Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. VIII. An intensive HST, IUE, and ground-based study of NGC 5548

K. T. Korista, D. Alloin, P. Barr, J. Clavel, R. D. Cohen, D. M. Crenshaw, I. N. Evans, K. Horne, A. P. Koratkar, G. A. Kriss, J. H. Krolik, M. A. Malkan, S. L. Morris, H. Netzer, P. T. O'Brien, B. M. Peterson, G. A. Reichert, P. M. Rodríguez-Pascual, W. Wamsteker, K. S J Anderson & 92 others D. J. Axon, E. Benitez, P. Berlind, R. Bertram, J. H. Blackwell, N. G. Bochkarev, C. Boisson, M. Carini, R. Carrillo, T. E. Carone, F. Z. Cheng, J. A. Christensen, K. K. Chuvaev, M. Dietrich, J. J. Dokter, V. Doroshenko, D. Dultzin-Hacyan, M. N. England, B. R. Espey, A. V. Filippenko, C. M. Gaskell, M. R. Goad, L. C. Ho, J. P. Huchra, X. J. Jiang, S. Kaspi, W. Kollatschny, A. Laor, J. P. Luminet, G. M. Macalpine, J. W. Mackenty, Yu F. Malkov, D. Maoz, P. G. Martin, T. Matheson, B. Mccollum, N. Merkulova, L. Metik, M. Mignoli, H. R. Miller, M. G. Pastoriza, D. Pelat, J. Penfold, M. Perez, G. C. Perola, J. L. Persaud, J. Peters, R. Pitts, R. W. Pogge, I. Pronik, V. I. Pronik, R. L. Ptak, L. Rawley, M. C. Recondo-González, J. M. Rodríguez-Espinosa, W. Romanishin, A. C. Sadun, I. Salamanca, M. Santos-Lleó, K. Sekiguchi, S. G. Sergeev, A. I. Shapovalova, J. C. Shields, C. Shrader, J. M. Shull, N. A. Silbermann, M. L. Sitko, D. R. Skillman, H. A. Smith, S. M. Smith, M. A J Snijders, L. S. Sparke, G. M. Stirpe, R. E. Stoner, W. H. Sun, U. Thiele, S. Tokarz, Zlatan Tsvetanov, D. A. Turnshek, S. Veilleux, R. M. Wagner, S. J. Wagner, I. Wanders, T. Wang, W. F. Welsh, R. J. Weymann, R. J. White, B. J. Wilkes, B. J. Wills, C. Winge, H. Wu, Z. L. Zou

Research output: Contribution to journalArticle

210 Citations (Scopus)

Abstract

We present the data and initial results from a combined HST/IUE/ground-based spectroscopic monitoring campaign on the Seyfert 1 galaxy NGC 5548 that was undertaken in order to address questions that require both higher temporal resolution and higher signal-to-noise ratios than were obtained in our previous multiwavelength monitoring of this galaxy in 1988-1989. IUE spectra were obtained once every 2 days for a period of 74 days beginning on 1993 March 14. During the last 39 days of this campaign, spectroscopic observations were also made with the HST Faint Object Spectrograph (FOS) on a daily basis. Ground-based observations, consisting of 165 optical spectra and 77 photometric observations (both CCD imaging and aperture photometry), are reported for the period 1992 October-1993 September, although many of the data are concentrated around the time of the satellite-based program. These data constitute a fifth year of intensive optical monitoring of this galaxy. In this contribution we describe the acquisition and reduction of all of the satellite and ground-based data obtained in this program. We describe in detail various photometric problems with the FOS and explain how we identified and corrected for various anomalies. During the HST portion of the monitoring campaign, the 1350 Å continuum flux is found to have varied by nearly a factor of 2. In other wave bands, the continuum shows nearly identical behavior, except that the amplitude of variability is larger at shorter wavelengths, and the continuum light curves appear to show more short-timescale variability at shorter wavelengths. The broad emission lines also vary in flux, with amplitudes that are slightly smaller than the UV continuum variations and with a small time delay relative to the UV continuum. On the basis of simple time-series analysis of the UV and optical continuum and emission-line light curves, we find (1) that the ultraviolet and optical continuum variations are virtually simultaneous with any lag between the 1350 Å continuum and the 5100 Å continuum amounting to less than about 1 day; (2) that the variations in the highest ionization lines observed, He II λ1640 and N v λ1240, lag behind the continuum variations by somewhat less than 2 days; and (3) that the velocity field of the C IV-emitting region is not dominated by radial motion. The results on the C IV velocity field are preliminary and quite uncertain, but there are some weak indications that the emission-line wings (|δv| ≥ 3000 km s-1) respond to continuum variations slightly more rapidly than does the core. The optical observations show that the variations in the broad Hβ line flux follow the continuum variations with a time lag of around 2 weeks, about twice the lag for Lyα and C IV, as in our previous monitoring campaign on this same galaxy. However, the lags measured for Lyα, C IV, and Hβ are each slightly smaller than previous determinations. We confirm two trends reported earlier, namely, (1) that the UV/optical continuum becomes "harder" as it gets brighter and (2) that the highest ionization emission lines have the shortest lags, thus indicating radial ionization stratification of a broad-line region that spans over an order of magnitude range in radius.

