Howard A. Smith

IDENTIFICATION OF THE INFRARED NON-THERMAL EMISSION IN BLAZARS

Blazars constitute the most interesting and enigmatic class of extragalactic γ-ray sources dominated by non-thermal emission. In this Letter, we show how the Wide Infrared Survey Explorer (WISE) infrared data make it possible to identify a distinct region of the [3.4]-[4.6]-[12] μm color-color diagram where the sources dominated by the thermal radiation are separated from those dominated by non-thermal emission, in particular the blazar population. This infrared non-thermal region, which we indicate as the WISE blazar strip (WBS), will constitute a new powerful diagnostic tool when the full WISE survey data are released. The WBS can be used to extract new blazar candidates, to identify those of uncertain type and also to search for the counterparts of unidentified γ-ray sources. We show one example of the value of the use of the WBS identifying the TeV source VER J0648+152, recently discovered by VERITAS.

INFRARED COLORS OF THE GAMMA-RAY-DETECTED BLAZARS

Blazars constitute the most enigmatic class of extragalactic γ-ray sources, and their observational features have been ascribed to a relativistic jet closely aligned to the line of sight. They are generally divided in two main classes: the BL Lac objects (BL Lacs) and the flat-spectrum radio quasars (FSRQs). In the case of BL Lacs the double-bumped spectral energy distribution (SED) is generally described by the synchrotron self-Compton (SSC) emission, while for the FSRQs it is interpreted as due to external Compton (EC) emission. Recently, we showed that in the [3.4]-[4.6]-[12] μm color-color diagram the blazar population covers a distinct region (i.e., the WISE blazar Strip (WBS)) clearly separated from the other extragalactic sources that are dominated by thermal emission. In this paper, we investigate the relation between the infrared and γ-ray emission for a subset of confirmed blazars from the literature, associated with Fermi sources, for which WISE archival observations are available. This sample is a proper subset of the sample of sources used previously, and the availability of Fermi data is critical to constrain the models on the emission mechanisms for the blazars. We found that the selected blazars also lie on the WBS covering a narrower region of the infrared color-color planes than the overall blazar population. We then found an evident correlation between the IR and γ-ray spectral indices expected in the SSC and EC frameworks. Finally, we determined the ratio between their γ-ray and infrared fluxes, a surrogate of the ratio of powers between the inverse Compton and the synchrotron SED components, and used such parameter to test different blazar emitting scenarios.

The WISE gamma-ray strip parametrization: the nature of the gamma-ray Active Galactic Nuclei of Uncertain type

Despite the large number of discoveries made recently by Fermi, the origins of the so called unidentied -ray sources remain unknown. The large number of these sources suggests that among them there could be a population that signicantly contributes to the isotropic gamma-ray background and is therefore crucial to understand their nature. The rst step toward a complete comprehension of the unidentied -ray source population is to identify those that can be associated with blazars, the most numerous class of extragalactic sources in the -ray sky. Recently, we discovered that blazars can be recognized and separated from other extragalactic sources using the infrared (IR) WISE satellite colors. The blazar population delineates a remarkable and distinctive region of the IR color-color space, the WISE blazar strip. In particular, the subregion delineated by the -ray emitting blazars is even narrower and we named it as the WISE Gamma-ray Strip (WGS). In this paper we parametrize the WGS on the basis of a single parameters that we then use to determine if -ray Active Galactic Nuclei of the uncertain type (AGUs) detected by Fermi are consistent with the WGS and so can be considered blazar candidates. We nd that 54 AGUs out of a set 60 analyzed have IR colors consistent with the WGS; only 6 AGUs are outliers. This result implies that a very high percentage (i.e., in this sample about 90%) of the AGUs detected by Fermi are indeed blazar candidates.

