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Balser D.S.,U.S. National Radio Astronomy Observatory | Roshi D.A.,U.S. National Radio Astronomy Observatory | Jeyakumar S.,University of Guanajuato | Bania T.M.,Boston University | And 3 more authors.
Astrophysical Journal | Year: 2016

We measure carbon radio recombination line (RRL) emission at 5.3 GHz toward four H II regions with the Green Bank Telescope to determine the magnetic field strength in the photodissociation region (PDR) that surrounds the ionized gas. Roshi suggests that the non-thermal line widths of carbon RRLs from PDRs are predominantly due to magneto-hydrodynamic waves, thus allowing the magnetic field strength to be derived. We model the PDR with a simple geometry and perform the non-LTE radiative transfer of the carbon RRL emission to solve for the PDR physical properties. Using the PDR mass density from these models and the carbon RRL non-thermal line width we estimate total magnetic field strengths of B ∼ 100300 μG in W3 and NGC 6334A. Our results for W49 and NGC 6334D are less well constrained with total magnetic field strengths between B ∼ 2001000 μG. H I and OH Zeeman measurements of the line of sight magnetic field strength (Blos), taken from the literature, are between a factor of ∼0.51 of the lower bound of our carbon RRL magnetic field strength estimates. Since ?Blos ? ≤ B, our results are consistent with the magnetic origin of the non-thermal component of carbon RRL widths. © 2016. The American Astronomical Society. All rights reserved. Source

Wehrle A.E.,Space Science Institute | Grupe D.,Morehead State University | Grupe D.,Swift Mission Operation Center | Jorstad S.G.,Boston University | And 9 more authors.
Astrophysical Journal | Year: 2016

BL Lac, the eponymous blazar, flared to historically high levels at millimeter, infrared, X-ray, and gamma-ray wavelengths in 2012. We present observations made with Herschel, Swift, NuSTAR, Fermi, the Submillimeter Array, CARMA, and the VLBA in 2012-2013, including three months with nearly daily sampling at several wavebands. We have also conducted an intensive campaign of 30 hr with every-orbit observations by Swift and NuSTAR, accompanied by Herschel, and Fermi observations. The source was highly variable at all bands. Time lags, correlations between bands, and the changing shapes of the spectral energy distributions can be explained by synchrotron radiation and inverse Compton emission from nonthermal seed photons originating from within the jet. The passage of four new superluminal very long baseline interferometry knots through the core and two stationary knots about 4 pc downstream accompanied the high flaring in 2012-2013. The seed photons for inverse Compton scattering may arise from the stationary knots and from a Mach disk near the core where relatively slow-moving plasma generates intense nonthermal radiation. The 95 spectral energy distributions obtained on consecutive days form the most densely sampled, broad wavelength coverage for any blazar. The observed spectral energy distributions and multi-waveband light curves are similar to simulated spectral energy distributions and light curves generated with a model in which turbulent plasma crosses a conical shock with a Mach disk. © 2016. The American Astronomical Society. All rights reserved. Source

Barriere N.M.,University of California at Berkeley | Tomsick J.A.,University of California at Berkeley | Baganoff F.K.,Massachusetts Institute of Technology | Boggs S.E.,University of California at Berkeley | And 13 more authors.
Astrophysical Journal | Year: 2014

Sagittarius A* harbors the supermassive black hole that lies at the dynamical center of our Galaxy. Sagittarius A* spends most of its time in a low luminosity emission state but flares frequently in the infrared and X-ray, increasing up to a few hundred fold in brightness for up to a few hours at a time. The physical processes giving rise to the X-ray flares are uncertain. Here we report the detection with the NuSTAR observatory in Summer and Fall 2012 of four low to medium amplitude X-ray flares to energies up to 79 keV. For the first time, we clearly see that the power-law spectrum of Sagittarius A* X-ray flares extends to high energy, with no evidence for a cutoff. Although the photon index of the absorbed power-law fits are in agreement with past observations, we find a difference between the photon index of two of the flares (significant at the 95% confidence level). The spectra of the two brightest flares (∼55 times quiescence in the 2-10 keV band) are compared to simple physical models in an attempt to identify the main X-ray emission mechanism, but the data do not allow us to significantly discriminate between them. However, we confirm the previous finding that the parameters obtained with synchrotron models are, for the X-ray emission, physically more reasonable than those obtained with inverse Compton models. One flare exhibits large and rapid (<100 s) variability, which, considering the total energy radiated, constrains the location of the flaring region to be within ∼10 Schwarzschild radii of the black hole. © 2014. The American Astronomical Society. All rights reserved. Source

Lohfink A.M.,University of Cambridge | Reynolds C.S.,University of Maryland College Park | Pinto C.,University of Cambridge | Alston W.,University of Cambridge | And 13 more authors.
Astrophysical Journal | Year: 2016

We present a multi-epoch X-ray spectral analysis of the Seyfert 1 galaxy Fairall 9. Our analysis shows that Fairall 9 displays unique spectral variability in that its ratio residuals to a simple absorbed power law in the 0.5-10 keV band remain constant with time in spite of large variations in flux. This behavior implies an unchanging source geometry and the same emission processes continuously at work at the timescale probed. With the constraints from NuSTAR on the broad-band spectral shape, it is clear that the soft excess in this source is a superposition of two different processes, one being blurred ionized reflection in the innermost parts of the accretion disk, and the other a continuum component such as a spatially distinct Comptonizing region. Alternatively, a more complex primary Comptonization component together with blurred ionized reflection could be responsible. © 2016. The American Astronomical Society. All rights reserved. Source

Barriere N.M.,University of California at Berkeley | Krivonos R.,University of California at Berkeley | Tomsick J.A.,University of California at Berkeley | Bachetti M.,CNRS Astrophysics and Planetology Research Institute | And 11 more authors.
Astrophysical Journal | Year: 2015

We report on two NuSTAR observations of GRS 1741.9-2853, a faint neutron star (NS) low-mass X-ray binary burster located 10′ away from the Galactic center. NuSTAR detected the source serendipitously as it was emerging from quiescence: its luminosity was 6 × 1034 erg s-1 on 2013 July 31 and 5 × 1035 erg s-1 in a second observation on 2013 August 3. A bright, 800 s long, H-triggered mixed H/He thermonuclear Type I burst with mild photospheric radius expansion (PRE) was present during the second observation. Assuming that the luminosity during the PRE was at the Eddington level, an H mass fraction X = 0.7 in the atmosphere, and an NS mass M = 1.4 M⊙, we determine a new lower limit on the distance for this source of 6.3 ± 0.5 kpc. Combining with previous upper limits, this places GRS 1741.9-2853 at a distance of 7 kpc. Energy independent (achromatic) variability is observed during the cooling of the NS, which could result from the disturbance of the inner accretion disk by the burst. The large dynamic range of this burst reveals a long power-law decay tail. We also detect, at a 95.6% confidence level (1.7σ), a narrow absorption line at 5.46 ± 0.10 keV during the PRE phase of the burst, reminiscent of the detection by Waki et al. We propose that the line, if real, is formed in the wind above the photosphere of the NS by a resonant Kα transition from H-like Cr gravitationally redshifted by a factor 1 + z = 1.09, corresponding to a radius range of 29.0-41.4 km for a mass range of 1.4-2.0 M⊙. © 2015. The American Astronomical Society. All rights reserved. Source

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