Original languageEnglish
Pages (from-to)285-330
Number of pages46
JournalAstrophysical Journal, Supplement Series
Volume97
Issue number2
Publication statusPublished - Apr 1995
Externally publishedYes

Fingerprint

IUE
active galactic nuclei
continuums
monitoring
ionization
time lag
wavelength
time series analysis
galaxies
faint objects
signal-to-noise ratio
stratification
light curve
spectrographs
timescale
anomaly
velocity distribution
H lines
temporal resolution
wavelengths

Keywords

  • Galaxies: active
  • Galaxies: individual (NGC 5548)
  • Galaxies: nuclei
  • Galaxies: photometry
  • Galaxies: Seyfert
  • Ultraviolet: Galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. VIII. An intensive HST, IUE, and ground-based study of NGC 5548. / Korista, K. T.; Alloin, D.; Barr, P.; Clavel, J.; Cohen, R. D.; Crenshaw, D. M.; Evans, I. N.; Horne, K.; Koratkar, A. P.; Kriss, G. A.; Krolik, J. H.; Malkan, M. A.; Morris, S. L.; Netzer, H.; O'Brien, P. T.; Peterson, B. M.; Reichert, G. A.; Rodríguez-Pascual, P. M.; Wamsteker, W.; Anderson, K. S J; Axon, D. J.; Benitez, E.; Berlind, P.; Bertram, R.; Blackwell, J. H.; Bochkarev, N. G.; Boisson, C.; Carini, M.; Carrillo, R.; Carone, T. E.; Cheng, F. Z.; Christensen, J. A.; Chuvaev, K. K.; Dietrich, M.; Dokter, J. J.; Doroshenko, V.; Dultzin-Hacyan, D.; England, M. N.; Espey, B. R.; Filippenko, A. V.; Gaskell, C. M.; Goad, M. R.; Ho, L. C.; Huchra, J. P.; Jiang, X. J.; Kaspi, S.; Kollatschny, W.; Laor, A.; Luminet, J. P.; Macalpine, G. M.; Mackenty, J. W.; Malkov, Yu F.; Maoz, D.; Martin, P. G.; Matheson, T.; Mccollum, B.; Merkulova, N.; Metik, L.; Mignoli, M.; Miller, H. R.; Pastoriza, M. G.; Pelat, D.; Penfold, J.; Perez, M.; Perola, G. C.; Persaud, J. L.; Peters, J.; Pitts, R.; Pogge, R. W.; Pronik, I.; Pronik, V. I.; Ptak, R. L.; Rawley, L.; Recondo-González, M. C.; Rodríguez-Espinosa, J. M.; Romanishin, W.; Sadun, A. C.; Salamanca, I.; Santos-Lleó, M.; Sekiguchi, K.; Sergeev, S. G.; Shapovalova, A. I.; Shields, J. C.; Shrader, C.; Shull, J. M.; Silbermann, N. A.; Sitko, M. L.; Skillman, D. R.; Smith, H. A.; Smith, S. M.; Snijders, M. A J; Sparke, L. S.; Stirpe, G. M.; Stoner, R. E.; Sun, W. H.; Thiele, U.; Tokarz, S.; Tsvetanov, Zlatan; Turnshek, D. A.; Veilleux, S.; Wagner, R. M.; Wagner, S. J.; Wanders, I.; Wang, T.; Welsh, W. F.; Weymann, R. J.; White, R. J.; Wilkes, B. J.; Wills, B. J.; Winge, C.; Wu, H.; Zou, Z. L.

