Publications by Garret Cotter


Infrared spectroscopy of nearby radio active elliptical galaxies

Astrophysical Journal, Supplement Series 203 (2012)

J Mould, T Reynolds, T Readhead, D Floyd, B Jannuzi, G Cotter, L Ferrarese, K Matthews, D Atlee, M Brown

In preparation for a study of their circumnuclear gas we have surveyed 60% of a complete sample of elliptical galaxies within 75 Mpc that are radio sources. Some 20% of our nuclear spectra have infrared emission lines, mostly Paschen lines, Brackett γ, and [Fe II]. We consider the influence of radio power and black hole mass in relation to the spectra. Access to the spectra is provided here as a community resource. © 2012. The American Astronomical Society. All rights reserved..


The first billion years: report of a study program

(2012)

J Bowman, S Furlanetto, D Jones, A Readhead, Y Ali-Hamimoud, J Bock, G Bower, M Bradford, C Carilli, T Chang, R Chary, J Chluba, S Church, G Cotter, A Cooray, D DeBoer, A Oliveira-Costa, R Dean, O Dore, T Gaier, K Grainge, J Gunderson, C Hirata, ME Jones, G Keating, C Lawrence, L Levenson, J lazio, P Lubin, T Pearson, J Pritchard, A Pullen, S Rawlings, D Reichers, L Samoska, M Seiffert, AC Taylor


Spectroscopy of broad-line blazars from 1LAC

Astrophysical Journal 748 (2012)

MS Shaw, RW Romani, G Cotter, SE Healey, PF Michelson, ACS Readhead, JL Richards, W Max-Moerbeck, OG King, WJ Potter

We report on optical spectroscopy of 165 flat spectrum radio quasars (FSRQs) in the Fermi 1LAC sample, which have helped allow a nearly complete study of this population. Fermi FSRQs show significant evidence for non-thermal emission even in the optical; the degree depends on the γ-ray hardness. They also have smaller virial estimates of hole mass than the optical quasar sample. This appears to be largely due to a preferred (axial) view of the γ-ray FSRQ and non-isotropic (H/R ∼ 0.4) distribution of broad-line velocities. Even after correction for this bias, the Fermi FSRQs show higher mean Eddington ratios than the optical population. A comparison of optical spectral properties with Owens Valley Radio Observatory radio flare activity shows no strong correlation. © 2012 The American Astronomical Society. All rights reserved.


Synchrotron and inverse-Compton emission from blazar jets - I. A uniform conical jet model

Monthly Notices of the Royal Astronomical Society (2012)

WJ Potter, G Cotter


Blazars in the Fermi era: The ovro 40 m telescope monitoring program

Astrophysical Journal, Supplement Series 194 (2011)

JL Richards, W Max-Moerbeck, V Pavlidou, OG King, TJ Pearson, ACS Readhead, R Reeves, MC Shepherd, MA Stevenson, LC Weintraub, L Fuhrmann, E Angelakis, J Anton Zensus, SE Healey, RW Romani, MS Shaw, K Grainge, M Birkinshaw, K Lancaster, DM Worrall, GB Taylor, G Cotter, R Bustos

The Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope provides an unprecedented opportunity to study gamma-ray blazars. To capitalize on this opportunity, beginning in late 2007, about a year before the start of LAT science operations, we began a large-scale, fast-cadence 15GHz radio monitoring program with the 40 m telescope at the Owens Valley Radio Observatory. This program began with the 1158 northern (δ > -20°) sources from the Candidate Gamma-ray Blazar Survey and now encompasses over 1500 sources, each observed twice per week with about 4mJy (minimum) and 3% (typical) uncertainty. Here, we describe this monitoring program and our methods, and present radio light curves from the first two years (2008 and 2009). As a first application, we combine these data with a novel measure of light curve variability amplitude, the intrinsic modulation index, through a likelihood analysis to examine the variability properties of subpopulations of our sample. We demonstrate that, with high significance (6σ), gamma-ray-loud blazars detected by the LAT during its first 11 months of operation vary with almost a factor of two greater amplitude than do the gamma-ray-quiet blazars in our sample. We also find a significant (3σ) difference between variability amplitude in BL Lacertae objects and flat-spectrum radio quasars (FSRQs), with the former exhibiting larger variability amplitudes. Finally, low-redshift (z < 1) FSRQs are found to vary more strongly than high-redshift FSRQs, with 3σ significance. These findings represent an important step toward understanding why some blazars emit gamma-rays while others, with apparently similar properties, remain silent. © 2011. The American Astronomical Society. All rights reserved..


