BL Lacertae (BL Lac) objects are a subclass of blazar, distinguished by their featureless optical spectrum. The featureless spectrum presents a challenge in measuring the redshift of the BL Lacs. In this paper, we measure the redshift of BL Lacs using the photometric dropout technique. The space-based telescope Swift and the ground-based SARA telescopes are employed to provide magnitudes in the uvw 2 , uvm 2 , uvw 1 , u , b , v , g ′ , r ′ , i ′ , and z ′ filters. We observe 60 BL Lacs and report reliable redshift upper limits for 41 of them. We discover three new high- z BL Lacs ( z > 1.3) at redshifts of 1.74 − 0.08 + 0.05 , 1.88 − 0.03 + 0.07 , and 2.10 − 0.04 + 0.03 , bringing the number of high- z BL Lacs found by this method up to 19. Discussions are made on the implications for the blazar sequence, the Fermi blazar divide, and the gamma-ray horizon based on an analysis of the 4LAC catalog and all high- z BL Lacs found with the photo- z technique.

Active galactic nuclei with their relativistic jets pointed toward the observer are a class of luminous gamma-ray sources commonly known as blazars. The study of this source class is essential to unveil the physical processes powering these extreme jets, to understand their cosmic evolution, as well as to indirectly probe the extragalactic background light. To do so, however, one needs to correctly classify and measure a redshift for a large sample of these sources. The Third Fermi–LAT Catalog of High-Energy Sources (3FHL) contains 1212 blazars detected at energies greater than 10 GeV. However, ∼25% of these sources are unclassified and ∼56% lack redshift information. To increase the optical completeness of blazars in the 3FHL catalog, we devised an optical spectroscopic follow-up campaign using 4 m and 8 m telescopes. In this paper, we present the results of the last part of this campaign, where we observed 5 blazars using the 8 m Gemini-S telescope in Chile. We report all the 5 sources to be classified as BL Lacs, a redshift lower limit for 2 sources, and featureless spectra for the remaining 3 sources. We also performed a one-zone leptonic fit to the two sources with the redshift lower limits.

The Third Catalog of Hard Fermi Large Area Telescope Sources (3FHL) reports the detection of 1556 objects at E > 10 GeV. However, 177 sources remain unassociated and 23 are associated with a ROSAT X-ray detection of unknown origin. Pointed X-ray observations were conducted on 30 of these unassociated and unknown sources with Swift-XRT. A bright X-ray source counterpart was detected in 21 out of 30 fields. In five of these 21 fields, we detected more than one X-ray counterpart, totaling 26 X-ray sources analyzed. Multiwavelength data was compiled for each X-ray source detected. We find that 21 out of the 26 X-ray sources detected display the multiwavelength properties of blazars, while one X-ray source displays the characteristics of a Galactic source. Using trained decision tree, random forest, and support vector machine models, we predict all 21 blazar counterpart candidates to be BL Lacertae objects (BL Lacs). This is in agreement with BL Lacs being the most populous source class in the 3FHL.

As a follow-up to the optical spectroscopic campaign aimed at achieving completeness in the Third Catalog of Hard Fermi-LAT Sources (3FHL), we present here the results of a sample of 28 blazars of an uncertain type observed using the 4 m telescope at Cerro Tololo Inter-American Observatory in Chile. Out of these 28 sources, we find that 25 are BL Lacertae objects (BL Lacs) and 3 are flat-spectrum radio quasars (FSRQs). We measure redshifts or lower limits for 16 of these blazars, and it is observed that the 12 remaining blazars have featureless optical spectra. These results are part of a more extended optical spectroscopy follow-up campaign for 3FHL blazars, where, until now, 51 blazars of an uncertain type have been classified into BL Lac and FSRQ categories. Furthermore, this campaign has resulted in redshift measurements and lower limits for 15 of these sources. Our results contribute toward attaining a complete sample of blazars above 10 GeV, which then will be crucial in extending our knowledge on blazar emission mechanisms and the extragalactic background light.

