Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy

S. Mathur; A. Gupta; K. Page; R. W. Pogge; Y. Krongold; M. R. Goad; S. M. Adams; M. D. Anderson; P. Arévalo; A. J. Barth; C. Bazhaw; T. G. Beatty; M. C. Bentz; A. Bigley; S. Bisogni; G. A. Borman; T. A. Boroson; M. C. Bottorff; W. N. Brandt; A. A. Breeveld; J. E. Brown; J. S. Brown; E. M. Cackett; G. Canalizo; M. T. Carini; K. I. Clubb; J. M. Comerford; C. T. Coker; E. M. Corsini; D. M. Crenshaw; S. Croft; K. V. Croxall; E. Dalla Bontà; A. J. Deason; K. D. Denney; A. De Lorenzo-Cáceres; G. De Rosa; M. Dietrich; R. Edelson; J. Ely; M. Eracleous; P. A. Evans; M. M. Fausnaugh; G. J. Ferland; A. V. Filippenko; K. Flatland; O. D. Fox; E. L. Gates; N. Gehrels; S. Geier; J. M. Gelbord; V. Gorjian; J. E. Greene; C. J. Grier; D. Grupe; P. B. Hall; C. B. Henderson; S. Hicks; E. Holmbeck; T. W.S. Holoien; D. Horenstein; Keith Horne; T. Hutchison; M. Im; J. J. Jensen; C. A. Johnson; M. D. Joner; J. Jones; J. Kaastra; S. Kaspi; B. C. Kelly; P. L. Kelly; J. A. Kennea; M. Kim; S. Kim; S. C. Kim; A. King; S. A. Klimanov; C. S. Kochanek; K. T. Korista; G. A. Kriss; M. W. Lau; J. C. Lee; D. C. Leonard; M. Li; P. Lira; Z. Ma; F. Macinnis; E. R. Manne-Nicholas; M. A. Malkan; J. C. Mauerhan; R. McGurk; I. M. McHardy; C. Montouri; L. Morelli; A. Mosquera; D. Mudd; F. Muller-Sanchez; R. Musso; S. V. Nazarov; H. Netzer; M. L. Nguyen; R. P. Norris; J. A. Nousek; P. Ochner; D. N. Okhmat; B. Ou-Yang; A. Pancoast; I. Papadakis; J. R. Parks; L. Pei; B. M. Peterson; A. Pizzella; R. Poleski; J. U. Pott; S. E. Rafter; H. W. Rix; J. Runnoe; D. A. Saylor; J. S. Schimoia; K. Schnülle; S. G. Sergeev; B. J. Shappee; I. Shivvers; M. Siegel; G. V. Simonian; A. Siviero; A. Skielboe; G. Somers; M. Spencer; D. Starkey; D. J. Stevens; H. I. Sung; J. Tayar; N. Tejos; C. S. Turner; P. Uttley; J. Van Saders; M. Vestergaard; L. Vican; S. Villanueva; C. Villforth; Y. Weiss; J. H. Woo; H. Yan; S. Young; H. Yuk; W. Zheng; W. Zhu; Y. Zu

