FRB 121102 is the only known repeating fast radio burst source. Here we analyze a wide-frequency-range (1-8 GHz) sample of high signal-to-noise, coherently dedispersed bursts detected using the ...Arecibo and Green Bank telescopes. These bursts reveal complex time-frequency structures that include subbursts with finite bandwidths. The frequency-dependent burst structure complicates the determination of a dispersion measure (DM); we argue that it is appropriate to use a DM metric that maximizes frequency-averaged pulse structure, as opposed to peak signal-to-noise, and find DM = 560.57 0.07 pc cm−3 at MJD 57,644. After correcting for dispersive delay, we find that the subbursts have characteristic frequencies that typically drift lower at later times in the total burst envelope. In the 1.1-1.7 GHz band, the ∼0.5-1 ms subbursts have typical bandwidths ranging from 100 to 400 MHz, and a characteristic drift rate of ∼200 MHz ms−1 toward lower frequencies. At higher radio frequencies, the subburst bandwidths and drift rate are larger, on average. While these features could be intrinsic to the burst emission mechanism, they could also be imparted by propagation effects in the medium local to the source. Comparison of the burst DMs with previous values in the literature suggests an increase of ΔDM ∼ 1-3 pc cm−3 in 4 yr; though, this could be a stochastic variation as opposed to a secular trend. This implies changes in the local medium or an additional source of frequency-dependent delay. Overall, the results are consistent with previously proposed scenarios in which FRB 121102 is embedded in a dense nebula.
The millisecond-duration radio flashes known as fast radio bursts (FRBs) represent an enigmatic astrophysical phenomenon. Recently, the sub-arcsecond localization (∼100 mas precision) of FRB 121102 ...using the Very Large Array has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. Here, we report very-long-baseline radio interferometric observations using the European VLBI Network and the 305 m Arecibo telescope, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of 40 pc ( 12 mas angular separation, at 95% confidence). We detect consistent angular broadening of the bursts and persistent radio source (∼2-4 mas at 1.7 GHz), which are both similar to the expected Milky Way scattering contribution. The persistent radio source has a projected size constrained to be 0.7 pc ( 0.2 mas angular extent at 5.0 GHz) and a lower limit for the brightness temperature of T b 5 × 10 7 K . Together, these observations provide strong evidence for a direct physical link between FRB 121102 and the compact persistent radio source. We argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB 121102 that best match the observed data.
Massive outflows of neutral atomic hydrogen (H
I
) have been observed in absorption in a number of radio galaxies and are considered a signature of active galactic nucleus (AGN) feedback. These ...outflows on kiloparsec scales have not been investigated in great detail as they require high-angular-resolution observations to be spatially resolved. In some radio AGN, they are likely to be the result of the radio jets interacting with the interstellar medium. We have used the global very-long-baseline-interferometry (VLBI) array to map the H
I
outflow in a small sample of young and restarted radio galaxies that we previously observed with the Very Large Array and the Westerbork Synthesis Radio Telescope at a lower resolution. Here we report on our findings for
4C 52.37
and
3C 293
and we discuss the sample including the previously published
4C 12.50
and
3C 236
. For
4C 52.37
, we present the first ever H
I
VLBI observations, which recovered the majority of the outflowing H
I
gas in the form of clouds toward the central 100 pc of the AGN. The clouds are blueshifted by up to ∼600 km s
−1
with respect to the systemic velocity.
3C 293
is largely resolved out in our VLBI observation, but toward the VLBI core we detect some outflowing H
I
gas blueshifted with respect to the systemic velocity by up to ∼300 km s
−1
. We also find indications of outflowing gas toward the other parts of the western lobe suggesting that the H I outflow is extended. Overall, we find that the fraction of H
I
gas recovered by our VLBI observations varies significantly within our sample, ranging from complete (
4C 12.50
) to marginal (
3C 293
). However, in all cases we find evidence for a clumpy structure of both the outflowing and the quiescent gas, consistent with predictions from numerical simulations. All the outflows include at least a component of relatively compact clouds with masses in the range of 10
4
− 10
5
M
⊙
. The outflowing clouds are often already observed at a few tens of parsecs (in projection) from the core. We find indications that the H
I
outflow might have a diffuse component, especially in larger sources. Our results support the interpretation that we observe these AGNs at different stages in the evolution of the interaction between the jet and the interstellar medium and this is reflected in the properties of the outflowing gas as predicted by numerical simulations.