In: Astrophysical Journal, Supplement Series, Vol. 97, No. 2, 04.1995, p. 285-330.

Research output: Contribution to journalArticle

Korista, KT, Alloin, D, Barr, P, Clavel, J, Cohen, RD, Crenshaw, DM, Evans, IN, Horne, K, Koratkar, AP, Kriss, GA, Krolik, JH, Malkan, MA, Morris, SL, Netzer, H, O'Brien, PT, Peterson, BM, Reichert, GA, Rodríguez-Pascual, PM, Wamsteker, W, Anderson, KSJ, Axon, DJ, Benitez, E, Berlind, P, Bertram, R, Blackwell, JH, Bochkarev, NG, Boisson, C, Carini, M, Carrillo, R, Carone, TE, Cheng, FZ, Christensen, JA, Chuvaev, KK, Dietrich, M, Dokter, JJ, Doroshenko, V, Dultzin-Hacyan, D, England, MN, Espey, BR, Filippenko, AV, Gaskell, CM, Goad, MR, Ho, LC, Huchra, JP, Jiang, XJ, Kaspi, S, Kollatschny, W, Laor, A, Luminet, JP, Macalpine, GM, Mackenty, JW, Malkov, YF, Maoz, D, Martin, PG, Matheson, T, Mccollum, B, Merkulova, N, Metik, L, Mignoli, M, Miller, HR, Pastoriza, MG, Pelat, D, Penfold, J, Perez, M, Perola, GC, Persaud, JL, Peters, J, Pitts, R, Pogge, RW, Pronik, I, Pronik, VI, Ptak, RL, Rawley, L, Recondo-González, MC, Rodríguez-Espinosa, JM, Romanishin, W, Sadun, AC, Salamanca, I, Santos-Lleó, M, Sekiguchi, K, Sergeev, SG, Shapovalova, AI, Shields, JC, Shrader, C, Shull, JM, Silbermann, NA, Sitko, ML, Skillman, DR, Smith, HA, Smith, SM, Snijders, MAJ, Sparke, LS, Stirpe, GM, Stoner, RE, Sun, WH, Thiele, U, Tokarz, S, Tsvetanov, Z, Turnshek, DA, Veilleux, S, Wagner, RM, Wagner, SJ, Wanders, I, Wang, T, Welsh, WF, Weymann, RJ, White, RJ, Wilkes, BJ, Wills, BJ, Winge, C, Wu, H & Zou, ZL 1995, 'Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. VIII. An intensive HST, IUE, and ground-based study of NGC 5548', Astrophysical Journal, Supplement Series, vol. 97, no. 2, pp. 285-330.
Korista, K. T. ; Alloin, D. ; Barr, P. ; Clavel, J. ; Cohen, R. D. ; Crenshaw, D. M. ; Evans, I. N. ; Horne, K. ; Koratkar, A. P. ; Kriss, G. A. ; Krolik, J. H. ; Malkan, M. A. ; Morris, S. L. ; Netzer, H. ; O'Brien, P. T. ; Peterson, B. M. ; Reichert, G. A. ; Rodríguez-Pascual, P. M. ; Wamsteker, W. ; Anderson, K. S J ; Axon, D. J. ; Benitez, E. ; Berlind, P. ; Bertram, R. ; Blackwell, J. H. ; Bochkarev, N. G. ; Boisson, C. ; Carini, M. ; Carrillo, R. ; Carone, T. E. ; Cheng, F. Z. ; Christensen, J. A. ; Chuvaev, K. K. ; Dietrich, M. ; Dokter, J. J. ; Doroshenko, V. ; Dultzin-Hacyan, D. ; England, M. N. ; Espey, B. R. ; Filippenko, A. V. ; Gaskell, C. M. ; Goad, M. R. ; Ho, L. C. ; Huchra, J. P. ; Jiang, X. J. ; Kaspi, S. ; Kollatschny, W. ; Laor, A. ; Luminet, J. P. ; Macalpine, G. M. ; Mackenty, J. W. ; Malkov, Yu F. ; Maoz, D. ; Martin, P. G. ; Matheson, T. ; Mccollum, B. ; Merkulova, N. ; Metik, L. ; Mignoli, M. ; Miller, H. R. ; Pastoriza, M. G. ; Pelat, D. ; Penfold, J. ; Perez, M. ; Perola, G. C. ; Persaud, J. L. ; Peters, J. ; Pitts, R. ; Pogge, R. W. ; Pronik, I. ; Pronik, V. I. ; Ptak, R. L. ; Rawley, L. ; Recondo-González, M. C. ; Rodríguez-Espinosa, J. M. ; Romanishin, W. ; Sadun, A. C. ; Salamanca, I. ; Santos-Lleó, M. ; Sekiguchi, K. ; Sergeev, S. G. ; Shapovalova, A. I. ; Shields, J. C. ; Shrader, C. ; Shull, J. M. ; Silbermann, N. A. ; Sitko, M. L. ; Skillman, D. R. ; Smith, H. A. ; Smith, S. M. ; Snijders, M. A J ; Sparke, L. S. ; Stirpe, G. M. ; Stoner, R. E. ; Sun, W. H. ; Thiele, U. ; Tokarz, S. ; Tsvetanov, Zlatan ; Turnshek, D. A. ; Veilleux, S. ; Wagner, R. M. ; Wagner, S. J. ; Wanders, I. ; Wang, T. ; Welsh, W. F. ; Weymann, R. J. ; White, R. J. ; Wilkes, B. J. ; Wills, B. J. ; Winge, C. ; Wu, H. ; Zou, Z. L. / Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. VIII. An intensive HST, IUE, and ground-based study of NGC 5548. In: Astrophysical Journal, Supplement Series. 