doi:10.3847/1538-4357/aa832b

Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy

S. Mathur, A. Gupta, K. Page, R. W. Pogge, Y. Krongold, M. R. Goad, S. M. Adams, M. D. Anderson, P. Arévalo, A. J. Barth, C. Bazhaw, T. G. Beatty, M. C. Bentz, A. Bigley, S. Bisogni, G. A. Borman, T. A. Boroson, M. C. Bottorff, W. N. Brandt, A. A. BreeveldJ. E. Brown, J. S. Brown, E. M. Cackett, G. Canalizo, M. T. Carini, K. I. Clubb, J. M. Comerford, C. T. Coker, E. M. Corsini, D. M. Crenshaw, S. Croft, K. V. Croxall, E. Dalla Bontà, A. J. Deason, K. D. Denney, A. De Lorenzo-Cáceres, G. De Rosa, M. Dietrich, R. Edelson, J. Ely, M. Eracleous, P. A. Evans, M. M. Fausnaugh, G. J. Ferland, A. V. Filippenko, K. Flatland, O. D. Fox, E. L. Gates, N. Gehrels, S. Geier, J. M. Gelbord, V. Gorjian, J. E. Greene, C. J. Grier, D. Grupe, P. B. Hall, C. B. Henderson, S. Hicks, E. Holmbeck, T. W.S. Holoien, D. Horenstein, Keith Horne, T. Hutchison, M. Im, J. J. Jensen, C. A. Johnson, M. D. Joner, J. Jones, J. Kaastra, S. Kaspi, B. C. Kelly, P. L. Kelly, J. A. Kennea, M. Kim, S. Kim, S. C. Kim, A. King, S. A. Klimanov, C. S. Kochanek, K. T. Korista, G. A. Kriss, M. W. Lau, J. C. Lee, D. C. Leonard, M. Li, P. Lira, Z. Ma, F. Macinnis, E. R. Manne-Nicholas, M. A. Malkan, J. C. Mauerhan, R. McGurk, I. M. McHardy, C. Montouri, L. Morelli, A. Mosquera, D. Mudd, F. Muller-Sanchez, R. Musso, S. V. Nazarov, H. Netzer, M. L. Nguyen, R. P. Norris, J. A. Nousek, P. Ochner, D. N. Okhmat, B. Ou-Yang, A. Pancoast, I. Papadakis, J. R. Parks, L. Pei, B. M. Peterson, A. Pizzella, R. Poleski, J. U. Pott, S. E. Rafter, H. W. Rix, J. Runnoe, D. A. Saylor, J. S. Schimoia, K. Schnülle, S. G. Sergeev, B. J. Shappee, I. Shivvers, M. Siegel, G. V. Simonian, A. Siviero, A. Skielboe, G. Somers, M. Spencer, D. Starkey, D. J. Stevens, H. I. Sung, J. Tayar, N. Tejos, C. S. Turner, P. Uttley, J. Van Saders, M. Vestergaard, L. Vican, S. Villanueva, C. Villforth, Y. Weiss, J. H. Woo, H. Yan, S. Young, H. Yuk, W. Zheng, W. Zhu, Y. Zu

Research output: Contribution to journal › Article › peer-review

34 Scopus citations

Abstract

During the Space Telescope and Optical Reverberation Mapping Project observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the six-month-long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with the Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all three observations: the UV emission-line flux decrease, the soft-excess increase, and the emission-line anomaly.

Original language	English (US)
Article number	55
Journal	Astrophysical Journal
Volume	846
Issue number	1
DOIs	https://doi.org/10.3847/1538-4357/aa832b
State	Published - Sep 1 2017

Bibliographical note

Funding Information:
We are grateful to Christine Done for discussions on the warm Comptonization model. Support for this work was provided by the National Aeronautics and Space Administration (NASA) through Chandra Award Number G04-15114X to S.M. issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. Support for HST program GO-13330 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. C.S.K. is supported in part by NSF grants AST-1515876 and AST-1515927. K.H. acknowledges support from STFC grant ST/ M001296/1. K.L.P. and P.A.E. ackowledge support from the UK Space Agency. A.V.F.ʼs group at UC Berkeley is grateful for financial assistance from NSF grant AST-1211916, the TABASGO Foundation, and the Christopher R. Redlich Fund. Y.K. acknowledges support from the grant PAIIPIT IN104215 and CONACYT grant168519. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester (see Evans et al. 2009).

Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.

Keywords

X-rays: galaxies
X-rays: individual (NGC 5548)
galaxies: active
galaxies: individual (NGC 5548)

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10.3847/1538-4357/aa832b

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Mathur, S., Gupta, A., Page, K., Pogge, R. W., Krongold, Y., Goad, M. R., Adams, S. M., Anderson, M. D., Arévalo, P., Barth, A. J., Bazhaw, C., Beatty, T. G., Bentz, M. C., Bigley, A., Bisogni, S., Borman, G. A., Boroson, T. A., Bottorff, M. C., Brandt, W. N., ... Zu, Y. (2017). Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy. Astrophysical Journal, 846(1), Article 55. https://doi.org/10.3847/1538-4357/aa832b

