Browsing by Author "Walton, D. J."
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- ItemA GROWTH-RATE INDICATOR FOR COMPTON-THICK ACTIVE GALACTIC NUCLEI(2016) Brightman, M.; Masini, A.; Ballantyne, D. R.; Balokovic, M.; Brandt, W. N.; Chen, C. -T.; Comastri, A.; Farrah, D.; Gandhi, P.; Harrison, F. A.; Ricci, C.; Stern, D.; Walton, D. J.Due to their heavily obscured central engines, the growth rate of Compton-thick (CT) active galactic nuclei (AGNs) is difficult to measure. A statistically significant correlation between the Eddington ratio, lambda(Edd), and the X-ray power-law index, Gamma, observed in unobscured AGNs offers an estimate of their growth rate from X-ray spectroscopy (albeit with large scatter). However, since X-rays undergo reprocessing by Compton scattering and photoelectric absorption when the line of sight to the central engine is heavily obscured, the recovery of the intrinsic Gamma is challenging. Here we study a sample of local, predominantly CT megamaser AGNs, where the black hole mass, and thus Eddington luminosity, are well known. We compile results of the X-ray spectral fitting of these sources with sensitive high-energy (E > 10 keV) NuSTAR data, where X-ray torus models, which take into account the reprocessing effects have been used to recover the intrinsic Gamma values and X-ray luminosities, L-X. With a simple bolometric correction to L-X to calculate lambda(Edd), we find a statistically significant correlation between Gamma and lambda(Edd) (p = 0.007). A linear fit to the data yields Gamma = (0.41 +/- 0.18)log(10)lambda(Edd) + (2.38 +/- 0.20), which is statistically consistent with results for unobscured AGNs. This result implies that torus modeling successfully recovers the intrinsic AGN parameters. Since the megamasers have low-mass black holes (M-BH approximate to 10(6)-10(7) M-circle dot) and are highly inclined, our results extend the Gamma-lambda(Edd) relationship to lower masses and argue against strong orientation effects in the corona, in support of AGN unification. Finally this result supports the use of Gamma as a growth-rate indicator for accreting black holes, even for CT AGNs.
- ItemAn extremely luminous and variable ultraluminous X-ray source in the outskirts of circinus observed with nustar(2013) Walton, D. J.; Fürst, F.; Harrison, F. A.; Stern, D.; Bachetti, Matteo; Barret, Didier; Bauer, Franz Erik; Boggs, S. E.
- ItemBroad-band X-ray spectral analysis of the Seyfert 1 galaxy GRS 1734-292(2017) Tortosa, A.; Marinucci, A.; Matt, G.; Bianchi, S.; La Franca, F.; Ballantyne, D. R.; Boorman, P. G.; Fabian, A. C.; Farrah, D.; Ricci, Claudio; Fuerst, F.; Gandhi, P.; Harrison, F. A.; Koss, M. J.; Stern, D.; Ursini, F.; Walton, D. J.
- ItemDetermining the covering factor of compton-thick active galactic nuclei with NuSTAR(2015) Brightman, M.; Balokovic, M.; Stern, D.; Arevalo, P.; Ballantyne, D. R.; Bauer, Franz Erik; Bogg, S. E.; Craig, W. W.; Christensen, F. E.; Comastri, A.; Gandhi, P.; Hailey, C. J.; Harrison, F. A.; Hickox, R. C.; Koss, M.; Lamassa, S.; Fuerst, F.; Puccetti, S.; Rivers, E.; Vasudevan, R.; Walton, D. J.; Zhang, W. W.