1995 ; Vol. 97, No. 2. pp. 285-330.
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title = "Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. VIII. An intensive HST, IUE, and ground-based study of NGC 5548",
abstract = "We present the data and initial results from a combined HST/IUE/ground-based spectroscopic monitoring campaign on the Seyfert 1 galaxy NGC 5548 that was undertaken in order to address questions that require both higher temporal resolution and higher signal-to-noise ratios than were obtained in our previous multiwavelength monitoring of this galaxy in 1988-1989. IUE spectra were obtained once every 2 days for a period of 74 days beginning on 1993 March 14. During the last 39 days of this campaign, spectroscopic observations were also made with the HST Faint Object Spectrograph (FOS) on a daily basis. Ground-based observations, consisting of 165 optical spectra and 77 photometric observations (both CCD imaging and aperture photometry), are reported for the period 1992 October-1993 September, although many of the data are concentrated around the time of the satellite-based program. These data constitute a fifth year of intensive optical monitoring of this galaxy. In this contribution we describe the acquisition and reduction of all of the satellite and ground-based data obtained in this program. We describe in detail various photometric problems with the FOS and explain how we identified and corrected for various anomalies. During the HST portion of the monitoring campaign, the 1350 {\AA} continuum flux is found to have varied by nearly a factor of 2. In other wave bands, the continuum shows nearly identical behavior, except that the amplitude of variability is larger at shorter wavelengths, and the continuum light curves appear to show more short-timescale variability at shorter wavelengths. The broad emission lines also vary in flux, with amplitudes that are slightly smaller than the UV continuum variations and with a small time delay relative to the UV continuum. On the basis of simple time-series analysis of the UV and optical continuum and emission-line light curves, we find (1) that the ultraviolet and optical continuum variations are virtually simultaneous with any lag between the 1350 {\AA} continuum and the 5100 {\AA} continuum amounting to less than about 1 day; (2) that the variations in the highest ionization lines observed, He II λ1640 and N v λ1240, lag behind the continuum variations by somewhat less than 2 days; and (3) that the velocity field of the C IV-emitting region is not dominated by radial motion. The results on the C IV velocity field are preliminary and quite uncertain, but there are some weak indications that the emission-line wings (|δv| ≥ 3000 km