On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

Abstract.Using the Effelsberg 100-m telescope, detections of four extragalactic water vapor masers are reported. Isotropic luminosities are ∼50, 1000, 1 and 230 L for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 and Arp 299, respectively. Mrk 34 contains by far the most distant and one of the most luminous water vapor megamasers so far reported in a Seyfert galaxy. The interacting system Arp 299 appears to show two maser hotspots separated by approximately 20 . With these new results and even more recent data from Braatz et al. (2004, ApJ, 617, L29), the detection rate in our sample of Seyferts with known jet-Narrow Line Region interactions becomes 50% (7/14), while in star forming galaxies with high (S 100 µm > 50 Jy) far infrared fluxes the detection rate is 22% (10/45). The jet-NLR interaction sample may not only contain "jet-masers" but also a significant number of accretion "disk-masers" like those seen in NGC 4258. A statistical analysis of 53 extragalactic H 2 O sources (excluding the Galaxy and the Magellanic Clouds) indicates (1) that the correlation between IRAS Point Source and H 2 O luminosities, established for individual star forming regions in the galactic disk, also holds for AGN-dominated megamaser galaxies; (2) that maser luminosities are not correlated with 60 µm/100 µm color temperatures; and (3) that only a small fraction of the luminous megamasers (L H 2 O > 100 L ) detectable with 100-m sized telescopes have so far been identified. The H 2 O luminosity function (LF) suggests that the number of galaxies with 1 L < L H 2 O < 10 L , the transition range between "kilomasers" (mostly star formation) and "megamasers" (active galactic nuclei), is small. The overall slope of the LF, ∼−1.5, indicates that the number of detectable masers is almost independent of their luminosity. If the LF is not steepening at very high maser luminosities and if it is possible to find suitable candidate sources, H 2 O megamasers at significant redshifts should be detectable even with present day state-of-the-art facilities.

Water masers are found in dense molecular clouds closely associated with supermassive black holes at the centres of active galaxies. On the basis of the understanding of the local water-maser luminosity function, it was expected that masers at intermediate and high redshifts would be extremely rare. However, galaxies at redshifts z> 2 might be quite different from those found locally, not least because of more frequent mergers and interaction events. Here we use gravitational lensing to search for masers at higher redshifts than would otherwise be possible, and find a water maser at redshift 2.64 in the dust- and gas-rich, gravitationally lensed type-1 quasar MG J0414+0534 (refs 6-13). The isotropic luminosity is 10,000 (, solar luminosity), which is twice that of the most powerful local water maser and half that of the most distant maser previously known. Using the locally determined luminosity function, the probability of finding a maser this luminous associated with any single active galaxy is 10(-6). The fact that we see such a maser in the first galaxy we observe must mean that the volume densities and luminosities of masers are higher at redshift 2.64.

We have analyzed the NVSS and SUMSS data at 1.4 GHz and 843 MHz for a well defined complete sample of hard X-ray AGN observed by INTEGRAL. A large number (70/79) of sources are detected in the radio band, showing a wide range of radio morphologies, from unresolved or slightly resolved cores to extended emission over several hundreds of kpc scales. The radio fluxes have been correlated with the 2-10 keV and 20-100 keV emission, revealing significant correlations with slopes consistent with those expected for radiatively efficient accreting systems. The high energy emission coming from the inner accretion regions correlates with the radio emission averaged over hundreds of kpc scales (i.e., thousands of years).

Large column densities, derived from X-ray studies, are typically measured towards AGN hosting water masers, especially when the H 2 O emission is associated with the nuclear accretion disk. In addition, possible correlations between the intrinsic X-ray luminosity and the characteristics of the H 2 O maser emission have been put forward that, however, require confirmation. We have performed high-sensitivity XMM-Newton observations of a sample of five H 2 O maser sources confidently detected in our ongoing survey with the Swift satellite of all known water masers in AGN, in order to obtain detailed X-ray information of these promising targets and to set up a systematic detailed study of the X-ray/H 2 O-maser relation in AGN. For three galaxies, NGC 613, VII Zw 73, and IRAS 16288+3929, the amount of intrinsic absorption has been estimated, indicating column densities of 4-6×10 23 cm −2 . For UGC 3789 and NGC 6264 (the two confirmed disk-maser galaxies in our sample), column densities in excess of 1×10 24 cm −2 are inferred from the large EW of the Fe Kα line. By adding our results to those obtained in past similar studies, we find that the percentage of water masers sources that host highly-obscured (N H > 10 23 cm −2 ) and Compton-thick AGN is 96% (45/47) and 57% (27/47), respectively. In addition, 86%, 18/21 of disk maser galaxies host Compton-thick AGN. The correlation between the galaxies' bolometric luminosity and accretion disk radius, suggested in previous works, is also confirmed.

