Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://hea-www.harvard.edu/~pgreen/Papers/aox_composites98.pdf Äàòà èçìåíåíèÿ: Mon Sep 19 19:23:34 2005 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 03:31:01 2012 Êîäèðîâêà: koi8-r Ïîèñêîâûå ñëîâà: dust disk

THE ASTROPHYSICAL JOURNAL, 498 : 170 õ 180, 1998 May 1
( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

DIFFERENCES BETWEEN THE OPTICAL/ULTRAVIOLET SPECTRA OF X-RAY BRIGHT AND X-RAY FAINT QSOs PAUL J. GREEN
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 ; pgreen=cfa.harvard.edu Received 1997 July 29 ; accepted 1997 December 4

ABSTRACT We contrast measurements of composite optical and ultraviolet (UV) spectra constructed from samples of QSOs deïned by their soft X-ray brightness. X-ray bright (XB) composites show stronger emission lines in general, but particularly from the narrow-line region. The dierence in the [O III]/Hb ratio is particularly striking, and is even more so when blended Fe II emission is properly subtracted. The correlation of this ratio with X-ray brightness was a principal component of QSO spectral diversity found by Boroson & Green. We ïnd here that other, much weaker narrow optical forbidden lines ([O II] and [Ne V]) are enhanced by factors of 2 to 3 in our XB composites, and that narrow line emission is also strongly enhanced in the XB UV composite. Broad permitted-line ÿuxes are slightly larger for all XB spectra, but the narrow-/broad-line ratio stays similar or increases strongly with X-ray brightness for all strong permitted lines except Hb. Spectral dierences between samples divided by their relative X-ray brightness (as measured by a ) ox exceed those seen between complementary samples divided by luminosity or radio loudness. We propose that the Baldwin eect may be a secondary correlation to the primary relationship between a and emission line equivalent width. We conclude that either (1) W depends primarily on the shape of ox ionizing the j continuum, as crudely characterized here by a , or (2) both W and a are related to some third paramox ox eter characterizing the QSO physics. One such possibility isj intrinsic warm absorption ; a soft X-ray absorber situated between the broad and narrow line regions can successfully account for many of the properties observed. Subject headings : galaxies : active õ quasars : emission lines õ quasars : general õ ultraviolet : galaxies õ X-rays : galaxies
1

. INTRODUCTION

Most QSOs have been discovered by virtue of their strong optical/UV emission lines or nonstellar colors in this bandpass. Our understanding to date of the violent inner regions of active galactic nuclei (AGNe) also derive in large part from their optical/ultraviolet (OUV) spectra. The production of QSO emission lines is widely attributed to photoionization and heating of the emitting gas by the UV to X-ray continuum (see, e.g., Ferland & Shields 1985 ; Krolik & Kallman 1988). Studies investigating the relationship of emission lines to continuum radiation have a long history in the ïeld, but several strong observational relationships remain unexplained. If the proportionality between line and continuum strength were linear, then diagnostics such as line ratios and W would be independent of continuum luminosity. j Baldwin (1977) ïrst noticed that, in high-redshift quasars, the W of the C IV 1550 ñ emission line in quasars decreases j with increasing UV (1450 ñ) luminosity. The Baldwin eect (BE, hereafter) was also found to be strong for ions such as O VI, N V, He II, C III], Mg II, and Lya (see, e.g., Tytler & Fan 1992 ; Zamorani et al. 1992). The initial excitement about the potential for the BE as a standard candle and cosmological probe has faded ; the dispersion in the relationship is too large. Neither the source of that dispersion nor the cause of the BE itself have been deïnitively identiïed. However, some possible explanations for the BE have been oered, one being a dependence of the continuum spectral energy distribution (SED) on luminosity (Zheng & Malkan 1993 ; Green 1996). Many important lines respond primarily to the extreme ultraviolet (EUV) or soft X-ray continuum. Unfortunately, 170