s-1) respond to continuum variations slightly more rapidly than does the core. The optical observations show that the variations in the broad Hβ line flux follow the continuum variations with a time lag of around 2 weeks, about twice the lag for Lyα and C IV, as in our previous monitoring campaign on this same galaxy. However, the lags measured for Lyα, C IV, and Hβ are each slightly smaller than previous determinations. We confirm two trends reported earlier, namely, (1) that the UV/optical continuum becomes {"}harder{"} as it gets brighter and (2) that the highest ionization emission lines have the shortest lags, thus indicating radial ionization stratification of a broad-line region that spans over an order of magnitude range in radius.",
keywords = "Galaxies: active, Galaxies: individual (NGC 5548), Galaxies: nuclei, Galaxies: photometry, Galaxies: Seyfert, Ultraviolet: Galaxies",
author = "Korista, {K. T.} and D. Alloin and P. Barr and J. Clavel and Cohen, {R. D.} and Crenshaw, {D. M.} and Evans, {I. N.} and K. Horne and Koratkar, {A. P.} and Kriss, {G. A.} and Krolik, {J. H.} and Malkan, {M. A.} and Morris, {S. L.} and H. Netzer and O'Brien, {P. T.} and Peterson, {B. M.} and Reichert, {G. A.} and Rodr{\'i}guez-Pascual, {P. M.} and W. Wamsteker and Anderson, {K. S J} and Axon, {D. J.} and E. Benitez and P. Berlind and R. Bertram and Blackwell, {J. H.} and Bochkarev, {N. G.} and C. Boisson and M. Carini and R. Carrillo and Carone, {T. E.} and Cheng, {F. Z.} and Christensen, {J. A.} and Chuvaev, {K. K.} and M. Dietrich and Dokter, {J. J.} and V. Doroshenko and D. Dultzin-Hacyan and England, {M. N.} and Espey, {B. R.} and Filippenko, {A. V.} and Gaskell, {C. M.} and Goad, {M. R.} and Ho, {L. C.} and Huchra, {J. P.} and Jiang, {X. J.} and S. Kaspi and W. Kollatschny and A. Laor and Luminet, {J. P.} and Macalpine, {G. M.} and Mackenty, {J. W.} and Malkov, {Yu F.} and D. Maoz and Martin, {P. G.} and T. Matheson and B. Mccollum and N. Merkulova and L. Metik and M. Mignoli and Miller, {H. R.} and Pastoriza, {M. G.} and D. Pelat and J. Penfold and M. Perez and Perola, {G. C.} and Persaud, {J. L.} and J. Peters and R. Pitts and Pogge, {R. W.} and I. Pronik and Pronik, {V. I.} and Ptak, {R. L.} and L. Rawley and Recondo-Gonz{\'a}lez, {M. C.} and Rodr{\'i}guez-Espinosa, {J. M.} and W. Romanishin and Sadun, {A. C.} and I. Salamanca and M. Santos-Lle{\'o} and K. Sekiguchi and Sergeev, {S. G.} and Shapovalova, {A. I.} and Shields, {J. C.} and C. Shrader and Shull, {J. M.} and Silbermann, {N. A.} and Sitko, {M. L.} and Skillman, {D. R.} and Smith, {H. A.} and Smith, {S. M.} and Snijders, {M. A J} and Sparke, {L. S.} and Stirpe, {G. M.} and Stoner, {R. E.} and Sun, {W. H.} and U. Thiele and S. Tokarz and Zlatan Tsvetanov and Turnshek, {D. A.} and S. Veilleux and Wagner, {R. M.} and Wagner, {S. J.} and I. Wanders and T. Wang and Welsh, {W. F.} and Weymann, {R. J.} and White, {R. J.} and Wilkes, {B. J.} and Wills, {B. J.} and C. Winge and H. Wu and Zou, {Z. L.}",
year = "1995",
month = "4",
language = "English",
volume = "97",
pages = "285--330",
journal = "Astrophysical Journal, Supplement Series",
issn = "0067-0049",
publisher = "IOP Publishing Ltd.",
number = "2",