Characteristics of Gamma-ray loud blazars in the VLBA Imaging and Polarimetry Survey

Astrophysical Journal 726 (2011)

JD Linford, GB Taylor, RW Romani, SE Healey, JF Helmboldt, ACS Readhead, R Reeves, JL Richards, G Cotter


Characteristics of Gamma-ray loud blazars in the VLBA Imaging and Polarimetry Survey

Astrophysical Journal Letters 726 (2011)

JD Linford, GB Taylor, RW Romani, SE Healey, JF Helmboldt, ACS Readhead, R Reeves, JL Richards, G Cotter

The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed as part of the VLBA Imaging and Polarimetry Survey. This large, flux-limited sample of active galactic nuclei (AGNs) provides insights into the mechanism that produces strong γ-ray emission. At lower flux levels, radio flux density does not directly correlate with γ-ray flux. We find that the LAT-detected BL Lac objects tend to be similar to the non-LAT BL Lac objects, but that the LAT-detected FSRQs are often significantly different from the non-LAT FSRQs. The differences between the γ-ray loud and quiet FSRQs can be explained by Doppler boosting; these objects appear to require larger Doppler factors than those of the BL Lac objects. It is possible that the γ-ray loud FSRQs are fundamentally different from the γ-ray quiet FSRQs. Strong polarization at the base of the jet appears to be a signature for γ-ray loud AGNs. © 2011. The American Astronomical Society. All rights reserved. Printedin the U.S.A.


Characteristics of Gamma-ray loud blazars in the VLBA Imaging and Polarimetry Survey

Astrophysical Journal 726 (2011)

JD Linford, GB Taylor, RW Romani, SE Healey, JF Helmboldt, ACS Readhead, R Reeves, JL Richards, G Cotter

The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed as part of the VLBA Imaging and Polarimetry Survey. This large, flux-limited sample of active galactic nuclei (AGNs) provides insights into the mechanism that produces strong γ-ray emission. At lower flux levels, radio flux density does not directly correlate with γ-ray flux. We find that the LAT-detected BL Lac objects tend to be similar to the non-LAT BL Lac objects, but that the LAT-detected FSRQs are often significantly different from the non-LAT FSRQs. The differences between the γ-ray loud and quiet FSRQs can be explained by Doppler boosting; these objects appear to require larger Doppler factors than those of the BL Lac objects. It is possible that the γ-ray loud FSRQs are fundamentally different from the γ-ray quiet FSRQs. Strong polarization at the base of the jet appears to be a signature for γ-ray loud AGNs. © 2011. The American Astronomical Society. All rights reserved. Printedin the U.S.A.


Design concepts for the Cherenkov Telescope Array CTA: An advanced facility for ground-based high-energy gamma-ray astronomy