the EUV band is severely obscured by Galactic absorption. However, constraints on the ionizing continuum are available through analysis of the adjacent UV and soft X-ray windows. In a small, uniform sample of optically selected QSOs (Laor et al. 1997), the strongest correlation found between X-ray continuum and optical emission line parameters was of the soft X-ray spectral slope a (where f P X l l~aX) with the FWHM of the Hb emission line. Strong correlations between a , L , Fe II/Hb, and the X *O III+ [O III]/Hb ratio were seen both there and in previous QSO studies (e.g., Boroson & Green 1992). The latter authors found that most of the variation in the observed properties of low-redshift QSOs can be represented in a principal component analysis by eigenvectors linking Fe II, [O III], Hb, and He II emission line properties and continuum properties, such as radio loudness, and the relative strength of X-ray emission, as characterized by a (deïned below). A ox recent, and possibly related result is that Seyfert 1 galaxies with broad Hb emission lines tend to have hard (ÿat) X-ray spectral slopes (see, e.g., Brandt, Mathur, & Elvis 1997). In a hard X-ray selected sample of (mostly Seyfert) AGNe, narrow [O III] ÿux correlates well with X-ray ÿux, while broad Balmer lines do not (Grossan 1992). The physical origin of these diverse and interrelated correlations has yet to be determined. We are launching a large-scale eort to probe these eects in large samples, using both data and analysis as homogeneous as possible. Many physically informative trends intrinsic to QSOs may be masked by dispersion in the data caused by either low signal-to-noise ratios or variability. An important tool for studying global properties of QSOs is the coaddition of data for samples of QSOs. In this


SPECTRA OF X-RAY BRIGHT AND X-RAY FAINT QSOs paper, we concentrate on an analysis of composite optical/ UV spectra of subsamples of QSOs grouped by the relative strength of their soft X-ray emission.
2

171

. ANALYSIS

2.1. Constructing Comparison Samples Although the signal-to-noise ratio (S/N) for the individual spectra in both samples we study here tends to be about 10 or less per resolution element, the coaddition (averaging) of spectra with similar continuum properties allow us to increase the S/N and constrain the properties of the average QSO. Through averaging, a greater number of emission lines, with a wider range of ionization energies, and ïner details in emission line proïles become measurable. This technique has been applied in several recent studies (see, e.g., Cristiani & Vio 1990 ; Francis et al. 1991 ; Osmer, Porter, & Green 1994 ; Zheng et al. 1996). What is lost is a reliable measure of the intrinsic dispersion in the observed correlations. However, it is important to ïrst discover the correlations intrinsic to the average QSO. The sources of dispersion in the relationship can later be studied if data of adequate S/N are available for a large enough sample. Table 1 summarizes mean continuum properties for the X-ray bright and X-ray faint subsamples we culled from the Large Bright Quasar Survey and IUE QSO samples described below. Optical and X-ray luminosities are taken from Green et al. (1995) and Green (1996) and assume H \ 50 km s~1 Mpc~1 and q \ 0.5. The slope of a hypo0 thetical power law connecting 0 2500 ñ and 2 keV is deïned as a \ 0.384 log (l /l ), so that a is larger for objects ox withox stronger opticalopt X relative to X-ray emission.
2.1.1. L BQS Sample

The Large Bright Quasar Survey (LBQS ; Hewett, Foltz, & Chaee 1995) is a sample of more than 1000 QSOs, uniformly selected over a wide range of redshifts. LBQS QSO candidates were selected using the Automatic Plate Mea-