We undertook coordinated campaigns with the Green Bank, Effelsberg, and Arecibo radio telescopes during Chandra X-ray Observatory and XMM-Newton observations of the repeating fast radio burst FRB ...121102 to search for simultaneous radio and X-ray bursts. We find 12 radio bursts from FRB 121102 during 70 ks total of X-ray observations. We detect no X-ray photons at the times of radio bursts from FRB 121102 and further detect no X-ray bursts above the measured background at any time. We place a 5 upper limit of 3 × 10−11 erg cm−2 on the 0.5-10 keV fluence for X-ray bursts at the time of radio bursts for durations ms, which corresponds to a burst energy of 4 × 1045 erg at the measured distance of FRB 121102. We also place limits on the 0.5-10 keV fluence of 5 × 10−10 and 1 × 10−9 erg cm−2 for bursts emitted at any time during the XMM-Newton and Chandra observations, respectively, assuming a typical X-ray burst duration of 5 ms. We analyze data from the Fermi Gamma-ray Space Telescope Gamma-ray Burst Monitor and place a 5 upper limit on the 10-100 keV fluence of 4 × 10−9 erg cm−2 (5 × 1047 erg at the distance of FRB 121102) for gamma-ray bursts at the time of radio bursts. We also present a deep search for a persistent X-ray source using all of the X-ray observations taken to date and place a 5 upper limit on the 0.5-10 keV flux of 4 × 10−15 erg s−1 cm−2 (3 × 1041 erg s−1 at the distance of FRB 121102). We discuss these non-detections in the context of the host environment of FRB 121102 and of possible sources of fast radio bursts in general.
Abstract
Double-peaked emission-line AGN (DPAGN) have been regarded as binary black hole candidates. We present here results from parsec-scale radio observations with the Very Long Baseline Array ...(VLBA) of five DPAGN belonging to the KISSR sample of emission-line galaxies. This work concludes our pilot study of nine type 2 Seyfert and LINER DPAGN from the KISSR sample. In the nine sources, dual compact cores are only detected in the “offset AGN,” KISSR 102. However, the overall incidence of jets in the eight sources that were detected with the VLBA is ≥60%. We find a difference in the “missing flux density” going from the Very Large Array to VLBA scales between Seyferts and LINERs, with LINERs showing less missing flux density on parsec-scales. Using the emission-line modeling code, MAPPINGS III, we find that the emission lines are likely to be influenced by jets in 5/9 sources. Jet-medium interaction is the likely cause of the emission-line splitting observed in the SDSS spectra of these sources. Jets in radio-quiet AGN are therefore energetically capable of influencing their parsec- and kpc-scale environments, which makes them agents of “radio AGN feedback” (similar to radio-loud AGN).
Fast radio bursts (FRBs) are brief, bright, extragalactic radio flashes
. Their physical origin remains unknown, but dozens of possible models have been postulated
. Some FRB sources exhibit repeat ...bursts
. Although over a hundred FRB sources have been discovered
, only four have been localized and associated with a host galaxy
, and just one of these four is known to emit repeating FRBs
. The properties of the host galaxies, and the local environments of FRBs, could provide important clues about their physical origins. The first known repeating FRB, however, was localized to a low-metallicity, irregular dwarf galaxy, and the apparently non-repeating sources were localized to higher-metallicity, massive elliptical or star-forming galaxies, suggesting that perhaps the repeating and apparently non-repeating sources could have distinct physical origins. Here we report the precise localization of a second repeating FRB source
, FRB 180916.J0158+65, to a star-forming region in a nearby (redshift 0.0337 ± 0.0002) massive spiral galaxy, whose properties and proximity distinguish it from all known hosts. The lack of both a comparably luminous persistent radio counterpart and a high Faraday rotation measure
further distinguish the local environment of FRB 180916.J0158+65 from that of the single previously localized repeating FRB source, FRB 121102. This suggests that repeating FRBs may have a wide range of luminosities, and originate from diverse host galaxies and local environments.
