Person: Aldcroft, Thomas
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Aldcroft
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Aldcroft, Thomas
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Publication A Full Year's Chandra Exposure on Sloan Digital Sky Survey Quasars From the Chandra Multiwavelength Project(IOP Publishing, 2008) Green, Paul J.; Aldcroft, Thomas; Richards, G. T.; Barkhouse, W. A.; Constantin, Ana-Maria; Haggard, D.; Karovska, Margarita; Kim, D.-W.; Kim, M.; Viklinin, Alexey; Anderson, S. F.; Mossman, Amy; Kashyap, Vinay; Myers, A. C.; Silverman, J. D.; Wilkes, Belinda; Tananbaum, HarveyWe study the spectral energy distributions and evolution of a large sample of optically selected quasars from the Sloan Digital Sky Survey that were observed in 323 Chandra images analyzed by the Chandra Multiwavelength Project. Our highest-confidence matched sample includes 1135 X-ray detected quasars in the redshift range 0.2 3, substantially expanding the known sample. We find no evidence for evolution out to z ∼ 5 for either the X-ray photon index Γ or for the ratio of optical/UV to X-ray flux αox. About 10% of detected QSOs show best-fit intrinsic absorbing columns greater than 1022 cm−2, but the fraction might reach ∼1/3 if most nondetections are absorbed. We confirm a significant correlation between αox and optical luminosity, but it flattens or disappears for fainter (MB −23) active galactic nucleus (AGN) alone. We report significant hardening of Γ both toward higher X-ray luminosity, and for relatively X-ray loud quasars. These trends may represent a relative increase in nonthermal X-ray emission, and our findings thereby strengthen analogies between Galactic black hole binaries and AGN. For uniformly selected subsamples of narrow-line Seyfert 1s and narrow absorption line QSOs, we find no evidence for unusual distributions of either αox or Γ.Publication Chandra Multiwavelength Project: Normal Galaxies at Intermediate Redshift(IOP Publishing, 2006) Kim, D.‐W.; Barkhouse, W. A.; Romero‐Colmenero, E.; Green, P. J.; Kim, M.; Mossman, Amy; Schlegel, E.; Silverman, J. D.; Aldcroft, Thomas; Anderson, C.; Ivezic, Z.; Kashyap, V.; Tananbaum, Harvey; Wilkes, BelindaWe have investigated 136 Chandra extragalactic sources, including 93 galaxies with narrow emission lines (NELGs) and 43 with only absorption lines (ALGs). Based on fX/fO, LX, X-ray spectral hardness, and optical emission-line diagnostics, we have conservatively classified 36 normal galaxies and 71 AGNs. Their redshift ranges from 0.01 to 1.2, with normal galaxies in the range z = 0.01-0.3. Our normal galaxies appear to share characteristics with local galaxies, as expected from the X-ray binary populations and the hot interstellar matter (ISM). In conjunction with normal galaxies found in other surveys, we found no statistically significant evolution in LX/LB, within the limited z range (lesssim0.1). The best-fit slope of our log(N)-log(S) relationship is -1.5 for both S (0.5-2 keV) and B (0.5-8 keV) energy bands, which is considerably steeper than that of the AGN-dominated cosmic background sources, but slightly flatter than the previous estimate, indicating that normal galaxies will not exceed the AGN population until fX(0.5-2.0 keV) ~ 2 × 10-18 ergs s-1 cm-2 (a factor of ~5 lower than the previous estimate). A group of NELGs appear to be heavily obscured in X-rays. After correcting for intrinsic absorption, their X-ray luminosities could be LX > 1044 ergs s-1, making them type 2 quasar candidates. While most X-ray-luminous ALGs do not appear to be significantly absorbed, we found two heavily obscured objects that could be as luminous as an unobscured broad-line quasar. Among 43 ALGs, we found two E+A galaxy candidates. The X-ray spectra of both galaxies are soft, and one of them has a nearby close companion galaxy, supporting the merger/interaction scenario rather than the dusty starburst hypothesis.Publication Discovery of a Jetlike Structure at the High-Redshift QSO CXOMP J084128.3+131107(IOP Publishing, 2004) Schwartz, D. A.; Silverman, J.; Birkinshaw, M.; Karovska, Margarita; Aldcroft, Thomas; Barkhouse, W.; Green, P.; Kim, D.