Experimental Astronomy 32 (2011) 193-316

M Actis, G Agnetta, F Aharonian, A Akhperjanian, J Aleksić, E Aliu, D Allan, I Allekotte, F Antico, LA Antonelli, P Antoranz, A Aravantinos, T Arlen, H Arnaldi, S Artmann, K Asano, H Asorey, J Bähr, A Bais, C Baixeras, S Bajtlik, D Balis, A Bamba, C Barbier, M Barceló, A Barnacka, J Barnstedt, UB de Almeida, JA Barrio, S Basso, D Bastieri, C Bauer, J Becerra, Y Becherini, K Bechtol, J Becker, V Beckmann, W Bednarek, B Behera, M Beilicke, M Belluso, M Benallou, W Benbow, J Berdugo, K Berger, T Bernardino, K Bernlöhr, A Biland, S Billotta, T Bird, E Birsin, E Bissaldi, S Blake, O Blanch, AA Bobkov, L Bogacz, M Bogdan, C Boisson, J Boix, J Bolmont, G Bonanno, A Bonardi, T Bonev, J Borkowski, O Botner, A Bottani, M Bourgeat, C Boutonnet, A Bouvier, S Brau-Nogué, I Braun, T Bretz, MS Briggs, P Brun, L Brunetti, JH Buckley, V Bugaev, R Bühler, T Bulik, G Busetto, S Buson, K Byrum, M Cailles, R Cameron, R Canestrari, S Cantu, E Carmona, A Carosi, J Carr, PH Carton, M Casiraghi, H Castarede, O Catalano, S Cavazzani, S Cazaux, B Cerruti, M Cerruti, PM Chadwick, J Chiang, M Chikawa

Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA. © 2011 The Author(s).


The Balmer-dominated bow shock and wind nebula structure of γ-ray pulsar PSR J1741-2054

Astrophysical Journal 724 (2010) 908-914

RW Romani, MS Shaw, F Camilo, G Cotter, GR Sivakoff

We have detected an Hα bow shock nebula around PSR J1741-2054, a pulsar discovered through its GeV γ-ray pulsations. The pulsar is only ∼1'.5 behind the leading edge of the shock. Optical spectroscopy shows that the nebula is non-radiative, dominated by Balmer emission. The Hα images and spectra suggest that the pulsar wind momentum is equatorially concentrated and implies a pulsar space velocity ≈150kms-1, directed 15° ± 10° out of the plane of the sky. The complex Hα profile indicates that different portions of the post-shock flow dominate line emission as gas moves along the nebula and provide an opportunity to study the structure of this unusual slow non-radiative shock under a variety of conditions. CXO ACIS observations reveal an X-ray pulsar wind nebula within this nebula, with a compact ∼2.5 equatorial structure and a trail extending several arcminutes behind. Together these data support a close (≤0.5 kpc) distance, a spin geometry viewed edge-on, and highly efficient γ-ray production for this unusual, energetic pulsar. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.


The first catalog of active galactic nuclei detected by the Fermi large area telescope

Astrophysical Journal 715 (2010) 429-457

AA Abdo, M Ackermann, M Ajello, A Allafort, E Antolini, WB Atwood, M Axelsson, L Baldini, J Ballet, G Barbiellini, D Bastieri, BM Baughman, K Bechtol, R Bellazzini, B Berenji, RD Blandford, ED Bloom, JR Bogart, E Bonamente, AW Borgland, A Bouvier, J Bregeon, A Brez, M Brigida, P Bruel, R Buehler, TH Burnett, S Buson, GA Caliandro, RA Cameron, A Cannon, PA Caraveo, S Carrigan, JM Casandjian, E Cavazzuti, C Cecchi, O Çelik, A Celotti, E Charles, A Chekhtman, AW Chen, CC Cheung, J Chiang, S Ciprini, R Claus, J Cohen-Tanugi, J Conrad, L Costamante, G Cotter, S Cutini, V D'Elia, CD Dermer, A De Angelis, F De Palma, A De Rosa, SW Digel, E Do Couto E Silva, PS Drell, R Dubois, D Dumora, L Escande, C Farnier, C Favuzzi, SJ Fegan, EC Ferrara, WB Focke, P Fortin, M Frailis, Y Fukazawa, S Funk, P Fusco, F Gargano, D Gasparrini, N Gehrels, S Germani, B Giebels, N Giglietto, P Giommi, F Giordano, M Giroletti, T Glanzman, G Godfrey, P Grandi, IA Grenier, MH Grondin, JE Grove, S Guiriec, D Hadasch, AK Harding, M Hayashida, E Hays, SE Healey, AB Hill, D Horan, RE Hughes, G Iafrate, R Itoh, G Jóhannesson, AS Johnson, RP Johnson