UNVEILING THE NATURE OF UNIDENTIFIED GAMMA-RAY SOURCES. I. A NEW METHOD FOR THE ASSOCIATION OF GAMMA-RAY BLAZARS

We present a new method for identifying blazar candidates by examining the locus, i.e., the region occupied by the Fermi γ-ray blazars in the three-dimensional color space defined by the WISE infrared colors. This method is a refinement of our previous approach that made use of the two-dimensional projection of the distribution of WISE γ-ray-emitting blazars (the Strip) in the three WISE color-color planes. In this paper, we define the three-dimensional locus by means of a principal component analysis of the color distribution of a large sample of blazars composed of all the ROMA-BZCAT sources with counterparts in the WISE All-Sky Catalog associated with γ-ray sources in the second Fermi-LAT catalog (2FGL; the WISE Fermi blazars sample, WFB). Our new procedure yields a total completeness of c tot ~ 81% and a total efficiency of e tot ~ 97%. We also obtain local estimates of the efficiency and completeness as functions of the WISE colors and galactic coordinates of the candidate blazars. The catalog of all WISE candidate blazars associated with the WFB sample is also presented, complemented by archival multi-frequency information for the alternative associations. Finally, we apply the new association procedure to all γ-ray blazars in the 2FGL and provide a catalog containing all the γ-ray candidate blazars selected according to our procedure.

UNVEILING THE NATURE OF UNIDENTIFIED GAMMA-RAY SOURCES. II. RADIO, INFRARED, AND OPTICAL COUNTERPARTS OF THE GAMMA-RAY BLAZAR CANDIDATES

F. Massaro SLAC National Laboratory and Kavli Institute for Particle Astrophysics and Cosmology, 2575 Sand Hill Road, Menlo Park, CA 94025, USA R. D’Abrusco, A. Paggi, H. A. Smith Harvard Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA N. Masetti INAF Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129, Bologna, Italy M. Giroletti INAF Istituto di Radioastronomia, via Gobetti 101, 40129, Bologna, Italy G. Tosti Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, ItalyA significant fraction (~30%) of the high-energy gamma-ray sources listed in the second Fermi Large Area Telescope catalog (2FGL) are still of unknown origin, being not yet associated with counterparts at low energies. We recently developed a new association method to identify if there is a ?-ray blazar candidate within the positional uncertainty region of a generic 2FGL source. This method is entirely based on the discovery that blazars have distinct infrared colors with respect to other extragalactic sources found, thanks to the Wide-field Infrared Survey Explorer (WISE) all-sky observations. Several improvements have also been performed to increase the efficiency of our method in recognizing ?-ray blazar candidates. In this paper we applied our method to two different samples, the first constituted by unidentified ?-ray sources (UGSs), and the second by active galaxies of uncertain type, both listed in the 2FGL. We present a catalog of IR counterparts for ~20% of the UGSs investigated. Then, we also compare our results for the associated sources with those present in the literature. In addition, we illustrate the extensive archival research carried out to identify the radio, infrared, optical, and X-ray counterparts of the WISE-selected, ?-ray blazar candidates. Finally, we discuss the future developments of our method based on ground-based follow-up observations.

Unveiling the Nature of the Unidentified Gamma-Ray Sources. IV. The Swift Catalog of Potential X-Ray Counterparts

A significant fraction (~30%) of the high-energy γ-ray sources listed in the second Fermi Large Area Telescope (LAT) catalog are still of unknown origin, having not yet been associated with counterparts at lower energies. To investigate the nature of these enigmatic sources, we present an extensive search of X-ray sources lying in the positional uncertainty region of a selected sample of these unidentified gamma-ray sources (UGSs) that makes use of all available observations performed by the Swift X-ray Telescope before 2013 March 31, available for 205 UGSs. To detect the fainter sources, we merged all the observations covering the Fermi LAT positional uncertainty region at a 95% level of confidence of each UGS. This yields a catalog of 357 X-ray sources, finding candidate X-ray counterparts for ~70% of the selected sample. In particular, 25% of the UGSs feature a single X-ray source within their positional uncertainty region, while 45% have multiple X-ray sources. For each X-ray source, we also looked in the corresponding Swift UVOT merged images for optical and ultraviolet counterparts, also performing source photometry. We found ultraviolet-optical correspondences for ~70% of the X-ray sources. We searched several major radio, infrared, optical, and ultraviolet surveys for possible counterparts within the positional error of the sources in the X-ray catalog to obtain additional information on their nature. Applying the kernel density estimation technique to infrared colors of Wide-Field Infrared Survey Explorer counterparts of our X-ray sources we select six γ-ray blazar candidates. In addition, comparing our results with previous analyses, we select 11 additional γ-ray blazar candidates.

