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Acciari V.A.,Harvard - Smithsonian Center for Astrophysics | Aliu E.,Barnard College | Beilicke M.,Washington University in St. Louis | Benbow W.,Harvard - Smithsonian Center for Astrophysics | And 66 more authors.
Astrophysical Journal Letters | Year: 2010

We present results from an intensive VERITAS monitoring campaign of the high-frequency peaked BL Lac object 1ES 1218+304 in 2008/2009. Although 1ES 1218+304 was detected previously by MAGIC and VERITAS at a persistent level of ∼ 6% of the Crab Nebula flux, the new VERITAS data reveal a prominent flare reaching ∼ 20% of the Crab. While very high energy (VHE) flares are quite common in many nearby blazars, the case of 1ES 1218+304 (redshift z = 0.182) is particularly interesting since it belongs to a group of blazars that exhibit unusually hard VHE spectra considering their redshifts. When correcting the measured spectra for absorption by the extragalactic background light, 1ES 1218+304 and a number of other blazars are found to have differential photon indices Γ≤ 1.5. The difficulty in modeling these hard spectral energy distributions in blazar jets has led to a range of theoretical γ-ray emission scenarios, one of which is strongly constrained by these new VERITAS observations. We consider the implications of the observed light curve of 1ES 1218+304, which shows day scale flux variations, for shock acceleration scenarios in relativistic jets, and in particular for the viability of kiloparsec-scale jet emission scenarios. © 2010. The American Astronomical Society. All rights reserved.

Arlen T.,University of California at Los Angeles | Aune T.,University of California at Santa Cruz | Beilicke M.,Washington University in St. Louis | Benbow W.,Harvard - Smithsonian Center for Astrophysics | And 85 more authors.
Astrophysical Journal | Year: 2012

Observations of radio halos and relics in galaxy clusters indicate efficient electron acceleration. Protons should likewise be accelerated and, on account of weak energy losses, can accumulate, suggesting that clusters may also be sources of very high energy (VHE; E > 100GeV) gamma-ray emission. We report here on VHE gamma-ray observations of the Coma galaxy cluster with the VERITAS array of imaging Cerenkov telescopes, with complementing Fermi Large Area Telescope observations atGeV energies. No significant gamma-ray emission from the Coma Cluster was detected. Integral flux upper limits at the 99% confidence level were measured to be on the order of (2-5) × 10 -8 photons m -2 s -1 (VERITAS, >220 GeV) and 2 × 10-6 photons m -2 s -1 (Fermi, 1-3 GeV), respectively. We use the gamma-ray upper limits to constrain cosmic rays (CRs) and magnetic fields in Coma. Using an analytical approach, the CR-to-thermal pressure ratio is constrained to be <16% from VERITAS data and <1.7% from Fermi data (averaged within the virial radius). These upper limits are starting to constrain the CR physics in self-consistent cosmological cluster simulations and cap the maximum CR acceleration efficiency at structure formation shocks to be <50%. Alternatively, this may argue for non-negligible CR transport processes such as CR streaming and diffusion into the outer cluster regions. Assuming that the radio-emitting electrons of the Coma halo result from hadronic CR interactions, the observations imply a lower limit on the central magnetic field in Coma of (2-5.5) μG, depending on the radial magnetic field profile and on the gamma-ray spectral index. Since these values are below those inferred by Faraday rotation measurements in Coma (for most of the parameter space), this renders the hadronic model a very plausible explanation of the Coma radio halo. Finally, since galaxy clusters are dark matter (DM) dominated, the VERITAS upper limits have been used to place constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the DM particles, 〈σv〉. © 2012. The American Astronomical Society. All rights reserved.

Yusef-Zadeh F.,Northwestern University | Wardle M.,Macquarie University | Miller-Jones J.C.A.,NRAO | Miller-Jones J.C.A.,Curtin University Australia | And 3 more authors.
Astrophysical Journal | Year: 2011

SgrA* exhibits flares in radio, millimeter, and submillimeter wavelengths with durations of ∼1hr. Using structure function, power spectrum, and autocorrelation function analysis, we investigate the variability of SgrA* on timescales ranging from a few seconds to several hours and find evidence for subminute timescale variability at radio wavelengths. These measurements suggest a strong case for continuous variability from subminute to hourly timescales. This short timescale variability constrains the size of the emitting region to be less than 0.1AU. Assuming that the minute timescale fluctuations of the emission at 7mm arise through the expansion of regions of optically thick synchrotron-emitting plasma, this suggests the presence of explosive, energetic expansion events at speeds close to c. The required rates of mass processing and energy loss of this component are estimated to be ≳6 × 10-10 M⊙yr-1 and 400 L ⊙, respectively. The inferred scale length corresponding to 1 minute light travel time is comparable to the time-averaged spatially resolved 0.1AU scale observed at 1.3mm emission of SgrA*. This steady component from SgrA* is interpreted mainly as an ensemble average of numerous weak and overlapping flares that are detected on short timescales. The nature of such short timescale variable emission or quiescent variability is not understood but could result from fluctuations in the accretion flow of SgrA* that feed the base of an outflow or jet. © 2011. The American Astronomical Society. All rights reserved.