Mathur, S, Gupta, A, Page, K, Pogge, RW, Krongold, Y, Goad, MR, Adams, SM, Anderson, MD, Arévalo, P, Barth, AJ, Bazhaw, C, Beatty, TG, Bentz, MC, Bigley, A, Bisogni, S, Borman, GA, Boroson, TA, Bottorff, MC, Brandt, WN, Breeveld, AA, Brown, JE, Brown, JS, Cackett, EM, Canalizo, G, Carini, MT, Clubb, KI, Comerford, JM, Coker, CT, Corsini, EM, Crenshaw, DM, Croft, S, Croxall, KV, Dalla Bontà, E, Deason, AJ, Denney, KD, De Lorenzo-Cáceres, A, De Rosa, G, Dietrich, M, Edelson, R, Ely, J, Eracleous, M, Evans, PA, Fausnaugh, MM, Ferland, GJ, Filippenko, AV, Flatland, K, Fox, OD, Gates, EL, Gehrels, N, Geier, S, Gelbord, JM, Gorjian, V, Greene, JE, Grier, CJ, Grupe, D, Hall, PB, Henderson, CB, Hicks, S, Holmbeck, E, Holoien, TWS, Horenstein, D, Horne, K, Hutchison, T, Im, M, Jensen, JJ, Johnson, CA, Joner, MD, Jones, J, Kaastra, J, Kaspi, S, Kelly, BC, Kelly, PL, Kennea, JA, Kim, M, Kim, S, Kim, SC, King, A, Klimanov, SA, Kochanek, CS, Korista, KT, Kriss, GA, Lau, MW, Lee, JC, Leonard, DC, Li, M, Lira, P, Ma, Z, Macinnis, F, Manne-Nicholas, ER, Malkan, MA, Mauerhan, JC, McGurk, R, McHardy, IM, Montouri, C, Morelli, L, Mosquera, A, Mudd, D, Muller-Sanchez, F, Musso, R, Nazarov, SV, Netzer, H, Nguyen, ML, Norris, RP, Nousek, JA, Ochner, P, Okhmat, DN, Ou-Yang, B, Pancoast, A, Papadakis, I, Parks, JR, Pei, L, Peterson, BM, Pizzella, A, Poleski, R, Pott, JU, Rafter, SE, Rix, HW, Runnoe, J, Saylor, DA, Schimoia, JS, Schnülle, K, Sergeev, SG, Shappee, BJ, Shivvers, I, Siegel, M, Simonian, GV, Siviero, A, Skielboe, A, Somers, G, Spencer, M, Starkey, D, Stevens, DJ, Sung, HI, Tayar, J, Tejos, N, Turner, CS, Uttley, P, Van Saders, J, Vestergaard, M, Vican, L, Villanueva, S, Villforth, C, Weiss, Y, Woo, JH, Yan, H, Young, S, Yuk, H, Zheng, W, Zhu, W & Zu, Y 2017, 'Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy', Astrophysical Journal, vol. 846, no. 1, 55. https://doi.org/10.3847/1538-4357/aa832b

@article{2368efbee78c42e089de63a8222495aa,

title = "Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy",

abstract = "During the Space Telescope and Optical Reverberation Mapping Project observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the six-month-long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with the Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all three observations: the UV emission-line flux decrease, the soft-excess increase, and the emission-line anomaly.",

keywords = "X-rays: galaxies, X-rays: individual (NGC 5548), galaxies: active, galaxies: individual (NGC 5548)",

author = "S. Mathur and A. Gupta and K. Page and Pogge, {R. W.} and Y. Krongold and Goad, {M. R.} and Adams, {S. M.} and Anderson, {M. D.} and P. Ar{\'e}valo and Barth, {A. J.} and C. Bazhaw and Beatty, {T. G.} and Bentz, {M. C.} and A. Bigley and S. Bisogni and Borman, {G. A.} and Boroson, {T. A.} and Bottorff, {M. C.} and Brandt, {W. N.} and Breeveld, {A. A.} and Brown, {J. E.} and Brown, {J. S.} and Cackett, {E. M.} and G. Canalizo and Carini, {M. T.} and Clubb, {K. I.} and Comerford, {J. M.} and Coker, {C. T.} and Corsini, {E. M.} and Crenshaw, {D. M.} and S. Croft and Croxall, {K. V.} and {Dalla Bont{\`a}}, E. and Deason, {A. J.} and Denney, {K. D.} and {De Lorenzo-C{\'a}ceres}, A. and {De Rosa}, G. and M. Dietrich and R. Edelson and J. Ely and M. Eracleous and Evans, {P. A.} and Fausnaugh, {M. M.} and Ferland, {G. J.} and Filippenko, {A. V.} and K. Flatland and Fox, {O. D.} and Gates, {E. L.} and N. Gehrels and S. Geier and Gelbord, {J. M.} and V. Gorjian and Greene, {J. E.} and Grier, {C. J.} and D. Grupe and Hall, {P. B.} and Henderson, {C. B.} and S. Hicks and E. Holmbeck and Holoien, {T. W.S.} and D. Horenstein and Keith Horne and T. Hutchison and M. Im and Jensen, {J. J.} and Johnson, {C. A.} and Joner, {M. D.} and J. Jones and J. Kaastra and S. Kaspi and Kelly, {B. C.} and Kelly, {P. L.} and Kennea, {J. A.} and M. Kim and S. Kim and Kim, {S. C.} and A. King and Klimanov, {S. A.} and Kochanek, {C. S.} and Korista, {K. T.} and Kriss, {G. A.} and Lau, {M. W.} and Lee, {J. C.} and Leonard, {D. C.} and M. Li and P. Lira and Z. Ma and F. Macinnis and Manne-Nicholas, {E. R.} and Malkan, {M. A.} and Mauerhan, {J. C.} and R. McGurk and McHardy, {I. M.} and C. Montouri and L. Morelli and A. Mosquera and D. Mudd and F. Muller-Sanchez and R. Musso and Nazarov, {S. V.} and H. Netzer and Nguyen, {M. L.} and Norris, {R. P.} and Nousek, {J. A.} and P. Ochner and Okhmat, {D. N.} and B. Ou-Yang and A. Pancoast and I. Papadakis and Parks, {J. R.} and L. Pei and Peterson, {B. M.} and A. Pizzella and R. Poleski and Pott, {J. U.} and Rafter, {S. E.} and Rix, {H. W.} and J. Runnoe and Saylor, {D. A.} and Schimoia, {J. S.} and K. Schn{\"u}lle and Sergeev, {S. G.} and Shappee, {B. J.} and I. Shivvers and M. Siegel and Simonian, {G. V.} and A. Siviero and A. Skielboe and G. Somers and M. Spencer and D. Starkey and Stevens, {D. J.} and Sung, {H. I.} and J. Tayar and N. Tejos and Turner, {C. S.} and P. Uttley and {Van Saders}, J. and M. Vestergaard and L. Vican and S. Villanueva and C. Villforth and Y. Weiss and Woo, {J. H.} and H. Yan and S. Young and H. Yuk and W. Zheng and W. Zhu and Y. Zu",

