Cahill Center for Astronomy and Astrophysics

Pasadena, CA, United States

Cahill Center for Astronomy and Astrophysics

Pasadena, CA, United States
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Balokovic M.,Cahill Center for Astronomy and Astrophysics | Comastri A.,National institute for astrophysics | Harrison F.A.,Cahill Center for Astronomy and Astrophysics | Alexander D.M.,Durham University | And 29 more authors.
Astrophysical Journal | Year: 2014

We present X-ray spectral analyses for three Seyfert 2 active galactic nuclei (AGNs), NGC 424, NGC 1320, and IC 2560, observed by NuSTAR in the 3-79 keV band. The high quality hard X-ray spectra allow detailed modeling of the Compton reflection component for the first time in these sources. Using quasi-simultaneous NuSTAR and Swift/XRT data, as well as archival XMM-Newton data, we find that all three nuclei are obscured by Compton-thick material with column densities in excess of 5 × 1024 cm-2, and that their X-ray spectra above 3 keV are dominated by reflection of the intrinsic continuum on Compton-thick material. Due to the very high obscuration, absorbed intrinsic continuum components are not formally required by the data in any of the sources. We constrain the intrinsic photon indices and the column density of the reflecting medium through the shape of the reflection spectra. Using archival multi-wavelength data we recover the intrinsic X-ray luminosities consistent with the broadband spectral energy distributions. Our results are consistent with the reflecting medium being an edge-on clumpy torus with a relatively large global covering factor and overall reflection efficiency of the order of 1%. Given the unambiguous confirmation of the Compton-thick nature of the sources, we investigate whether similar sources are likely to be missed by commonly used selection criteria for Compton-thick AGNs, and explore the possibility of finding their high-redshift counterparts. © 2014. The American Astronomical Society. All rights reserved..


Bachetti M.,Toulouse 1 University Capitole | Bachetti M.,CNRS Institute for research in astrophysics and planetology | Harrison F.A.,Cahill Center for Astronomy and Astrophysics | Cook R.,Cahill Center for Astronomy and Astrophysics | And 22 more authors.
Astrophysical Journal | Year: 2015

Timing of high-count-rate sources with the NuSTAR Small Explorer Mission requires specialized analysis techniques. NuSTAR was primarily designed for spectroscopic observations of sources with relatively low count rates rather than for timing analysis of bright objects. The instrumental dead time per event is relatively long (∼2.5 msec) and varies event-to-event by a few percent. The most obvious effect is a distortion of the white noise level in the power density spectrum (PDS) that cannot be easily modeled with standard techniques due to the variable nature of the dead time. In this paper, we show that it is possible to exploit the presence of two completely independent focal planes and use the cospectrum, the real part of the cross PDS, to obtain a good proxy of the white-noise-subtracted PDS. Thereafter, one can use a Monte Carlo approach to estimate the remaining effects of dead time, namely, a frequency-dependent modulation of the variance and a frequency-independent drop of the sensitivity to variability. In this way, most of the standard timing analysis can be performed, albeit with a sacrifice in signal-to-noise ratio relative to what would be achieved using more standard techniques. We apply this technique to NuSTAR observations of the black hole binaries GX 339-4, Cyg X-1, and GRS 1915+105. © 2015. The American Astronomical Society. All rights reserved.


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.


Balokovic M.,Cahill Center for Astronomy and Astrophysics | Matt G.,Third University of Rome | Harrison F.A.,Cahill Center for Astronomy and Astrophysics | Zoghbi A.,University of Maryland University College | And 17 more authors.
Astrophysical Journal | Year: 2015