UNVEILING THE NATURE OF THE UNIDENTIFIED GAMMA-RAY SOURCES V: ANALYSIS OF THE RADIO CANDIDATES WITH THE KERNEL DENSITY ESTIMATION

(WISE) and then we analyze their IR colors in comparison with those of the known -ray blazars. 14 We propose a new approach, based on a 2-dimensional kernel density estimation (KDE) technique in 15

A Near-Infrared Study of NGC 7538 IRS 1, 2, and 3

?????We present images of the cluster of embedded young stars designated NGC 7538 IRS 1, 2, and 3 in the J (1.25 ?m), H (1.64 ?m), and K (2.16 ?m) broad bands; the 1.64 ?m line of [Fe II]; the 2.12 ?m and 2.25 ?m lines of H2; the 2.17 ?m Brackett ? H-recombination line; and the 3.29 ?m unidentified dust feature. The NGC 7538 IRS 2 compact H II region, as seen in Brackett ? line emission, possesses a cometary morphology. We find evidence for a shell-like distribution of [Fe II] line emission along the periphery of the compact H II region. This morphology and the observed intensity ratio I([Fe II] 1.64 ?m)/I(Br?) imply that the [Fe II] line emission emanates from shocked stellar wind material. We observe arcs of H2 line emission immediately exterior to the [Fe II] line emission shell and suggest that these arcs delineate molecular cloud material shocked via collision with the stellar wind. We observe larger arcs with centers of curvature displaced from each other and IRS 2, in broad-band-continuum, H2-line, and 3.29 ?m feature emission, and suggest that these arcs represent molecular gas (either shocked or UV-excited) and dust swept up during distinct episodes of stellar mass ejection. We submit that the morphological and energetic evidence favors a stellar wind bow shock interpretation, in which the motion of the central O star with respect to the surrounding molecular cloud, in conjunction with the stars high-velocity stellar wind and episodic mass ejection, gives rise to the observed general cometary morphology and circum-H II region emission.

Refining the Associations of the Fermi Large Area Telescope Source Catalogs

The Fermi-Large Area Telescope (LAT) First Source Catalog (1FGL) was released in 2010 February and the Fermi-LAT 2-Year Source Catalog (2FGL) appeared in 2012 April, based on data from 24 months of operation. Since they were released, many follow up observations of unidentified γ-ray sources have been performed and new procedures for associating γ-ray sources with potential counterparts at other wavelengths have been developed. Here we review and characterize all of the associations as published in the 1FGL and 2FGL catalogs on the basis of multifrequency archival observations. In particular, we located 177 spectra for the low-energy counterparts that were not listed in the previous Fermi catalogs, and in addition we present new spectroscopic observations of eight γ-ray blazar candidates. Based on our investigations, we introduce a new counterpart category of “candidate associations” and propose a refined classification for the candidate low-energy counterparts of the Fermi sources. We compare the 1FGL-assigned counterparts with those listed in 2FGL to determine which unassociated sources became associated in later releases of the Fermi catalogs. We also search for potential counterparts to all of the remaining unassociated Fermi sources. Finally, we prepare a refined and merged list of all of the associations of 1FGL plus 2FGL that includes 2219 unique Fermi objects. This is the most comprehensive and systematic study of all the associations collected for the γ-ray sources available to date. We conclude that 80% of the Fermi sources have at least one known plausible γ-ray emitter within their positional uncertainty regions.

Large Proper-Motion Infrared [Fe II] Emission-Line Features in GGD 37

We report the detection of proper motions in [Fe II] 1.644 μm emission among the southeasternmost features in the Herbig-Haro object complex GGD 37. Imaging observations were made over a period of 5 yr, at epochs 1993.67, 1996.71, and 1998.68, that reveal proper-motion velocities of 400 ± 200 km s-1 at GGD 37:W2 and 850 ± 200 km s-1 at a newly identified object, designated GGD 37:RWPF 1. We believe the proper-motion velocity of GGD 37:RWPF 1 is the highest found to date for a Herbig-Haro object. The direction of GGD 37:RWPF 1s proper motion is almost due west, away from the nearby young stellar object complex in Cep A East. GGD 37:W2s direction of proper motion is north/northwest, which does not lead back to any of the compact objects in Cep A East.