suring Machine (Irwin & Trimble 1984) to scan UK Schmidt direct photographic and objective prism plates. A combination of quantiïable selection techniques were used, including color selection, selection of objects with strong emission lines, and selection of objects having redshifted absorption features or continuum breaks. The technique appears to be highly efficient at ïnding QSOs with 0.2 \ z \ 3.3, a signiïcantly broader range than in past work. Follow-up (6 õ10 ñ resolution) optical spectra with S/N B 10 (in the continuum at D4500 ñ) were obtained at the Multiple Mirror Telescope (MMT) and the 2.5 m duPont telescope. The digital spectra used here were graciously provided to the author by Craig Foltz. Soft X-ray data for LBQS QSOs was selected from the ROSAT All-Sky Survey (RASS) as detailed in Green et al. (1995). Of the 908 QSOs in the LBQS/RASS sample, 92 are detected in X-rays. For the nondetections, we assign an upper limit of 4 p to the raw counts. Before generating composite spectra, we remove QSOs with known broad absorption lines (BAL QSOs) from the LBQS/RASS sample. Above z \ 1, an insufficient fraction of QSOs are detected to construct X-ray bright and faint subsamples of comparable size. We therefore exclude QSOs with redshifts [1, for which the RASS data are insufficiently sensitive to be of use. To create composite spectra of similar S/N, we choose a dividing point of a that results in ox similarly sized subsamples. We construct the X-ray bright sample (XB ; 60 QSOs) of only detected objects, with a \ ox 1.475. The X-ray faint sample (XF ; 54 QSOs) includes both detections and lower limits with a º 1.475. QSOs with lower limits below that value could ox rightly belong either to the XB or XF sample and so are excluded from consideration. Note also that the value of 1.475 does not imply that a is measured to such accuracy. Rather, it provides a conox venient dividing line near the median of the small range of a values (D1.2õ1.6) typically measured in QSOs. oxContinuum properties of the ïnal XF and XB (a -selecox ted) samples are listed in Table 1. In survival analysis, if the

TABLE 1 CONTINUUM PARAMETERS FOR QSO SUBSAMPLES SAMPLE SIZE PARAMETER Redshift ...... SUBSAMPLE LBQS/XF LBQS/XB IUE/XF IUE/XB LBQS/XF LBQS/XB IUE/XF IUE/XB LBQS/XF LBQS/XB IUE/XF IUE/XB LBQS/XF LBQS/XB IUE/XF IUE/XB N total 54 60 23 25 54 60 23 25 54 60 23 25 54 60 23 25 N limits 0 0 0 0 0 0 0 0 42 0 4 0 42 0 0 0 a MEDIAN 0.46 0.45 0.35 0.21 30.41 30.30 31.03 30.50 1.61 1.34 1.57 1.31 22.55 26.78 26.91 27.17 MEAN 0.49 0.51 0.57 0.34 30.44 30.30 31.05 30.57 1.61 1.35 1.60 1.28 25.96 26.78 26.78 27.25 RMS 0.03 0.03 0.09 0.03 0.06 0.06 0.15 0.08 0.01 0.01 0.03 0.02 0.08 0.06 0.20 0.09 TRUNCATEDb No No No No No No No No 1.67 No No No 25.64 No No No PROBABILITYc (%) 43 2.5 17 25 \10~4 \10~4 \10~4 1

log l

opt

.......

a ............ ox

log l ......... X

NOTE.õFor LBQS/XF, z\1 and a º 1.475 ; for LBQS/XB, z \ 1 and a \ 1.475, detections only ; for IUE/XF, ox ox a º 1.4 ; and for IUE/XB, a \ 1.4, detections only. ox a All limits are upper limits except for a . ox ox b This column lists the value of an outlying nondetection, if it was necessarily redeïned, truncating the distribution to allow normalization. The resulting mean value is biased. c Probability that the two subsample distributions for this parameter are randomly drawn from the same parent distribution, from ASURV logrank test.