BL Lacertae objects (BL Lacs) represent a large fraction (22\%) of gamma-ray sources in the Third Fermi Large Area Telescope catalog (3FGL). Nearly half of the BL Lac population remains without a redshift because of their featureless optical spectra. We aim to increase the number of BL Lacs with a redshift measurement by using the photometric technique. For this work, we have used 6 Swift-UVOT filters and SDSS g,r,i,z optical filters mounted on the 0.65 m SARA-CTIO located in Chile and the 1.0 m SARA-ORM in Canary Islands. A sample of 45 sources was selected from the 3FGL catalog for which photometry was performed in 10 optical and UV filters to obtain redshift measurements. We found 3 sources with z>1.3 while reliable upper limits have been derived for 17 sources. The results presented here bring the total number of high-z (z>1.3) BL Lacs to 29.

The Fermi Large Area Telescope (Fermi-LAT) 3FHL catalog is the latest catalog of >10 GeV sources and will remain an important resource for the high-energy community for the foreseeable future. Therefore, it is crucial that this catalog is made complete by providing associations for most sources. In this paper, we present the results of the X-ray analysis of 38 3FHL sources. We found a single bright X-ray source in 20 fields, two sources each in two fields, and none for the remaining 16. The analysis of the properties of the 22 3FHL fields with X-ray sources led us to believe that most (∼19/22) are of extragalactic origin. A machine-learning algorithm was used to determine the source type and we find that 15 potential blazars are likely BL Lacertae objects (BL Lac objects). This is consistent with the fact that BL Lac objects are by far the most numerous population detected above >10 GeV in the 3FHL.

With bolometric luminosities exceeding 10(48) erg s(-1), powerful jets, and supermassive black holes at their center, MeV blazars are some of the most extreme sources in the universe. Recently, the Fermi-Large Area Telescope detected five new gamma-ray emitting MeV blazars beyond redshift z = 3.1. With the goal of precisely characterizing the jet properties of these extreme sources, we started a multiwavelength campaign to follow them up with joint Nuclear Spectroscopic Telescope Array, Swift, and the Southeastern Association for Research in Astronomy's optical telescopes. We observe six high-redshift quasars, four of them belonging to the new gamma-ray emitting MeV blazars. Thorough X-ray analysis reveals spectral flattening at soft X-ray for three of these objects. The source NVSS J151002+570243 also shows a peculiar rehardening of the X-ray spectrum at energies E > 6 keV. Adopting a one-zone leptonic emission model, this combination of hard X-rays and gamma-rays enables us to determine the location of the Inverse Compton peak and to accurately constrain the jet characteristics. In the context of the jetaccretion disk connection, we find that all six sources have jet powers exceeding accretion disk luminosity, seemingly validating this positive correlation even beyond z > 3. Our six sources are found to have 10(9) M-circle dot black holes, further raising the space density of supermassive black holes in the redshift bin z = [3, 4].

High synchrotron peak (HSP; v(sy)(pk) > 10(15) Hz) BL Lac objects are some of the most extreme accelerators in the universe. Those found at high redshifts (z > 1) challenge our understanding of blazar evolution models and are crucial for cosmological measurements of the extragalactic background light. In this paper, we study a high-z BL Lac object, 4FGL J2146.5-1344, detected to be at z = 1.34 using the photometric dropout technique. We collected multiwavelength data for this source from optical up to gamma-rays, in order to study its spectral energy distribution (SED). In particular, this source was observed for the first time with the Nuclear Spectroscopic Telescope Array, which accurately measures the synchrotron emission of this blazar up to 50 keV. Despite being classified as an HSP BL Lac object, the modeling of the SED reveals that this source likely belongs to the "masquerading BL Lac" class, which comprises flat spectrum radio quasars appearing as disguised BL Lac objects.