We present high-resolution spectral line and continuum VLBI and VLA observations of the nuclear region of NGC 253 at 22 GHz. While the water vapor masers in this region were detected on arcsecond and milliarcsecond scales, we could not detect any compact continuum emission with a 5σ upper limit of ∼1 mJy. The observations reveal that the water maser emission is not related to a possible low-luminosity active galactic nucleus but is almost certainly associated with star-formation activity. Not detecting any compact continuum source on milliarcsecond scales also questions the presence of a -previously assumed -active nucleus in NGC 253.

Context. Narrow-line Seyfert 1 (NLS1) galaxies are a class of active galactic nuclei (AGN) that have all the properties of type 1 Seyfert galaxies but show peculiar characteristics, including the narrowest Balmer lines, strongest Fe II emission, and extreme properties in the X-rays. Line and continuum radio observations provide an optimal tool to access the (often) optically obscured innermost regions of AGN and reveal the kinematics of the gas around their central engines. Aims. We investigate the interplay between the peculiar NLS1 class of AGN and the maser phenomenon, to help us understand the nature of the maser emission in some NLS1s where water maser emission has been detected. Methods. We observed a sample of NLS1 galaxies with the Green Bank Telescope in a search for water maser emission at 22 GHz. We also reduced and analysed archival Green Bank Telescope and Very Large Array data and produced 22-GHz spectra for the five NLS1 galaxies with detected maser emission. In particular, we imaged the maser and nuclear radio continuum of NGC 5506 at subarcsec scales with the Very Large Array. Results. We discovered maser emission in two NLS1 galaxies: IGR J16385-2057, and IRAS 03450+0055. In addition to the three previously known maser detections in the NLS1s Mrk 766, NGC 4051, and NGC 5506, this yields a water maser detection rate in NLS1 galaxies of ∼7% (5/71). This value rises significantly to ∼21% (5/24) when considering only NLS1 galaxies at recessional velocities less than 10 000 km s −1 . For NGC 4051 and NGC 5506, we find that the water maser emission is located within 5 and 12 pc, respectively, of nuclear radio continuum knots, which are interpreted as core-jet structures. Conclusions. The water maser detection rate in NLS1s is surprisingly high, much higher than the detection rate obtained for type 1 AGN and similar to those in Seyfert 2 and low-ionization nuclear emission-line region galaxies. The masers in NGC 4051 and NGC 5506 are nuclear and associated with the AGN, either with an accretion disk, a radio jet, or a nuclear outflow. The apparent lack of high-velocity maser features and evidence, recently reported, of radiative outflows and radio jets in the host galaxies seems to favour interpretation as a jet or an outflow. A similar association is also seemingly true for the maser in Mrk 766, IGR J16385-2057, and IRAS 03450+0055, although, in these cases, without radio interferometric measurements we cannot rule out an off-nuclear origin of the emission.

Context. Recently, a relationship between the water maser detection rate and far infrared (FIR) flux density has been found as a result of a 22 GHz maser survey in a sample comprised of northern galaxies with 100 µm flux density >50 Jy and a declination >−30 • . Aims. The survey has been extended toward galaxies with lower FIR flux densities in order to confirm this correlation and to discover additional maser sources for relevant follow-up interferometric studies. Methods. A sample of 41 galaxies with 30 Jy < S 100 µm < 50 Jy and δ > −30 • was observed with the 100-m telescope at Effelsberg in a search for the 22 GHz water vapor line. The average 3σ noise level of the survey is 40 mJy for a 1 km s −1 channel, corresponding to a detection threshold for the isotropic maser luminosity of ∼0.5 L at a distance of 25 Mpc. Results. Two detections are reported: a megamaser with an isotropic luminosity, L H 2 O , of ≈35 L in the Seyfert/Hii galaxy NGC 613 and a kilomaser with L H 2 O ≈ 1 L in the merger system NGC 520. The high luminosity and the presence of a Seyfert nucleus favor an association for NGC 613 with an active galactic nucleus. The kilomaser in NGC 520 was also detected with the Very Large Array, providing a position with subarcsecond accuracy. The H 2 O emission, originating from a < ∼ 0.02 pc sized region with a brightness temperature > ∼ 10 10 K (if the observed variations are intrinsic to the masing cloud(s)), is close to one of the two radio continuum sources located in the inner parsecs of NGC 520. The maser is most likely associated with a young supernova remnant (S NR), although an association with a low-luminosity AGN (LLAGN) cannot be ruled out. The maser detection rate, with 2 new maser sources out of 41 galaxies observed, is consistent with expectations extrapolated from the statistical properties of the S 100 µm > 50 Jy sample. The H 2 O kilomasers are "subluminous", while H 2 O megamasers tend to be "superluminous" with respect to the FIR luminosity of their parent galaxy, when compared with sites of massive star formation in the Milky Way.