172

GREEN
TABLE 2 COMPOSITE NORMALIZATION DEFINITIONS Wavelength Region j n Continuum Band LBQS Spectra 4150 3800 3200 2000 õ5100 õ 4800 õ 4000 õ3000 .......... .......... .......... .......... 4863 4102 3426 2798 4710 4020 3360 2645 õ õ õ õ 4780 4050 3400 2700 5040 4150 3450 3020 õ5090 õ 4270 õ3500 õ3100 Hb Hd [Ne V] Mg II Continuum Band Line Name

Vol. 498

IUE Spectra 1000 õ2000 .......... 1549 1450 õ1480 1680 õ1700 C IV

NOTE.õEach individual rest-frame QSO spectrum is divided by the ÿux of the ït line at the normalization wavelength j before addition to a composite spectrum. The continuum level n at j is found by ïtting a linear continuum between two points, the mean ÿux values in the n two continuum bands shown, at their central wavelength points.

lowest (highest) point in the data set is an upper (lower) limit, the mean is not well deïned since the distribution is not normalizable, and so the outlying censored point is redeïned as a detection. For the XF sample, we list here the value of that redeïned limit at which the distribution is truncated. The resulting mean value is biased, so that the true mean values of a and log l for the LBQS XF sample ox X are probably even more X-ray faint than those listed. We ïnd no signiïcant dierences in the distributions of redshift, NGal, or l between our XB and XF subsamples of opt the LBQS.HIn any case, the emission line properties of the LBQS as a whole show no strong dependence on either luminosity or redshift (Francis, Hooper, & Impey 1993).
2.1.2. IUE Sample

To explore changes in QSO UV spectra as a function of a , we include a previously compiled sample of QSOs ox
60 40 20 2000 60 40 20 3200 60 40 20 3800 60 40 20 4200 4400 4600 4800 5000 4000 4200 4400 4600 3400 3600 3800 4000 2200 2400 2600 2800

observed by both the International Ultraviolet Explorer (IUE) and Einstein. This sample was selected as described in Green (1996), by requiring that the QSOs in the IUE sample have Einstein soft X-ray data available. In Wilkes et al. (1994), we deïne the IUE/Einstein sample of 49 objects. Again, we remove all known BAL QSOs from the IUE sample. With this sample, because of selection eects, it is not possible to achieve similar-sized subsamples with similar redshift distributions by selecting an appropriate a ox value at which to divide the sample. Similar redshift distributions are obtained for a \ 1.3, but then the XB subsample contains only about aox dozen QSOs, compared to 37 in the XF subsample. Similar subsample sizes are obtained for a \ 1.4, for which, however, the requirement of a ox detection in the XB sample results in a lower mean redshift, due mostly to extra QSOs between 0.1 \ z \ 0.2. Since QSO spectral evolution between their mean redshifts (0.34 and 0.57, respectively) is negligible, we emphasize the subsampleîs split at a \ 1.4, which is in any case closer to the dividing value for ox LBQS subsamples. However, we also the check the subsampleîs split at a \ 1.3 to ensure that evoluox

FIG. 1.õHistograms of the number of QSOs whose spectra contribute to each of eight LBQS composite spectra. Four separate continuum points were used to generate LBQS composite spectra for XB and XF subsamples in four separate wavelength regions (see Table 2). Composites normalized at the four dierent continuum points have dierent histograms, since they require the rest frame continuum bands to be present in all contributing observed frame spectra. Dierent selection criteria for the XB (solid lines) and the XF subsamples (dotted lines) also yield slightly dierent histograms.

FIG. 2.õHistograms of the number of QSOs whose spectra contribute to the IUE composite spectra for the XB subsample (solid line) and the XF subsample (dotted line). The IUE composites were all normalized at the rest wavelength of C IV.


