We present the fourth Fermi Large Area Telescope catalog (4FGL) of γ-ray sources. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission in the energy range from 50MeV to 1TeV, it is the deepest yet in this energy range. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for the Galactic diffuse γ-ray emission, and two sets of light curves (one-year and two-month intervals). The 4FGL catalog includes 5064 sources above 4σ significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall, 358 sources are considered as identified based on angular extent, periodicity, or correlated variability observed at other wavelengths. For 1336 sources, we have not found plausible counterparts at other wavelengths. More than 3130 of the identified or associated sources are active galaxies of the blazar class, and 239 are pulsars.

We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the SOAR and Magellan telescopes; the UV spectrum was obtained with the Hubble Space Telescope at 5.5 days. Our data reveal a rapidly-fading blue component (T ≈ 5500 K at 1.5 days) that quickly reddens; spectra later than 4.5 days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at ∼ 7900 Å at t 4.5 days. The colours, rapid evolution and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light r-process nuclei with atomic mass number A 140. This indicates a sight-line within θ obs 45 • of the orbital axis. Comparison to models suggests ∼ 0.03 M of blue ejecta, with a velocity of ∼ 0.3c. The required lanthanide fraction is ∼ 10 −4 , but this drops to < 10 −5 in the outermost ejecta. The large velocities point to a dynamical origin, rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of 12 km. This mass also supports the idea that neutron star mergers are a major contributor to r-process nucleosynthesis. arXiv:1710.05456v1 [astro-ph.HE] 16 Oct 2017 2 NICHOLL ET AL.

Searching for low energy counterparts of γ-rays sources is one of the major challenges in modern γ-ray astronomy. In the third Fermi source catalog about 30% of detected sources are unidentified/unassociated Gamma-ray Sources (UGSs). We recently started an optical spectroscopic follow up campaign to confirm the blazar-like nature of candidates counterparts of UGSs. Here we report the spectra of 61 targets collected with the Southern Astrophysical Research Telescope (SOAR) between 2014 and the 2017. Our sample includes 33 potential counterparts of UGSs, selected on the basis of WISE colors, and 27 blazar candidates of uncertain type associated with gamma-ray sources of the last release of the Fermi catalog. We confirm the BZB nature of 20 sources lying within the positional uncertainty region of the UGSs. All the observed BCUs show blazar-like spectra, classified as 2 BZQs and 25 BZBs, for which we obtained 6 redshift estimates. Within the BCUs observations we report the redshift estimate for the BZB associated with, 3FGL J1106.4-3643 that is the second most distant BL Lac known to date, at zâ\u89¥ 1.084

The existence of "radio weak BL Lac objects" (RWBLs) has been an open question, still unsolved, since the discovery that quasars could be radio-quiet or radio-loud. Recently several groups identified RWBL candidates, mostly found while searching for low energy counterparts of the unidentified/unassociated gamma-ray sources listed in the Fermi catalogs. Confirming RWBLs is a challenging task since they could be confused with white dwarfs or weak emission line quasars when there are not sufficient data to precisely draw their broad band spectral energy distribution and their classification is mainly based on a featureless optical spectra. Motivated by the recent discovery that Fermi BL Lacs appear to have very peculiar mid-IR emission, we show that it is possible to distinguish between WDs, WELQs and BL Lacs using the [3.4]-[4.6]-[12]µm color-color plot built using the WISE magnitudes when the optical spectrum is available. On the basis of this analysis, we identify WISE J064459.38+603131 and WISE J141046.00+740511.2 as the first two genuine RWBLs, both potentially associated with Fermi sources. Finally, to strengthen our identification of these objects as true RWBLs, we present multifrequency observations for these two candidates to show that their spectral behavior is indeed consistent with those of the BL Lac population.

Context. Based on their overwhelming dominance among associated Fermi γ-ray catalogue sources, it is expected that a large fraction of the unidentified Fermi objects are blazars. Through crossmatching between the positions of unidentified γ-ray sources from the First Fermi Catalog of γ-ray sources emitting above 10 GeV (1FHL) and the ROSAT and Swift/XRT catalogues of X-ray objects and between pointed XRT observations, a sample of 36 potential associations was found in previous works with less than 15 arcsec of positional offset. One-third of them have recently been classified; the remainder, though believed to belong to the blazar class, still lack spectroscopic classifications. Aims. We study the optical spectrum of the putative counterparts of these unidentified gamma-ray sources in order to find their redshifts and to determine their nature and main spectral characteristics.Methods. An observational campaign was carried out on the putative counterparts of 13 1FHL sources using medium-resolution optical spectroscopy from the Osservatorio Astronomico di Bologna in Loiano, Italy; the Telescopio Nazionale Galileo and the Nordic Optical Telescope, both in the Canary Islands, Spain; and the Observatorio Astronómico Nacional San Pedro Mártir in Baja California, Mexico. Results. We were able to classify 14 new objects based on their continuum shapes and spectral features. Conclusions. Twelve new blazars were found, along with one new quasar and one new narrow line Seyfert 1 (NLS1) to be potentially associated with the 1FHL sources of our sample. Redshifts or lower limits were obtained when possible alongside central black hole mass and luminosity estimates for the NLS1 and the quasar.