We present the first catalog of active galactic nuclei (AGNs) detected by the Large Area Telescope (LAT), corresponding to 11 months of data collected in scientific operation mode. The First LAT AGN Catalog (1LAC) includes 671 γ-ray sources located at high Galactic latitudes (|b|>10°) that are detected with a test statistic greater than 25 and associated statistically with AGNs. Some LAT sources are associated with multiple AGNs, and consequently, the catalog includes 709 AGNs, comprising 300 BL Lacertae objects, 296 flat-spectrum radio quasars, 41 AGNs of other types, and 72 AGNs of unknown type. We also classify the blazars based on their spectral energy distributions as archival radio, optical, and X-ray data permit. In addition to the formal 1LAC sample, we provide AGN associations for 51 low-latitude LAT sources and AGN "affiliations" (unquantified counterpart candidates) for 104 high-latitude LAT sources without AGN associations. The overlap of the 1LAC with existing γ-ray AGN catalogs (LBAS, EGRET, AGILE, Swift, INTEGRAL, TeVCat) is briefly discussed. Various properties - such as γ-ray fluxes and photon power-law spectral indices, redshifts, γ-ray luminosities, variability, and archival radio luminosities - and their correlations are presented and discussed for the different blazar classes. We compare the 1LAC results with predictions regarding the γ-ray AGN populations, and we comment on the power of the sample to address the question of the blazar sequence. © 2010. The American Astronomical Society. All rights reserved..


The extraordinary radio galaxy MRC B1221-423: Probing deeper at radio and optical wavelengths

Monthly Notices of the Royal Astronomical Society 407 (2010) 721-733

HM Johnston, JW Broderick, G Cotter, R Morganti, RW Hunstead

We present optical spectra and high-resolution multiwavelength radio observations of the compact steep-spectrum radio source MRC B1221-423 (z = 0.1706). MRC B1221-423 is a very young (∼105 yr), powerful radio source which is undergoing a tidal interaction with a companion galaxy. We find strong evidence of interaction between the active galactic nucleus (AGN) and its environment. The radio morphology is highly distorted, showing a dramatic interaction between the radio jet and the host galaxy, with the jet being turned almost back on itself. H i observations show strong absorption against the nucleus at an infall velocity of ∼250 km s-1 compared to the stellar velocity, as well as a second, broader component which may represent gas falling into the nucleus. Optical spectra show that star formation is taking place across the whole system. Broad optical emission lines in the nucleus show evidence of outflow. Our observations confirm that MRC B1221-423 is a young radio source in a gas-rich nuclear environment, and that there was a time delay of a few times 100 Myr between the onset of star formation and the triggering of the AGN. © 2010 The Authors. Journal compilation © 2010 RAS.


Optical spectroscopy of bright fermi lat blazars

Astrophysical Journal 704 (2009) 477-484

MS Shaw, RW Romani, SE Healey, G Cotter, PF Michelson, ACS Readhead

We report on Hobby-Eberly Telescope and Palomar 5 m spectroscopy of recently identified γ-ray blazars in the Fermi LAT Bright Source List. These data provide identifications for 10 newly discovered γ-ray flat spectrum radio quasars and six new BL Lacertae (BL Lac) objects plus improved spectroscopy for six additional BL Lac objects. We substantially improve the identification completeness of the bright LAT blazars and give new redshifts and z constraints, new estimates of the black hole masses, and new measurements of the optical spectral energy distribution. © 2009 The American Astronomical Society. All rights reserved.