}

TY - JOUR

T1 - Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. VIII. An intensive HST, IUE, and ground-based study of NGC 5548

AU - Korista, K. T.

AU - Alloin, D.

AU - Barr, P.

AU - Clavel, J.

AU - Cohen, R. D.

AU - Crenshaw, D. M.

AU - Evans, I. N.

AU - Horne, K.

AU - Koratkar, A. P.

AU - Kriss, G. A.

AU - Krolik, J. H.

AU - Malkan, M. A.

AU - Morris, S. L.

AU - Netzer, H.

AU - O'Brien, P. T.

AU - Peterson, B. M.

AU - Reichert, G. A.

AU - Rodríguez-Pascual, P. M.

AU - Wamsteker, W.

AU - Anderson, K. S J

AU - Axon, D. J.

AU - Benitez, E.

AU - Berlind, P.

AU - Bertram, R.

AU - Blackwell, J. H.

AU - Bochkarev, N. G.

AU - Boisson, C.

AU - Carini, M.

AU - Carrillo, R.

AU - Carone, T. E.

AU - Cheng, F. Z.

AU - Christensen, J. A.

AU - Chuvaev, K. K.

AU - Dietrich, M.

AU - Dokter, J. J.

AU - Doroshenko, V.

AU - Dultzin-Hacyan, D.

AU - England, M. N.

AU - Espey, B. R.

AU - Filippenko, A. V.

AU - Gaskell, C. M.

AU - Goad, M. R.

AU - Ho, L. C.

AU - Huchra, J. P.

AU - Jiang, X. J.

AU - Kaspi, S.

AU - Kollatschny, W.

AU - Laor, A.

AU - Luminet, J. P.

AU - Macalpine, G. M.

AU - Mackenty, J. W.

AU - Malkov, Yu F.

AU - Maoz, D.

AU - Martin, P. G.

AU - Matheson, T.

AU - Mccollum, B.

AU - Merkulova, N.

AU - Metik, L.

AU - Mignoli, M.

AU - Miller, H. R.

AU - Pastoriza, M. G.

AU - Pelat, D.

AU - Penfold, J.

AU - Perez, M.

AU - Perola, G. C.

AU - Persaud, J. L.

AU - Peters, J.

AU - Pitts, R.

AU - Pogge, R. W.

AU - Pronik, I.

AU - Pronik, V. I.

AU - Ptak, R. L.

AU - Rawley, L.

AU - Recondo-González, M. C.

AU - Rodríguez-Espinosa, J. M.

AU - Romanishin, W.

AU - Sadun, A. C.

AU - Salamanca, I.

AU - Santos-Lleó, M.

AU - Sekiguchi, K.

AU - Sergeev, S. G.

AU - Shapovalova, A. I.

AU - Shields, J. C.

AU - Shrader, C.

AU - Shull, J. M.

AU - Silbermann, N. A.

AU - Sitko, M. L.

AU - Skillman, D. R.

AU - Smith, H. A.

AU - Smith, S. M.

AU - Snijders, M. A J

AU - Sparke, L. S.

AU - Stirpe, G. M.

AU - Stoner, R. E.

AU - Sun, W. H.

AU - Thiele, U.

AU - Tokarz, S.

AU - Tsvetanov, Zlatan

AU - Turnshek, D. A.

AU - Veilleux, S.

AU - Wagner, R. M.

AU - Wagner, S. J.

AU - Wanders, I.

AU - Wang, T.

AU - Welsh, W. F.

AU - Weymann, R. J.

AU - White, R. J.

AU - Wilkes, B. J.

AU - Wills, B. J.

AU - Winge, C.

AU - Wu, H.

AU - Zou, Z. L.

PY - 1995/4

Y1 - 1995/4

N2 - We present the data and initial results from a combined HST/IUE/ground-based spectroscopic monitoring campaign on the Seyfert 1 galaxy NGC 5548 that was undertaken in order to address questions that require both higher temporal resolution and higher signal-to-noise ratios than were obtained in our previous multiwavelength monitoring of this galaxy in 1988-1989. IUE spectra were obtained once every 2 days for a period of 74 days beginning on 1993 March 14. During the last 39 days of this campaign, spectroscopic observations were also made with the HST Faint Object Spectrograph (FOS) on a daily basis. Ground-based observations, consisting of 165 optical spectra and 77 photometric observations (both CCD imaging and aperture photometry), are reported for the period 1992 October-1993 September, although many of the data are concentrated around the time of the satellite-based program. These data constitute a fifth year of intensive optical monitoring of this galaxy. In this contribution we describe the acquisition and reduction of all of the satellite and ground-based data obtained in this program. We describe in detail various photometric problems with the FOS and explain how we identified and corrected for various anomalies. During the HST portion of the monitoring campaign, the 1350 Å continuum flux is found to have varied by nearly a factor of 2. In other wave bands, the continuum shows nearly identical behavior, except that the amplitude of variability is larger at shorter wavelengths, and the continuum light curves appear to show more short-timescale variability at shorter wavelengths. The broad emission lines also vary in flux, with amplitudes that are slightly smaller than the UV continuum variations and with a small time delay relative to the UV continuum. On the basis of simple time-series analysis of the UV and optical continuum and emission-line light curves, we find (1) that the ultraviolet and optical continuum variations are virtually simultaneous with any lag between the 1350 Å continuum and the 5100 Å continuum amounting to less than about 1 day; (2) that the variations in the highest ionization lines observed, He II λ1640 and N v λ1240, lag behind the continuum variations by somewhat less than 2 days; and (3) that the velocity field of the C IV-emitting region is not dominated by radial motion. The results on the C IV velocity field are preliminary and quite uncertain, but there are some weak indications that the emission-line wings (|δv| ≥ 3000 km s-1) respond to continuum variations slightly more rapidly than does the core. The optical observations show that the variations in the broad Hβ line flux follow the continuum variations with a time lag of around 2 weeks, about twice the lag for Lyα and C IV, as in our previous monitoring campaign on this same galaxy. However, the lags measured for Lyα, C IV, and Hβ are each slightly smaller than previous determinations. We confirm two trends reported earlier, namely, (1) that the UV/optical continuum becomes "harder" as it gets brighter and (2) that the highest ionization emission lines have the shortest lags, thus indicating radial ionization stratification of a broad-line region that spans over an order of magnitude range in radius.