The third Fermi source catalog lists 3033 γray sources above 4σ significance. More than 30% are classified as either unidentified/unassociated Gammaray sources (UGSs), with about 20% classified as Blazar candidates of uncertain types (BCUs). To confirm the blazar-like nature of candidate counterparts of UGSs and BCUs, we started in 2012 an optical spectroscopic follow up campaign. We report here the spectra of 36 targets with observations from the Observatorio Astronómico Nacional San Pedro Mártir, the Southern Astrophysical Research Observatory, the Kitt Peak National Observatory and the Northern Optical Telescope, between 2016 and 2017. We confirm the BL Lac nature of 23 sources, and the flat spectrum radio quasar nature of other 7 ones. We also provide redshift estimates for 19 out of these 30 confirmations, with only one being a lower limit due to spectral features ascribable to intervening systems along the line of sight. As in previous analyses, the largest fraction of now-classified BCUs belong to the class of BL Lac objects, that appear to be the most elusive class of active galactic nuclei. One of the BL Lacs identified in this work, associated with 3FGL J2213.6-4755, lies at a redshift of z >1.529, making it one of the few distant gamma-ray BL Lac objects.

Context. A significant fraction of all γ-ray sources detected by the Large Area Telescope aboard the Fermi satellite is still lacking a low-energy counterpart. In addition, there is still a large population of γ-ray sources with associated low-energy counterparts that lack firm classifications. In the last 10 years we have undertaken an optical spectroscopic campaign to address the problem of unassociated or unidentified γ-ray sources (UGSs), mainly devoted to observing blazars and blazar candidates because they are the largest population of γ-ray sources associated to date. Aims. Here we describe the overall impact of our optical spectroscopic campaign on sources associated in Fermi-LAT catalogs, coupled with objects found in the literature. In the literature search we kept track of efforts by different teams that presented optical spectra of counterparts or potential counterparts of Fermi-LAT catalog sources. Our summary includes an analysis of additional 30 newly collected optical spectra of counterparts or potential counterparts of Fermi-LAT sources of a previously unknown nature. Methods. New spectra were acquired at the Blanco 4 m and OAN-SPM 2.1 m telescopes, and those available in the Sloan Digital Sky Survey (data release 15) archive. Results. All new sources with optical spectra analyzed here are classified as blazars. Thanks to our campaign, altogether we discovered and classified 394 targets with an additional 123 objects collected from a literature search. We began our optical spectroscopic campaign between the release of the second and third Fermi-LAT source catalogs (2FGL and 3FGL, respectively), classified about 25% of the sources that had uncertain nature and discovered a blazar-like potential counterpart for ∼10% of UGSs listed therein. In the 4FGL catalog, about 350 Fermi-LAT sources have been classified to date thanks to our campaign. Conclusions. The most elusive class of blazars are found to be BL Lacs since the largest fraction of Fermi-LAT sources targeted in our observations showed a featureless optical spectrum. The same conclusion applied to the literature spectra. Finally, we confirm the high reliability of mid-IR color-based methods to select blazar-like candidate counterparts of unassociated or unidentified γ-ray sources.

We present the radio luminosity function (LF) of flat-spectrum radio quasars (FSRQ), using the the largest and most complete sample to date. Cross-matching between the FIRST 20 cm and GB6 6 cm radio surveys, we find 638 flat-spectrum radio sources above 220mJy at 1.4GHz; of these, 327 are are classified and verified using optical spectroscopy data, mainly from Sloan Digital Sky Survey Data Release 12. We also considered flatspectrum radio sources that lack both literature references and optical spectroscopy, and we identified 12 out of the 43 such sources to potentially be FSRQs, using their WISE colors. From the fully identified sample of 242 FSRQs, we derived the radio LF and cosmic evolution of blazars at 1.4GHz, finding good agreement with previous work at 5GHz. The number density of FSRQs increases dramatically to a redshift of z∼2 and then declines for higher redshifts. Furthermore, the redshift at which the quasar density peaks is clearly dependent on luminosity, with more luminous sources peaking at higher redshifts. The approximate best-fit LF for a luminosity-dependent evolutionary model is a broken power-law with slopes ∼ 0.7 and ∼1.7 below and above the break luminosity, L log 43.8 1.4 erg s −1 , respectively.