Acciari V.A.,Harvard - Smithsonian Center for Astrophysics | Arlen T.,University of California at Los Angeles | Aune T.,University of California at Santa Cruz | Beilicke M.,Washington University in St. Louis | And 74 more authors.
Astrophysical Journal | Year: 2010

Indirect dark matter searches with ground-based gamma-ray observatories provide an alternative for identifying the particle nature of dark matter that is complementary to that of direct search or accelerator production experiments. We present the results of observations of the dwarf spheroidal galaxies Draco, Ursa Minor, Bootes 1, and Willman 1 conducted by the Very Energetic Radiation Imaging Telescope Array System (VERITAS). These galaxies are nearby dark matter dominated objects located at a typical distance of several tens of kiloparsecs for which there are good measurements of the dark matter density profile from stellar velocity measurements. Since the conventional astrophysical background of very high energy gamma rays from these objects appears to be negligible, they are good targets to search for the secondary gamma-ray photons produced by interacting or decaying dark matter particles. No significant gamma-ray flux above 200 GeV was detected from these four dwarf galaxies for a typical exposure of ∼20 hr. The 95% confidence upper limits on the integral gamma-ray flux are in the range (0.4-2.2) x 10-12 photons cm-2 s -1. We interpret this limiting flux in the context of pair annihilation of weakly interacting massive particles (WIMPs) and derive constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the WIMPs (σv) ≲ 10 -23 cm3 s-1 for m? ≳ 300 GeV c-2). This limit is obtained under conservative assumptions regarding the dark matter distribution in dwarf galaxies and is approximately 3 orders of magnitude above the generic theoretical prediction for WIMPs in the minimal supersymmetric standard model framework. However, significant uncertainty exists in the dark matter distribution as well as the neutralino cross sections which under favorable assumptions could further lower this limit. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

Yusef-Zadeh F.,Northwestern University | Yusef-Zadeh F.,NASA | Lacy J.H.,University of Texas at Austin | Lacy J.H.,NASA | And 5 more authors.
Astrophysical Journal | Year: 2010

A group of four compact HII regions associated with the well-known 50 km s-1 molecular cloud is the closest site of on-going star formation to the dynamical center of the Galaxy, at a projected distance of ∼6 pc. We present a study of ionized gas based on the [Ne ii] (12.8 μm) line, as well as multi-frequency radio continuum, Hubble Space Telescope Paα, and Spitzer Infrared Array Camera observations of the most compact member of the HII group, Sgr A East HII D. The radio continuum image at 6 cm shows that this source breaks up into two equally bright ionized features, D1 and D2. The spectral energy distribution of the D source is consistent with it being due to a 25±3M⊙ star with a luminosity of 8±3×104 L⊙. The inferred mass, effective temperature of the UV source, and the ionization rate are compatible with a young O9-B0 star. The ionized features D1 and D2 are considered to be ionized by UV radiation collimated by an accretion disk.We consider that the central massive star photoevaporates its circumstellar disk on a timescale of 3×104 years giving a mass flux ∼3 ±10-5 M⊙ yr-1 and producing the ionized material in D1 and D2 expanding in an inhomogeneous medium. The ionized gas kinematics, as traced by the [Ne ii] emission, is difficult to interpret, but it could be explained by the interaction of a bipolar jet with surrounding gas along with what appears to be a conical wall of lower velocity gas. The other HII regions, Sgr A East A-C, have morphologies and kinematics that more closely resemble cometary flows seen in other compact HII regions, where gas moves along a paraboloidal surface formed by the interaction of a stellar wind with a molecular cloud.

Notis M.,Lehigh University | Newbury B.,ExxonMobil | Stephenson B.,Adler Planetarium and Astronomy Museum | Stephenson G.B.,Argonne National Laboratory
Applied Physics A: Materials Science and Processing | Year: 2013

The astrolabe is an ancient analogue astronomical computing device used for calculations relating to position and time of the observer's location. In its most common form (the planispheric astrolabe), it consists of an engraved plate or series of plates held together and pinned in a housing, the assembly usually being made of brass. The present study describes the use of X-ray diffraction (XRD) and X-ray fluorescence (XRF) in a synchrotron to elucidate the composition of, and fabrication techniques used for, the major component parts of the astrolabe. The synchrotron XRF studies are compared to similar studies made with a handheld XRF instrument and the advantages and disadvantages of both approaches are discussed. © 2012 Springer-Verlag Berlin Heidelberg.