- ItemHot Dust-obscured Galaxies with Excess Blue Light(2020) Assef, R. J.; Brightman, M.; Walton, D. J.; Stern, D.; Bauer, F. E.; Blain, A. W.; Diaz-Santos, T.; Eisenhardt, P. R. M.; Hickox, R. C.; Jun, H. D.; Psychogyios, A.; Tsai, C-W; Wu, J. W.Hot dust-obscured galaxies (Hot DOGs) are among the most luminous galaxies in the universe. Powered by highly obscured, possibly Compton-thick, active galactic nuclei (AGNs), Hot DOGs are characterized by spectral energy distributions that are very red in the mid-infrared yet dominated by the host galaxy stellar emission in the UV and optical. An earlier study identified a subsample of Hot DOGs with significantly enhanced UV emission. One target, W0204-0506, was studied in detail and, based on Chandra observations, it was concluded that the enhanced emission was most likely due to either extreme unobscured star formation (star formation rate > 1000Myr(-1)) or to light from the highly obscured AGN scattered by gas or dust into our line of sight. Here, we present a follow-up study of W0204-0506 as well as two more Hot DOGs with excess UV emission. For the two new objects we obtained Chandra/ACIS-S observations, and for all three targets we obtained Hubble Space Telescope/WFC3 F555W and F160W imaging. The analysis of these observations, combined with multiwavelength photometry and UV/optical spectroscopy suggests that UV emission is most likely dominated by light from the central highly obscured, hyperluminous AGN that has been scattered into our line of sight, by either gas or dust. We cannot decisively rule out, however, that star formation or a second AGN in the system may significantly contribute to the UV excess of these targets.
- ItemImaging Polarization of the Blue-excess Hot Dust-obscured Galaxy WISE J011601.41-050504.0(2022) Assef, R. J.; Bauer, F. E.; Blain, A. W.; Brightman, M.; Diaz-Santos, T.; Eisenhardt, P. R. M.; Jun, H. D.; Stern, D.; Tsai, C-W; Walton, D. J.; Wu, J. W.We report on VLT/FORS2 imaging polarimetry observations in the R (Special) band of WISE J011601.41-050504.0 (W0116-0505), a heavily obscured hyperluminous quasar at z = 3.173 classified as a Hot Dust-obscured Galaxy (Hot DOG) based on its mid-IR colors. Recently, Assef et al. identified W0116-0505 as having excess rest-frame optical/UV emission and concluded that this excess emission is most likely scattered light from the heavily obscured AGN. We find that the broadband rest-frame UV flux is strongly linearly polarized (10.8% +/- 1.9%, with a polarization angle of 74 degrees +/- 9 degrees), confirming this conclusion. We analyze these observations in the context of a simple model based on scattering either by free electrons or by optically thin dust, assuming a classical dust torus with polar openings. Both can replicate the degree of polarization and the luminosity of the scattered component for a range of geometries and column densities, but we argue that optically thin dust in the ISM is the more likely scenario. We also explore the possibility that the scattering medium corresponds to an outflow recently identified for W0116-0505. This is a feasible option if the outflow component is biconical with most of the scattering occurring at the base of the receding outflow. In this scenario, the quasar would still be obscured even if viewed face-on but might appear as a reddened type 1 quasar once the outflow has expanded. We discuss a possible connection between blue-excess Hot DOGs, extremely red quasars, reddened type 1 quasars, and unreddened quasars that depends on a combination of evolution and viewing geometry.
- ItemProbing the circumnuclear absorbing medium of the buried AGN in NGC 1068 through NuSTAR observations(OUP, 2019) Zaino, A.; Bianchi, S.; Marinucci, A.; Matt, G.; Bauer, Franz Erik; Brandt, W. N.; Gandhi, P.; Guainazzi, M.; Iwasawa, K.; Puccetti, S.; Ricci, Claudio; Walton, D. J.We present the results of the latest NuSTAR monitoring campaign of the Compton-thick Seyfert 2 galaxy NGC 1068, composed of four ∼50 ks observations performed between 2017 July and 2018 February to search for flux and spectral variability on time-scales from 1 to 6 months. We detect one unveiling and one eclipsing event with time-scales less than 27 and 91 d, respectively, ascribed to Compton-thick material with N H = (1.8 ± 0.8) × 10 24 and ≥ (2.4 ± 0.5) × 10 24 cm −2 moving across our line of sight. This gas is likely located in the innermost part of the torus or even further inward, thus providing further evidence of the clumpy structure of the circumnuclear matter in this source. Taking advantage of simultaneous Swift–XRT observations, we also detected a new flaring ULX, at a distance d ∼ 30 arcsec (i.e. ∼2 kpc) from the nuclear region of NGC 1068, with a peak X-ray intrinsic luminosity of (3.0 ± 0.4) × 10 40 erg s −1 in the 2–10 keV band.