Fast radio bursts (FRBs) are flashes of unknown physical origin
. The majority of FRBs have been seen only once, although some are known to generate multiple flashes
. Many models invoke magnetically ...powered neutron stars (magnetars) as the source of the emission
. Recently, the discovery
of another repeater (FRB 20200120E) was announced, in the direction of the nearby galaxy M81, with four potential counterparts at other wavelengths
. Here we report observations that localized the FRB to a globular cluster associated with M81, where it is 2 parsecs away from the optical centre of the cluster. Globular clusters host old stellar populations, challenging FRB models that invoke young magnetars formed in a core-collapse supernova. We propose instead that FRB 20200120E originates from a highly magnetized neutron star formed either through the accretion-induced collapse of a white dwarf, or the merger of compact stars in a binary system
. Compact binaries are efficiently formed inside globular clusters, so a model invoking them could also be responsible for the observed bursts.
The energetic feedback that is generated by radio jets in active galactic nuclei (AGNs) has been suggested to be able to produce fast outflows of atomic hydrogen (H I) gas, which can be studied in ...absorption at high spatial resolution. We have used the Very Large Array (VLA) and a global very long baseline interferometry (VLBI) array to locate and study in detail the H I outflow discovered with the Westerbork Synthesis Radio Telescope (WSRT) in the restarted radio galaxy 3C 236. Based on the VLA data, we confirm a blueshifted wing of the H I with a width of ~1000 km s−1. This H I outflow is partially recovered by the VLBI observation. In particular, we detect four clouds with masses of 0.28 − 1.5 × 104M⊙ with VLBI that do not follow the regular rotation of most of the H I. Three of these clouds are located, in projection, against the nuclear region on scales of ≲40 pc, while the fourth is cospatial to the southeast lobe at a projected distance of ~270 pc. Their velocities are between 150 and 640 km s−1 blueshifted with respect to the velocity of the disk-related H I. These findings suggest that the outflow is at least partly formed by clouds, as predicted by some numerical simulations, and that it originates already in the inner (few tens of pc) region of the radio galaxy. Our results indicate that the entire outflow might consist of many clouds, possibly with comparable properties as those clearly detected, but distributed at larger radii from the nucleus where the lower brightness of the lobe does not allow us to detect them. However, we cannot rule out a diffuse component of the outflow. Because 3C 236 is a low-excitation radio galaxy, it is less likely that the optical AGN is able to produce strong radiative winds. This leaves the radio jet as the main driver for the H I outflow.
Galaxies are believed to evolve through merging, which should lead to some hosting multiple supermassive black holes. There are four known triple black hole systems, with the closest black hole pair ...being 2.4 kiloparsecs apart (the third component in this system is at 3 kiloparsecs), which is far from the gravitational sphere of influence (about 100 parsecs for a black hole with mass one billion times that of the Sun). Previous searches for compact black hole systems concluded that they were rare, with the tightest binary system having a separation of 7 parsecs (ref. 10). Here we report observations of a triple black hole system at redshift z = 0.39, with the closest pair separated by about 140 parsecs and significantly more distant from Earth than any other known binary of comparable orbital separation. The effect of the tight pair is to introduce a rotationally symmetric helical modulation on the structure of the large-scale radio jets, which provides a useful way to search for other tight pairs without needing extremely high resolution observations. As we found this tight pair after searching only six galaxies, we conclude that tight pairs are more common than hitherto believed, which is an important observational constraint for low-frequency gravitational wave experiments.
The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow ...relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli-arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.
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