), with a 90% confidence error ellipse with radii ±2″ and ±1″ in R.A. and decl., respectively. The two bursts are polarized, and we find a Faraday rotation measure that is consistent with the low value of +0.6 rad m−2 reported by CHIME/FRB. The DSA-110 localization overlaps with the galaxy PSO J347.2702+48.7066 at a redshift z = 0.0771, which we identify as the likely host. PSO J347.2702+48.7066 has a stellar mass of approximately 1010 M ⊙, modest internal dust extinction, and a star formation rate likely in excess of 0.1 M ⊙ yr−1. The host-galaxy contribution to the dispersion measure is likely ≲50 pc cm−3. The FRB 20220912A source is therefore likely viewed along a tenuous plasma column through the host galaxy.
Journal Article