Measurements of the high-energy cut-off in the coronal continuum of active galactic nuclei have long been elusive for all but a small number of the brightest examples. We present a direct measurement of the cut-off energy in the nuclear continuum of the nearby Seyfert 1.9 galaxy MCG-05-23-016 with unprecedented precision. The high sensitivity of NuSTAR up to 79 keV allows us to clearly disentangle the spectral curvature of the primary continuum from that of its reflection component. Using a simple phenomenological model for the hard X-ray spectrum, we constrain the cut-off energy to 116-5 +6 keV with 90% confidence. Testing for more complex models and nuisance parameters that could potentially influence the measurement, we find that the cut-off is detected robustly. We further use simple Comptonized plasma models to provide independent constraints for both the kinetic temperature of the electrons in the corona and its optical depth. At the 90% confidence level, we find kTe = 29 ± 2 keV and τe = 1.23 ± 0.08 assuming a slab (disk-like) geometry, and kTe = 25 ± 2 keV and τe = 3.5 ± 0.2 assuming a spherical geometry. Both geometries are found to fit the data equally well and their two principal physical parameters are correlated in both cases. With the optical depth in the τe ≳ 1 regime, the data are pushing the currently available theoretical models of the Comptonized plasma to the limits of their validity. Since the spectral features and variability arising from the inner accretion disk have been observed previously in MCG-05-23-016, the inferred high optical depth implies that a spherical or disk-like corona cannot be homogeneous. © 2015. The American Astronomical Society. All rights reserved.


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.


Orienti M.,University of Bologna | Orienti M.,National institute for astrophysics | Koyama S.,Japan National Astronomical Observatory | Koyama S.,University of Tokyo | And 30 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We investigate the radio and γ -ray variability of the flat spectrum radio quasar PKS 1510-089 in the time range between 2010 November and 2012 January. In this period the source showed an intense activity, with two major γ -ray flares detected in 2011 July and October. During the latter episode both the γ -ray and the radio flux density reached their historical peak. Multiwavelength analysis shows a rotation of about 380{ring operator} of the optical polarization angle close in time with the rapid and strong γ -ray flare in 2011 July. An enhancement of the optical emission and an increase of the fractional polarization both in the optical and in radio bands are observed about three weeks later, close in time with another γ -ray outburst. On the other hand, after 2011 September a huge radio outburst has been detected, first in the millimetre regime followed with some time delay at centimetre down to decimetre wavelengths. This radio flare is characterized by a rising and a decaying stage, in agreement with the formation of a shock and its evolution, as a consequence of expansion and radiative cooling. If the γ -ray flare observed in 2011 October is related to this radio outburst, then this strongly indicates that the region responsible for the γ -ray variability is not within the broad line, but a few parsecs downstream along the jet. © 2012 The Authors.


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.


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.


Bachetti M.,Toulouse 1 University Capitole | Bachetti M.,CNRS Institute for research in astrophysics and planetology | Rana V.,Cahill Center for Astronomy and Astrophysics | Walton D.J.,Cahill Center for Astronomy and Astrophysics | And 19 more authors.
Astrophysical Journal | Year: 2013

We present the results of NuSTAR and XMM-Newton observations of the two ultraluminous X-ray sources: NGC 1313 X-1 and X-2. The combined spectral bandpass of the two satellites enables us to produce the first spectrum of X-1 between 0.3 and 30 keV, while X-2 is not significantly detected by NuSTAR above 10 keV. The NuSTAR data demonstrate that X-1 has a clear cutoff above 10 keV, whose presence was only marginally detectable with previous X-ray observations. This cutoff rules out the interpretation of X-1 as a black hole in a standard low/hard state, and it is deeper than predicted for the downturn of a broadened iron line in a reflection-dominated regime. The cutoff differs from the prediction of a single-temperature Comptonization model. Further, a cold disk-like blackbody component at ∼0.3 keV is required by the data, confirming previous measurements by XMM-Newton only. We observe a spectral transition in X-2, from a state with high luminosity and strong variability to a lower-luminosity state with no detectable variability, and we link this behavior to a transition from a super-Eddington to a sub-Eddington regime. © 2013. The American Astronomical Society. All rights reserved.


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.

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