AB - We present the data and initial results from a combined HST/IUE/ground-based spectroscopic monitoring campaign on the Seyfert 1 galaxy NGC 5548 that was undertaken in order to address questions that require both higher temporal resolution and higher signal-to-noise ratios than were obtained in our previous multiwavelength monitoring of this galaxy in 1988-1989. IUE spectra were obtained once every 2 days for a period of 74 days beginning on 1993 March 14. During the last 39 days of this campaign, spectroscopic observations were also made with the HST Faint Object Spectrograph (FOS) on a daily basis. Ground-based observations, consisting of 165 optical spectra and 77 photometric observations (both CCD imaging and aperture photometry), are reported for the period 1992 October-1993 September, although many of the data are concentrated around the time of the satellite-based program. These data constitute a fifth year of intensive optical monitoring of this galaxy. In this contribution we describe the acquisition and reduction of all of the satellite and ground-based data obtained in this program. We describe in detail various photometric problems with the FOS and explain how we identified and corrected for various anomalies. During the HST portion of the monitoring campaign, the 1350 Å continuum flux is found to have varied by nearly a factor of 2. In other wave bands, the continuum shows nearly identical behavior, except that the amplitude of variability is larger at shorter wavelengths, and the continuum light curves appear to show more short-timescale variability at shorter wavelengths. The broad emission lines also vary in flux, with amplitudes that are slightly smaller than the UV continuum variations and with a small time delay relative to the UV continuum. On the basis of simple time-series analysis of the UV and optical continuum and emission-line light curves, we find (1) that the ultraviolet and optical continuum variations are virtually simultaneous with any lag between the 1350 Å continuum and the 5100 Å continuum amounting to less than about 1 day; (2) that the variations in the highest ionization lines observed, He II λ1640 and N v λ1240, lag behind the continuum variations by somewhat less than 2 days; and (3) that the velocity field of the C IV-emitting region is not dominated by radial motion. The results on the C IV velocity field are preliminary and quite uncertain, but there are some weak indications that the emission-line wings (|δv| ≥ 3000 km s-1) respond to continuum variations slightly more rapidly than does the core. The optical observations show that the variations in the broad Hβ line flux follow the continuum variations with a time lag of around 2 weeks, about twice the lag for Lyα and C IV, as in our previous monitoring campaign on this same galaxy. However, the lags measured for Lyα, C IV, and Hβ are each slightly smaller than previous determinations. We confirm two trends reported earlier, namely, (1) that the UV/optical continuum becomes "harder" as it gets brighter and (2) that the highest ionization emission lines have the shortest lags, thus indicating radial ionization stratification of a broad-line region that spans over an order of magnitude range in radius.

KW - Galaxies: active

KW - Galaxies: individual (NGC 5548)

KW - Galaxies: nuclei

KW - Galaxies: photometry

KW - Galaxies: Seyfert

KW - Ultraviolet: Galaxies

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M3 - Article

VL - 97

SP - 285

EP - 330

JO - Astrophysical Journal, Supplement Series

JF - Astrophysical Journal, Supplement Series

SN - 0067-0049

IS - 2

ER -