CGRaBS: An all-sky survey of gamma-ray blazar candidates

Astrophysical Journal, Supplement Series 175 (2008) 97-104

SE Healey, RW Romani, G Cotter, PF Michelson, EF Schlafly, ACS Readhead, P Giommi, S Chaty, IA Grenier, LC Weintraub

We describe a uniform all-sky survey of bright blazars, selected primarily by their flat radio spectra, that is designed to provide a large catalog of likely γ-ray active galactic nuclei (AGNs). The defined sample has 1625 targets with radio and X-ray properties similar to those of the EGRET blazars, spread uniformly across the |b| > 10° sky. We also report progress toward optical characterization of the sample; of objects with known R < 23, 85% have been classified and 81% have measured redshifts. One goal of this program is to focus attention on the most interesting (e.g., high-redshift, high-luminosity,...) sources for intensive multiwavelength study during the observations by the Large Area Telescope (LAT) on GLAST. © 2008. The American Astronomical Society. All rights reserved.


Characteristics of EGRET blazars in the VLBA Imaging and Polarimetry Survey (VIPS)

Astrophysical Journal 671 (2007) 1355-1364

GB Taylor, SE Healey, JF Helmboldt, S Tremblay, CD Fassnacht, RC Walker, LO Sjouwerman, TJ Pearson, ACS Readhead, L Weintraub, N Gehrels, RW Romani, PF Michelson, RD Blandford, G Cotter

We examine the radio properties of EGRET-detected blazars observed as part of the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly an order of magnitude below the MOJAVE survey and most other samples that have been used to study the properties of EGRET blazars. At lower flux levels, radio flux density does not directly correlate with gamma-ray flux density. We do find that the EGRET-detected blazars tend to have higher brightness temperatures, greater core fractions, and possibly larger than average jet opening angles. A weak correlation is also found with jet length and with polarization. All of the well-established trends can be explained by systematically larger Doppler factors in the gamma-ray-loud blazars, consistent with the measurements of higher apparent velocities found in monitoring programs carried out at radio frequencies above 10 GHz. © 2007. The American Astronomical Society. All rights reserved.


The VLBA imaging and polarimetry survey at 5 GHz

Astrophysical Journal 658 (2007) 203-216

JF Helmboldt, GB Taylor, S Tremblay, CD Fassnacht, RC Walker, ST Myers, LO Sjouwerman, TJ Pearson, ACS Readhead, L Weintraub, N Gehrels, RW Romani, S Healey, PF Michelson, RD Blandford, G Cotter

We present the first results of the VLBA Imaging and Polarimetry Survey (VIPS), a 5 GHz VLBI survey of 1127 sources with flat radio spectra. Through automated data reduction and imaging routines, we have produced publicly available I, Q, and U images and have detected polarized flux density from 37% of the sources. We have also developed an algorithm to use each source's I image to automatically classify it as a pointlike source, a core jet, a compact symmetric object (CSO) candidate, or a complex source. Using data from the Sloan Digital Sky Survey (SDSS), we have found no significant trend between optical flux and 5 GHz flux density for any of the source categories. Using the velocity width of the Hβ emission line and the monochromatic luminosity at 5100 Å to estimate the central black hole mass, MBH, we have found a weak trend between MBH and 5 GHz luminosity density for objects with SDSS spectra. The mean ratio of the polarized to total 5 GHz flux density for VIPS sources with detected polarized flux density ranges from 1% to 20% with a median value of about 5%. We have also found significant evidence that the directions of the jets in core-jet systems tend to be perpendicular to the electric vector position angles (EVPAs). The data are consistent with a scenario in which ∼24% of the polarized core jets have EVPAs that are antialigned with the directions of their jet components and that have a substantial amount of Faraday rotation. In addition to these initial results, plans for future follow-up observations are discussed. © 2007. The American Astronomical Society. All rights reserved.