- ItemThe 2-79 keV X-Ray Spectrum of the Circinus Galaxy with NuSTAR, XMM-Newton, and Chandra: A Fully Compton-thick Active Galactic Nucleus(2014) Arévalo, Patricia.; Treister, Ezequiel; Bauer, Franz Erik; Puccetti, S.; Walton, D. J.; Koss, M.; Boggs, Steve E.; Brandt, W N.; Brightman, M.; Christensen, F. E.; Comastri, A.; Craig, W. W.
- ItemTHE BROADBAND SPECTRAL VARIABILITY OF MCG-6-30-15 OBSERVED BY NUSTAR AND XMM-NEWTON(2014) Marinucci, A.; Matt, G.; Miniutti, G.; Guainazzi, M.; Parker, M. L.; Brenneman, L.; Fabian, A. C.; Kara, E.; Arevalo, P.; Ballantyne, D. R.; Boggs, S. E.; Cappi, M.; Christensen, F. E.; Craig, W. W.; Elvis, M.; Hailey, C. J.; Harrison, F. A.; Reynolds, C. S.; Risaliti, G.; Stern, D. K.; Walton, D. J.; Zhang, W.MCG-6-30-15, at a distance of 37 Mpc (z = 0.008), is the archetypical Seyfert 1 galaxy showing very broad Fe K alpha emission. We present results from a joint NuSTAR and XMM-Newton observational campaign that, for the first time, allows a sensitive, time-resolved spectral analysis from 0.35 keV up to 80 keV. The strong variability of the source is best explained in terms of intrinsic X-ray flux variations and in the context of the light-bending model: the primary, variable emission is reprocessed by the accretion disk, which produces secondary, less variable, reflected emission. The broad Fe K alpha profile is, as usual for this source, well explained by relativistic effects occurring in the innermost regions of the accretion disk around a rapidly rotating black hole. We also discuss the alternative model in which the broadening of the Fe K alpha is due to the complex nature of the circumnuclear absorbing structure. Even if this model cannot be ruled out, it is disfavored on statistical grounds. We also detected an occultation event likely caused by broad-line region clouds crossing the line of sight.
- ItemThe Seyfert 2 galaxy NGC 2110 : hard X-ray emission observed by NuSTAR and variability of the iron Kα line(2015) Marinucci, A.; Matt, G.; Bianchi, S.; Lu, T. N.; Arévalo, P.; Balokovic, M.; Ballantyne, D.; Bauer, Franz Erik; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Gandhi, P.; Hailey, C. J.; Harrison, F.; Puccetti, S.; Rivers, E.; Walton, D. J.; Stern, D.; Zhang, W.
- ItemThe soft-X-ray emission of Ark 120. XMM-Newton, NuSTAR, and the importance of taking the broad view(OXFORD UNIV PRESS, 2014) Matt, G.; Marinucci, A.; Guainazzi, M.; Brenneman, L. W.; Elvis, M.; Lohfink, A.; Arevalo, P.; Boggs, S. E.; Cappi, M.; Christensen, F. E.; Craig, W. W.; Fabian, A. C.; Fuerst, F.; Hailey, C. J.; Harrison, F. A.; Parker, M.; Reynolds, C. S.; Stern, D.; Walton, D. J.; Zhang, W. W.We present simultaneous XMM-Newton and NuSTAR observations of the 'bare' Seyfert 1 galaxy, Ark 120, a system in which ionized absorption is absent. The NuSTAR hard-X-ray spectral coverage allows us to constrain different models for the excess soft-X-ray emission. Among phenomenological models, a cutoff power law best explains the soft-X-ray emission. This model likely corresponds to Comptonization of the accretion disc seed UV photons by a population of warm electrons: using Comptonization models, a temperature of similar to 0.3 keV and an optical depth of similar to 13 are found. If the UV-to-X-ray optxagnf model is applied, the UV fluxes from the XMM-Newton Optical Monitor suggest an intermediate black hole spin. Contrary to several other sources observed by NuSTAR, no high-energy cutoff is detected with a lower limit of 190 keV.