Masters K.L.,University of Portsmouth | Nichol R.C.,University of Portsmouth | Hoyle B.,University of Portsmouth | Hoyle B.,University of Barcelona | And 9 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present first results from Galaxy Zoo 2, the second phase of the highly successful Galaxy Zoo project. Using a volume-limited sample of 13665 disc galaxies (0.01 < z < 0.06 and Mr < -19.38), we study the fraction of galaxies with bars as a function of global galaxy properties like colour, luminosity and bulge prominence. Overall, 29.4 ± 0.5 per cent of galaxies in our sample have a bar, in excellent agreement with previous visually classified samples of galaxies (although this overall fraction is lower than that measured by automated bar-finding methods). We see a clear increase in the bar fraction with redder (g-r) colours, decreased luminosity and in galaxies with more prominent bulges, to the extent that over half of the red, bulge-dominated disc galaxies in our sample possess a bar. We see evidence for a colour bimodality for our sample of disc galaxies, with a 'red sequence' that is both bulge and bar dominated, and a 'blue cloud' which has little, or no, evidence for a (classical) bulge or bar. These results are consistent with similar trends for barred galaxies seen recently both locally and at higher redshift, and with early studies using the RC3. We discuss these results in the context of internal (secular) galaxy evolution scenarios and the possible links to the formation of bars and bulges in disc galaxies. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

Yusef-Zadeh F.,Northwestern University | Wardle M.,Macquarie University | Bushouse H.,STS Inc | Dowell C.D.,Jet Propulsion Laboratory | Roberts D.A.,Adler Planetarium and Astronomy Museum
Astrophysical Journal Letters | Year: 2010

We have investigated the nature of flare emission from Sgr A* during multi-wavelength observations of this source that took place in 2004, 2005, and 2006. We present evidence for dimming of submillimeter and radio flux during the peak of near-IR flares. This suggests that the variability of Sgr A* across its wavelength spectrum is phenomenologically related. The model explaining this new behavior of flare activity could be consistent with adiabatically cooling plasma blobs that are expanding but also partially eclipsing the background quiescent emission from Sgr A*.When a flare is launched, the plasma blob is most compact and is brightest in the optically thin regime whereas the emission in radio/submillimeter wavelengths has a higher opacity. Absorption in the observed light curve of Sgr A* at radio/submillimeter flux is due to the combined effects of lower brightness temperature of plasma blobs with respect to the quiescent brightness temperature and high opacity of plasma blobs. This implies that plasma blobs are mainly placed in the magnetosphere of a disk-like flow or further out in the flow. The depth of the absorption being larger in submillimeter than in radio wavelengths implies that the intrinsic size of the quiescent emission increases with increasing wavelength which is consistent with previous size measurements of Sgr A*. Lastly, we believe that occultation of the quiescent emission of Sgr A* at radio/submillimeter by IR flares can be used as a powerful tool to identify flare activity at its earliest phase of its evolution. © 2010. The American Astronomical Society. All rights reserved.

O'Malia K.K.J.,University of Colorado at Boulder | Thorburn J.A.,Carthage College | Hammergren M.,Adler Planetarium and Astronomy Museum | Snow T.P.,University of Colorado at Boulder | And 3 more authors.
Astrophysical Journal | Year: 2010

We present the search for both diffuse interstellar bands (DIBs) and molecules in Comet 17P (Holmes) and Comet C/2007 W1 (Boattini) occultation observations. Absorption spectra were taken during stellar occultations by Comet Holmes of 31 and β Persei, and the occultation of BD+22 216 by Comet Boattini. While no signature of the comets was detected, we present upper limits for some common cometary molecules such as C2, C3, CH, CN and for the most common DIBs. We did not detect either comet in absorption, most likely because of the large distance between the line of sight to the star and the nucleus of the comet. Interstellar sight lines with comparable reddening to what was measured in Comet Holmes have DIB equivalent widths between 5 and 50 mÅ. However, future observations with closer approaches to a background star have great potential for spatially mapping molecule distributions in comets, and in discovering DIBs, if they are present, in comets. Future observations could detect DIBs and molecules if they are done: (1) less than ∼104-103 km from the nucleus (2) with a signal to noise in the background star of ∼300 and (3) with a resolving power of at least 38,000. © 2010. The American Astronomical Society. All rights reserved.

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