Spectroscopic follow-up of a cluster candidate at z=1.45

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 382 (2007) 971-984

C van Breukelen, G Cotter, S Rawlings, T Readhead, D Bonfield, L Clewley, R Ivison, M Jarvis, C Simpson, M Watson


The discovery of a massive supercluster at z = 0.9 in the UKIDSS deep eXtragalactic survey

Monthly Notices of the Royal Astronomical Society 379 (2007) 1343-1351

AM Swinbank, AC Edge, I Smail, JP Stott, M Bremer, Y Sato, C Van Breukelen, M Jarvis, I Waddington, L Clewley, J Bergeron, G Cotter, S Dye, JE Geach, E Gonzalez-Solares, P Hirst, RJ Ivison, S Rawlings, C Simpson, GP Smith, A Verma, T Yamada

We analyse the first publicly released deep field of the UK Infrared Deep Sky Survey (UKIDSS) Deep eXtragalactic Survey to identify candidate galaxy overdensities at z ∼ 1 across ∼1 deg2 in the ELAIS-N1 field. Using I - K, J - K and K - 3.6 μm colours, we identify and spectroscopically follow up five candidate structures with Gemini/Gemini Multi-Object Spectrograph and confirm that they are all true overdensities with between five and 19 members each. Surprisingly, all five structures lie in a narrow redshift range at z = 0.89 ± 0.01, although they are spread across 30 Mpc on the sky. We also find a more distant overdensity at z = 1.09 in one of the spectroscopic survey regions. These five overdense regions lying in a narrow redshift range indicate the presence of a supercluster in this field and by comparing with mock cluster catalogues from N-body simulations we discuss the likely properties of this structure. Overall, we show that the properties of this supercluster are similar to the well-studied Shapley and Hercules superclusters at lower redshift. © 2007 RAS.


5-GHz MERLIN and VLBA observations of compact 9C sources

Monthly Notices of the Royal Astronomical Society 367 (2006) 323-330

RC Bolton, CJ Chandler, G Cotter, TJ Pearson, GG Pooley, ACS Readhead, JM Riley, EM Waldram

ABSTRACT In this paper, we present subarcsecond resolution observations of 36 compact sources from the 15 h region of the 15-GHz 9th Cambridge survey. These sources all have previously measured simultaneous continuum radio spectra spanning 1.4-43 GHz and we classify each source by fitting a quadratic function to its spectrum. Using the Multi-Element Radio-Linked Interferometer Network and the Very Long Baseline Array, both at 5 GHz, we resolve all six steep-spectrum objects and four of the 13 flat-spectrum objects. However, none of the 16 objects with convex spectra peaking above 2.5 GHz is resolved even at <3-mas resolution. These results, in combination with the findings of a 15-GHz variability study, suggest that emission from the high-frequency peaking objects is affected by relativistic beaming, and that these objects are not necessarily as young as the synchrotron self-absorption interpretation of their peak frequencies would imply. © 2006 RAS.


15-GHz variability of 9C sources

Monthly Notices of the Royal Astronomical Society 370 (2006) 1556-1564

RC Bolton, CJ Chandler, G Cotter, TJ Pearson, GG Pooley, ACS Readhead, JM Riley, EM Waldram

We present results from a 3-yr study of the 15-GHz variability of 51 9C sources. 48 of these sources make up a subsample of a larger one complete to 25 mJy in 9C, and as the sources are selected pseudo-randomly the results should be representative of the complete sample. 29 per cent of this subsample are found to be variable above the flux calibration uncertainties of ∼6 per cent. 50 per cent of the flat-spectrum objects are variable whilst none of the steep-spectrum objects or the objects with convex spectra peaking below 5 GHz are variable. Nine of the objects studied have convex spectra and peak frequencies above 5 GHz; eight of these were found to vary at 15 GHz, suggesting that the high-frequency peaking class in this sample is largely populated by objects with jets aligned close to the line of sight whose emission is dominated by beamed components. © 2006 RAS.

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