Active galactic nuclei (AGNs) with their relativistic jets pointed toward the observer form a subclass of luminous gamma-ray sources commonly known as blazars. The study of blazars is essential to improve our understanding of the AGNs emission mechanisms and evolution, as well as to map the extragalactic background light. To do so, however, one needs to classify and measure a redshift for a large sample of these sources correctly. The Third Fermi-LAT Catalog of High-Energy Sources (3FHL) contains approximate to 1160 blazars reported at energies greater than 10 GeV. However, similar to 25% of these sources are unclassified and similar to 50% lack redshift information. To increase the spectral completeness of the 3FHL catalog, we are working on an optical spectroscopic follow-up campaign using 4 m and 8 m telescopes. In this paper, we present the results of the second part of this campaign, where we observed 23 blazars using the 4 m telescope at Cerro Tololo Interamerican Observatory in Chile. We report that all the 23 sources are classified as BL Lacs, a confirmed redshift measurement for three sources, a redshift lower limit for two sources, and a tentative redshift measurement for three sources.

We consider the effect of resonant scattering of ≤keV surface emission in the closed zone of pulsar magnetospheres. For magnetospheric charge densities comparable to the corotation value, subtle effects are expected in the soft X-ray and visible/UV regimes. Radiation support of a dense closed-zone plasma may, however, give rise to large scattering-induced pulse modulation; we discuss limits to such plasma population. If closed-zone scattering can be detected, the resulting pulse profiles are sensitive to details of the neutron star gravity field. For millisecond pulsars, in particular, scattering pulse features can provide an interesting probe of relativistic gravity.

Using a Hartree-Fock formalism, we estimate energy levels and photon cross sections for atomic iron in magnetic fields B ~ 1013 G. Computing ionization equilibrium and normal mode opacities with these data, we construct LTE neutron star model atmospheres at 5.5 < log (Teff) < 6.5 and compute emergent spectra. We examine the dependence of the emergent spectra on Teff and B. We also show the spectral variation with the angle between the magnetic field and the atmosphere normal and describe the significant limb darkening in the X-ray band. These results are compared with recent detailed computations of neutron star H model atmospheres in high fields and with low-field Fe and H model atmospheres constructed from detailed opacities. The large spectral differences for different surface compositions may be discernible with present X-ray data; we also note improvements needed to allow comparison of Fe models with high quality spectra.

We compute model atmospheres and emergent spectra for low-field (B less than or similar to 10(10) G) neutron stars, using new opacity and equation of state data from the OPAL project. These computations, incorporating improved treatments of flux transport and convective stability, provide spectra for hydrogen, solar abundance, and iron atmospheres. We compare our results to high field magnetic atmospheres, available only for hydrogen. An application to apparently thermal flux from the low-field millisecond pulsar PSR J0437-4715 shows that H atmospheres fit substantially better than Fe models. We comment on extension to high fields and the implication of these results for neutron star luminosities and radii.

For massive black hole binaries produced in galactic mergers, we examine the possibility of inspiral induced by interaction with field stars. We model the evolution of such binaries for a range of galaxy core and binary parameters, using numerical results from the literature to compute the binary's energy and angular momentum loss rates due to stellar encounters and including the effect of back-action on the field stars. We find that only a small fraction of binary systems can merge within a Hubble time via unassisted stellar dynamics. External perturbations may, however, cause efficient inspiral. Averaging over a population of central black holes and galaxy mergers, we compute the expected background of gravitational radiation with periods Pw ~ 1-10 yr. Comparison with sensitivities from millisecond pulsar timing suggests that the strongest sources may be detectable with modest improvements to present experiments.

We perform Monte Carlo calculations of neutron star orbits in the Galaxy adopting the birth distribution of the slow moving radio pulsar population found by Narayan and Ostriker. We incorporate a smooth model of the interstellar medium and calculate the distribution of accreted masses onto neutron stars in the solar neighborhood. We also include appendices describing a general analytic approach to the accretion problem. This agrees very well with the Monte Carlo results and is valid under more general assumptions about the interstellar matter distribution and birth properties of neutron stars. These results may be used to test theories of high-energy sources which invoke interstellar accretion. In particular we explore the implications for a model of gamma-ray bursts based on electron capture-induced starquakes.