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 5 10 K b 7´. 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.

We present the results of H I and OH absorption measurements towards a sample of radio sources using the Arecibo 305-m telescope and the Giant Metrewave Radio Telescope (GMRT). In total, 27 radio sources were searched for associated 21-cm H I absorption. One totally new H I absorption system was detected against the radio galaxy 3C258, while five previously known H I absorption systems, and one galaxy detected in emission, were studied with improved frequency resolution and/or sensitivity. Our sample included 17 gigahertz peaked spectrum (GPS) and compact steep spectrum (CSS) objects, four of which exhibit H I absorption. This detection rate of ∼25 per cent compares with a value of ∼40 per cent by Vermeulen et al. for similar sources. We detected neither OH emission nor absorption towards any of the sources that were observed at Arecibo. We are, however, able to estimate a limit on the abundance ratio of N(H I)/N(OH) 4 × 10 6 for 3C258.We have combined our results with those from other available H I searches to compile a heterogeneous sample of 96 radio sources consisting of 27 GPS, 35 CSS, 13 compact flat spectrum (CFS) and 21 large (LRG) sources. The H I absorption detection rate is highest (∼45 per cent) for the compact GPS sources and least for the LRG sources. We find H I column density to be anticorrelated with source size, as reported earlier by Pihlström et al., a trend which is consistent with the results of optical spectroscopy. The H I column density shows no significant dependence on either redshift or luminosity, which are themselves strongly correlated. These results suggest that the environments of radio sources on GPS/CSS scales are similar at different redshifts. Further, in accordance with the unification scheme, the GPS/CSS galaxies have an H I detection rate of ∼40 per cent which is higher than the detection rate (∼20 per cent) towards the GPS/CSS quasars. Also, the principal (strongest) absorption component detected towards GPS sources appears blueshifted in ∼65 per cent of the cases. This is in agreement with the growing evidence for jet-cloud interactions playing an important role in determining the ionization and kinematical properties of the ambient gas.

Abstract. We present sensitive Giant Metrewave Radio Telescope (GMRT) and high-resolution Arecibo HI 21-cm observations of the damped Lyman-α absorber (DLA) at z = 0.2212 towards OI 363 (B2 0738+ 313). The GMRT and Arecibo spectra are in excellent agreement and yield a spin temperature Ts = 890 ± 160 K, consistent with earlier lower sensitivity observations of the system. This value of Ts is far higher than spin temperatures measured for the Milky Way and local spirals but is similar to Ts values obtained in the majority of damped absorbers (Ts > ∼ 1000 K). The high velocity resolution of the Arecibo spectra enables us to obtain estimates of physical conditions in the absorbing clouds by fitting multiple Gaussians to the absorption profile. The spectra are well fit by a three-component model with two narrow and one wide components, with temperatures T k 1 = 308 ± 24 K, T k 2 = 180 ± 30 K and T k 3 = 7600 ± 1250 K, respectively. The last of these is in excellent agreement with the expected temperatures for the WNM (5000−8000 K). Further, the mere fact that components are seen with lower temperatures than the estimated Ts implies that the absorber must have a multi-phase medium. We use the measured 21-cm optical depth and the above estimates of the kinetic temperature to obtain the HI column density in the various components. The total column density in the narrow components is found to be NHI(CNM) ≤ 1.9 ± 0.25 × 10 20 cm −2 , while that in the wide component is NHI(WNM) ≥ 1.26 ± 0.49 × 10 21 cm −2 . Thus, the WNM contains at least 75% of the total HI in the z = 0.2212 DLA, unlike our Galaxy, in which the CNM and WNM have equitable contributions. As conjectured earlier (Chengalur & Kanekar 2000), this accounts for the difference in the spin temperatures of the z = 0.2212 system and local spirals, suggesting that the DLA is probably a dwarf or LSB type galaxy; this is also in agreement with optical studies (Turnshek et al. 2001). Finally, the total column density in the DLA is found to be NHI ∼ 1.45 ± 0.49 × 10 21 cm −2 , which agrees within the errors with the value of NHI = 7.9 ± 1.4 × 10 20 cm −2 , obtained from the Lyman-α profile (Rao & Turnshek 1998). This reinforces our identification of the wide and narrow components as the WNM and CNM respectively.

Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of 10 11.5 K. Relativistic boosting can increase its observed value, but apparent brightness temperatures much in excess of 10 13 K are inaccessible using ground-based very long baseline interferometry (VLBI) at any wavelength. We present observations of the quasar 3C 273, made with the space VLBI mission RadioAstron on baselines up to 171,000 km, which directly reveal the presence of angular structure as small as 26 µas (2.7 light months) and brightness temperature in excess of 10 13 K. These measurements challenge our understanding of the non-thermal continuum emission in the vicinity of supermassive black holes and require a much higher Doppler factor than what is determined from jet apparent kinematics.

We present very long baseline interferometry (VLBI) observations 217 days after the -ray burst (GRB) of 2003 March 29. These observations provide further measurements of the size and position of GRB 030329 that are used to constrain the expansion rate and proper motion of this nearby GRB. The expansion rate appears to be slowing down with time, favoring expansion into a constant density interstellar medium, rather than a circumstellar wind with an r À2 density profile. We also present late-time Arecibo observations of the redshifted H i and OH absorption spectra toward GRB 030329. No absorption (or emission) is seen, allowing us to place limits on the atomic neutral hydrogen of N H < 8:5 ; 10 20 cm À2 and on molecular hydrogen of N H 2 < 1:4 ; 10 22 cm À2 . Finally, we present VLA limits on the radio polarization from the afterglow of <2% at late times.