note = "Funding Information: We are grateful to Christine Done for discussions on the warm Comptonization model. Support for this work was provided by the National Aeronautics and Space Administration (NASA) through Chandra Award Number G04-15114X to S.M. issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. Support for HST program GO-13330 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. C.S.K. is supported in part by NSF grants AST-1515876 and AST-1515927. K.H. acknowledges support from STFC grant ST/ M001296/1. K.L.P. and P.A.E. ackowledge support from the UK Space Agency. A.V.F.ʼs group at UC Berkeley is grateful for financial assistance from NSF grant AST-1211916, the TABASGO Foundation, and the Christopher R. Redlich Fund. Y.K. acknowledges support from the grant PAIIPIT IN104215 and CONACYT grant168519. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester (see Evans et al. 2009). Publisher Copyright: {\textcopyright} 2017. The American Astronomical Society. All rights reserved.",

year = "2017",

month = sep,

day = "1",

doi = "10.3847/1538-4357/aa832b",

language = "English (US)",

volume = "846",

journal = "Astrophysical Journal",

issn = "0004-637X",

publisher = "IOP Publishing Ltd.",

number = "1",

}

TY - JOUR

T1 - Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy

AU - Mathur, S.

AU - Gupta, A.

AU - Page, K.

AU - Pogge, R. W.

AU - Krongold, Y.

AU - Goad, M. R.

AU - Adams, S. M.

AU - Anderson, M. D.

AU - Arévalo, P.

AU - Barth, A. J.

AU - Bazhaw, C.

AU - Beatty, T. G.

AU - Bentz, M. C.

AU - Bigley, A.

AU - Bisogni, S.

AU - Borman, G. A.

AU - Boroson, T. A.

AU - Bottorff, M. C.

AU - Brandt, W. N.

AU - Breeveld, A. A.

AU - Brown, J. E.

AU - Brown, J. S.

AU - Cackett, E. M.

AU - Canalizo, G.

AU - Carini, M. T.

AU - Clubb, K. I.

AU - Comerford, J. M.

AU - Coker, C. T.

AU - Corsini, E. M.

AU - Crenshaw, D. M.

AU - Croft, S.

AU - Croxall, K. V.

AU - Dalla Bontà, E.

AU - Deason, A. J.

AU - Denney, K. D.

AU - De Lorenzo-Cáceres, A.

AU - De Rosa, G.

AU - Dietrich, M.

AU - Edelson, R.

AU - Ely, J.

AU - Eracleous, M.

AU - Evans, P. A.

AU - Fausnaugh, M. M.

AU - Ferland, G. J.

AU - Filippenko, A. V.

AU - Flatland, K.

AU - Fox, O. D.

AU - Gates, E. L.

AU - Gehrels, N.

AU - Geier, S.

AU - Gelbord, J. M.

AU - Gorjian, V.

AU - Greene, J. E.

AU - Grier, C. J.

AU - Grupe, D.

AU - Hall, P. B.

AU - Henderson, C. B.

AU - Hicks, S.

AU - Holmbeck, E.

AU - Holoien, T. W.S.

