Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera-electron volts. Its arrival direction was consistent with the location of a known γ-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to γ-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy γ-rays. This observation of a neutrino in spatial coincidence with a γ-ray-emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes located in the Canary island of La Palma, Spain. During summer 2011 and 2012 it underwent a series of upgrades, involving the exchange of the MAGIC-I camera and its trigger system, as well as the upgrade of the readout system of both telescopes. We use observations of the Crab Nebula taken at low and medium zenith angles to assess the key performance parameters of the MAGIC stereo system. For low zenith angle observations, the standard trigger threshold of the MAGIC telescopes is ∼ 50 GeV. The integral sensitivity for point-like sources with Crab Nebula-like spectrum above 220 GeV is (0.66 ± 0.03)% of Crab Nebula flux in 50 h of observations. The angular resolution, defined as the σ of a 2-dimensional Gaussian distribution, at those energies is ≲ 0.07°, while the energy resolution is 16%. We also re-evaluate the effect of the systematic uncertainty on the data taken with the MAGIC telescopes after the upgrade. We estimate that the systematic uncertainties can be divided in the following components: < 15% in energy scale, 11%-18% in flux normalization and ± 0.15 for the energy spectrum power-law slope

We have used the Hubble Space T elescope (HST ) WFPC2 camera to survey 132 BL Lac objects comprising seven complete radio-, X-rayÈ, and optically selected samples. We obtained useful images for 110 targets spanning the redshift range These represent an unbiased subsample of the original 0 [ z [ 1.3. 132 since they were snapshots selected to Ðll random holes in the HST schedule. The exposure times ranged from a few hundred to D1000 s, increasing with redshift. Most images were taken in the F702W Ðlter ; those already observed in F814W during Cycle 5 were reobserved in F606W to give broader wavelength coverage. The data were analyzed uniformly, and both statistical and systematic errors were estimated (the latter dominate). In of the BL Lac images, host galaxies are detected, including nearly all 2 3 for z \ 0.5 (58 of 63). In contrast, only one-quarter of the BL Lac objects with z [ 0.5 (six of 22) were resolved because of the relatively short exposure times, and these tend to be very luminous host galaxies. The highest redshift host galaxy detected is in a BL Lac object at z \ 0.664. HST data add critical morphological information in the range a few tenths to a few arcseconds. In 58 of the 72 resolved host galaxies, a de Vaucouleurs proÐle is signiÐcantly preferred, at conÐdence, over a pure exponential Z99% disk ; the two Ðts are comparable in the remaining 14 cases because of their generally lower signal-tonoise ratios. These results limit the number of disk systems to at most 8% of BL Lac objects (at 99% conÐdence) and are consistent with all BL Lac host galaxies being ellipticals. The detected host galaxies are luminous ellipticals with a median absolute K-corrected magnitude of mag (rms M R D [23.7^0.6 dispersion), at least 1 mag brighter than M* and comparable to brightest cluster galaxies. The galaxy morphologies are generally smooth and undisturbed, with small or negligible ellipticities The (v [ 0.2). half-light surface brightness is anticorrelated with half-light radius in quantitatively the same way as other elliptical galaxies, indicating that apart from their highly active nuclei, BL Lac objects appear to be absolutely normal ellipticals. There is no correlation between host galaxy and observed nuclear magnitude or estimated jet power corrected for beaming. If black hole mass is correlated linearly with bulge mass in general, this implies a large range in Eddington ratio. The host galaxies of the radio-selected and X-rayÈselected BL Lac objects are comparable in both morphology and luminosity, strongly suggesting that nuclear properties do not have a dramatic e †ect on large-scale host galaxy properties, or vice versa. BL Lac objects have extended radio powers and host galaxy magnitudes very much like those of FR I galaxies, and quite distinct from FR IIÏs, which instead are more similar to quasars. Thus the present data strongly support the uniÐcation picture with FR I galaxies constituting the bulk of the parent population of BL Lac objects.

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The HST snapshot imaging survey of 110 BL Lac objects ( Urry et al.) has clearly shown that the host galaxies are massive and luminous ellipticals. The dispersion of the absolute magnitudes is sufficiently small that the measurement of the galaxy brightness becomes a valuable way of estimating their distance. This is illustrated by constructing a Hubble diagram of the 64 resolved objects with known redshift. By means of this relationship, we estimate the redshift of five resolved BL Lac objects of the survey that have no spectroscopic z. The adopted method also allows us to evaluate lower limits to the redshift for 13 objects of still unknown z using the lower limit on the host galaxy magnitude. This technique can be applied to any BL Lac object for which an estimate or a lower limit of the host galaxy magnitude is available. Finally, we show that the distribution of the nuclear luminosity of all the BL Lac objects of the survey indicates that the objects for which both the redshift and the host galaxy are undetected are among the most luminous, and possibly the most highly beamed.

Abstract. We performed an observational program with the X-ray satellite BeppoSAX, to study objects with extreme synchrotron peak frequencies (ν peak > 1 keV). Of the seven sources observed, five revealed or confirmed their extreme nature. Four showed peak frequencies in the range 1-5 keV, while one (1ES 1426+428) displayed a flat power law spectrum (αx = 0.92 ± 0.04) which locates its synchrotron peak at or above 100 keV. This is the third source of this type ever found, after Mkn 501 and 1ES 2344+514. In the context of the whole blazar class, the broad band properties of these objects confirm the scenario of a synchrotron peak smoothly spanning the IR -X-ray range, which explains the multi-frequency properties of the blazar class. Our data also confirm the large ν peak variability which seems to characterize this class of sources, compared with lower ν peak objects. Given the high synchrotron peak energies, which flag the presence of high energy electrons, these extreme BL Lacs are also good candidates for TeV emission, and therefore good probes of the IR background.

Very high energy (VHE) γ-ray emission from the flat spectrum radio quasar (FSRQ) PKS 1222+21 (4C 21.35, z = 0.432) was detected with the MAGIC Cherenkov telescopes during a short observation (∼0.5 hr) performed on 2010 June 17. The MAGIC detection coincides with high-energy MeV/GeV γ-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The VHE spectrum measured by MAGIC extends from about 70 GeV up to at least 400 GeV and can be well described by a power-law dN/dE ∝ E −Γ with a photon index Γ = 3.75 ± 0.27 stat ± 0.2 syst. The averaged integral flux above 100 GeV is (4.6 ± 0.5) × 10 −10 cm −2 s −1 (∼1 Crab Nebula flux). The VHE flux measured by MAGIC varies significantly within the 30 minute exposure implying a flux doubling time of about 10 minutes. The VHE and MeV/GeV spectra, corrected for the absorption by the extragalactic background light (EBL), can be described by a single power law with photon index 2.72 ± 0.34 between 3 GeV and 400 GeV, and is consistent with emission belonging to a single component in the jet. The

We study the dependence of the M BH -M host relation on the redshift up to z = 3 for a sample of 96 quasars, the host galaxy luminosities of which are known. Black hole masses were estimated assuming virial equilibrium in the broad-line regions, while the host galaxy masses were inferred from their luminosities. With these data, we are able to pin down the redshift dependence of the M BH -M host relation along 85 per cent of the Universe age. We show that, in the sampled redshift range, the M BH -L host relation remains nearly unchanged. Once we take into account the ageing of the stellar population, we find that the M BH /M host ratio ( ) increases by a factor of ∼7 from z = 0 to z = 3. We show that evolves with z regardless of the radio loudness and of the quasar luminosity. We propose that the most massive black holes, living their quasar phase at high redshift, become extremely rare objects in host galaxies of similar mass in the Local Universe.