-W.; Wilkes, Belinda; Worrall, DianaThe Chandra Multiwavelength Project has discovered a jetlike structure associated with a newly recognized QSO at redshift z = 1.866. The system was 9farcm4 off-axis during an observation of 3C 207. Although significantly distorted by the mirror point-spread function, we use both a ray trace and a nearby bright point source to show that the X-ray image must arise from some combination of point and extended sources, or else from a minimum of three distinct point sources. We favor the former situation, as three unrelated sources would have a small probability of occurring by chance in such a close alignment. We show that interpretation as a jet emitting X-rays via inverse Compton scattering on the cosmic microwave background is plausible. This would be a surprising and unique discovery of a radio-quiet QSO with an X-ray jet, since we have obtained upper limits of 100 μJy on the QSO emission at 8.46 GHz and limits of 200 μJy for emission from the putative jet.Publication The Chandra Multiwavelength Project: Optical Follow‐up of Serendipitous Chandra Sources(IOP Publishing, 2004) Green, P. J.; Silverman, J. D.; Cameron, R. A.; Kim, D.‐W.; Wilkes, Belinda; Barkhouse, W. A.; LaCluyze, A.; Morris, D.; Mossman, Amy; Ghosh, H.; Grimes, J. P.; Jannuzi, B. T.; Tananbaum, Harvey; Aldcroft, Thomas; Baldwin, J. A.; Chaffee, F. H.; Dey, A.; Dosaj, A.; Evans, Nancy; Fan, X.; Foltz, C.; Gaetz, Terrance; Hooper, E. J.; Kashyap, Vinay; Mathur, S.; McGarry, M. B.; Romero‐Colmenero, E.; Smith, M. G.; Smith, P. S.; Smith, R. C.; Torres, Guillermo; Viklinin, Alexey; Wik, D. R.We present follow-up optical g', r', and i' imaging and spectroscopy of serendipitous X-ray sources detected in six archival Chandra images included in the Chandra Multiwavelength Project (ChaMP). Of the 486 X-ray sources detected between 3 × 10-16 and 2 × 10-13 (with a median flux of 3 × 10-15) ergs cm-2 s-1, we find optical counterparts for 377 (78%), or 335 (68%) counting only unique counterparts. We present spectroscopic classifications for 125 objects, representing 75% of sources with r* < 21 optical counterparts (63% to r* = 22). Of all classified objects, 63 (50%) are broad-line active galactic nuclei (AGNs), which tend to be blue in (g*-r*) colors. X-ray information efficiently segregates these quasars from stars, which otherwise strongly overlap in these SDSS colors until z > 3.5. We identify 28 sources (22%) as galaxies that show narrow emission lines, while 22 (18%) are absorption line galaxies. Eight galaxies lacking broad-line emission have X-ray luminosities that require they host an AGN (logLX > 43). Half of these have hard X-ray emission suggesting that high gas columns obscure both the X-ray continuum and the broad emission line regions. We find objects in our sample that show signs of X-ray or optical absorption, or both, but with no strong evidence that these properties are coupled. ChaMP's deep X-ray and optical imaging enable multiband selection of small and/or high-redshift groups and clusters. In these six fields we have discovered three new clusters of galaxies, two with z > 0.4, and one with photometric evidence for a similar redshift.Publication The X‐Ray Warm Absorber in NGC 3516(IOP Publishing, 1997) Mathur, Smita; Wilkes, Belinda; Aldcroft, ThomasThe Seyfert 1 galaxy, NGC 3516 has been the subject of many absorption line studies at both ultraviolet and X-ray wavelengths. In the UV, strong, broad, variable associated metal line absorption with velocity width ~2000 km s-1 is thought to originate in gas with NH gsim 1019 cm-2 lying between 0.01 and 9 pc from the central active nucleus. The Ginga X-ray data are consistent with several possibilities: a warm absorber and a cold absorber combined either with partial covering or an unusually strong reflection spectrum. We present ROSAT observations of NGC 3516 which show a strong detection of a warm absorber dominated by a blend of O VII/O VIII edges at ~0.8 keV with NH ~ 7 × 1021 cm-2 and U:8-12. We argue that NGC 3516 contains an outflowing "XUV" absorber showing the presence of X-ray absorption edges which are consistent with the presence of broad absorption lines in the old IUE spectra and their disappearance in the new UV observations. Our dynamical model suggests that the O VII absorption edge will continue to weaken compared to the O VIII edge, an easily testable prediction with future missions like AXAF. Eventually the source would be transparent to the X-rays unless a new absorption system is produced.