AU - Horenstein, D.

AU - Horne, Keith

AU - Hutchison, T.

AU - Im, M.

AU - Jensen, J. J.

AU - Johnson, C. A.

AU - Joner, M. D.

AU - Jones, J.

AU - Kaastra, J.

AU - Kaspi, S.

AU - Kelly, B. C.

AU - Kelly, P. L.

AU - Kennea, J. A.

AU - Kim, M.

AU - Kim, S.

AU - Kim, S. C.

AU - King, A.

AU - Klimanov, S. A.

AU - Kochanek, C. S.

AU - Korista, K. T.

AU - Kriss, G. A.

AU - Lau, M. W.

AU - Lee, J. C.

AU - Leonard, D. C.

AU - Li, M.

AU - Lira, P.

AU - Ma, Z.

AU - Macinnis, F.

AU - Manne-Nicholas, E. R.

AU - Malkan, M. A.

AU - Mauerhan, J. C.

AU - McGurk, R.

AU - McHardy, I. M.

AU - Montouri, C.

AU - Morelli, L.

AU - Mosquera, A.

AU - Mudd, D.

AU - Muller-Sanchez, F.

AU - Musso, R.

AU - Nazarov, S. V.

AU - Netzer, H.

AU - Nguyen, M. L.

AU - Norris, R. P.

AU - Nousek, J. A.

AU - Ochner, P.

AU - Okhmat, D. N.

AU - Ou-Yang, B.

AU - Pancoast, A.

AU - Papadakis, I.

AU - Parks, J. R.

AU - Pei, L.

AU - Peterson, B. M.

AU - Pizzella, A.

AU - Poleski, R.

AU - Pott, J. U.

AU - Rafter, S. E.

AU - Rix, H. W.

AU - Runnoe, J.

AU - Saylor, D. A.

AU - Schimoia, J. S.

AU - Schnülle, K.

AU - Sergeev, S. G.

AU - Shappee, B. J.

AU - Shivvers, I.

AU - Siegel, M.

AU - Simonian, G. V.

AU - Siviero, A.

AU - Skielboe, A.

AU - Somers, G.

AU - Spencer, M.

AU - Starkey, D.

AU - Stevens, D. J.

AU - Sung, H. I.

AU - Tayar, J.

AU - Tejos, N.

AU - Turner, C. S.

AU - Uttley, P.

AU - Van Saders, J.

AU - Vestergaard, M.

AU - Vican, L.

AU - Villanueva, S.

AU - Villforth, C.

AU - Weiss, Y.

AU - Woo, J. H.

AU - Yan, H.

AU - Young, S.

AU - Yuk, H.

AU - Zheng, W.

AU - Zhu, W.

AU - Zu, Y.

N1 - Funding Information: We are grateful to Christine Done for discussions on the warm Comptonization model. Support for this work was provided by the National Aeronautics and Space Administration (NASA) through Chandra Award Number G04-15114X to S.M. issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. Support for HST program GO-13330 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. C.S.K. is supported in part by NSF grants AST-1515876 and AST-1515927. K.H. acknowledges support from STFC grant ST/ M001296/1. K.L.P. and P.A.E. ackowledge support from the UK Space Agency. A.V.F.ʼs group at UC Berkeley is grateful for financial assistance from NSF grant AST-1211916, the TABASGO Foundation, and the Christopher R. Redlich Fund. Y.K. acknowledges support from the grant PAIIPIT IN104215 and CONACYT grant168519. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester (see Evans et al. 2009). Publisher Copyright: © 2017. The American Astronomical Society. All rights reserved.

PY - 2017/9/1

Y1 - 2017/9/1

N2 - During the Space Telescope and Optical Reverberation Mapping Project observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the six-month-long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with the Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all three observations: the UV emission-line flux decrease, the soft-excess increase, and the emission-line anomaly.

AB - During the Space Telescope and Optical Reverberation Mapping Project observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the six-month-long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with the Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all three observations: the UV emission-line flux decrease, the soft-excess increase, and the emission-line anomaly.

KW - X-rays: galaxies

KW - X-rays: individual (NGC 5548)

KW - galaxies: active

KW - galaxies: individual (NGC 5548)

UR - http://www.scopus.com/inward/record.url?scp=85029067385&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85029067385&partnerID=8YFLogxK

U2 - 10.3847/1538-4357/aa832b

DO - 10.3847/1538-4357/aa832b

M3 - Article

AN - SCOPUS:85029067385

SN - 0004-637X

VL - 846

JO - Astrophysical Journal

JF - Astrophysical Journal

IS - 1

M1 - 55

ER -

Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy

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