No. 1, 1998

SPECTRA OF X-RAY BRIGHT AND X-RAY FAINT QSOs

173

tion or luminosity eects do not signiïcantly bias our results. 2.2. Constructing Composite Spectra We ïrst deredshift each individual spectrum by dividing the linear dispersion coefficients (initial wavelength and wavelength per pixel) by (1 ] z). An estimate of the rest frame continuum ÿux f at a chosen normalization point is c, l then derived by ïtting a linear continuum between two continuum bands. The widths and centers of these bands, typically chosen to straddle an important emission line, are listed for each wavelength region in Table 2. We then normalize the entire spectrum via division by f . c, l Each normalized spectrum is ïrst rebinned to the dispersion of the composite spectrum, conserving ÿux via interpolation. We choose 2.5 ñ bins, similar to the majority of the individual LBQS spectra. The IUE spectra are binned to 1.18 ñ, as in the original short-wavelength prime (SWP) camera spectra. Each normalized, rebinned spectrum is then stored as a vector in a two-dimensional array. Finally, for each pixel in the completed array, the number of spectra N with ÿuxes in that rest frame wavelength bin is tallied. If N º 3, the median and the mean of all the ÿux values in the bin are computed and stored in the ïnal one-dimensional composite spectra, as is N in the ïnal histogram array. A similar procedure is performed for each bin, until the spectra are completed. The strong sky line at j5577, sometimes poorly subtracted, was omitted from the LBQS composite. Geocoronal Lya lines were similarly excluded when building the IUE composites. We chose four separate continuum points to generate LBQS composite spectra in four wavelength regions (Table 2). Composites normalized at dierent continuum points may have dierent histograms, since they require the rest frame continuum bands to be present in all contributing observed frame spectra. Histograms of the number of QSOs contributing to each LBQS composite are shown in Figure 1. The IUE composites were all normalized at the wavelength of C IV, and the histograms for those spectra are shown in Figure 2. The median composites appear to be smoother than the means, since they are less aected by spikes and low S/N features in the individual spectra. As a result, the s2 values of model ïts are also lower for median composites. However, since the median omits QSOs with more extreme spectral properties, we prefer to analyze the average composites. We check the signiïcance of every strong dierence between XB and XF average composites by measurement of the median composites to determine whether outliers dominate the feature in question.
3

(normalized) line ÿuxes if desired. The intercept value reÿects the arbitrary normalization of individual spectra to unity at the chosen wavelength j shown in Table 2. n The emission line components are assumed to be Gaussian and symmetric (skew ïxed at unity), so output from the ïts includes ÿux, centroid, and FWHM for each line. Results from these ïts are shown in Table 3 and Figure 3. The errors listed in both Tables 3 and 4 are directly from the SPECFIT task, which assumes that the errors on the input spectrum follow a Gaussian distribution. The tabulated errors represent 1 p for a single interesting parameter. 3.1. TheHb and [O III] Region We ït a PL continuum using these relatively line-free regions : jj4020 õ 4050, 4150 õ 4270, 4420 õ 4450, 4710 õ 4780, and 5070 õ5130. The resulting ït parameters are ïxed, while the following single Gaussian components are ït to the strong emission lines between 4150 to 5100 ñ : (1) broad Hb ; (2) narrow Hb, with FWHM ïxed to that of [O III] j5007 ; (3) [O III] j5007 ; and (4) [O III] j4959, ïxed relative to the central wavelength and (1/3) ÿux of component (3). Finally, the blended iron emission above the PL continuum is summed over the wavelength range jj4434 õ 4684. As can be seen from Figure 3 and Table 3, by far the strongest eect is the great relative strength of [O III] emission in the XB composite ; [O III] emission is D2.5 times stronger in X-ray bright sample. Broad Hb emission is about 40% stronger in the XB composite, although the FWHM are similar at D5700 km s~1. The strengths of the narrow components of Hb are comparable. In both XB and XF composites, broad Hb is redshifted relative to rest (i.e., relative to [O III] j5007 or narrow Hb) by about 300 km s~1. Boroson & Green (1992 ; hereafter BG92), in a principal component analysis of low-redshift QSO spectra, found that the principal eigenvector of their sample (the linear combination of parameters that represents the largest variance in the sample) is primarily an anticorrelation between the strength of [O III] emission and optical Fe II strength. This trend is in the same direction as we ïnd here, with the large W ([O III]) XB sample having the lower Fe II j equivalent width, as judged by the iron emission near j4600 (Table 3). We can make more precise measurements in this region if we subtract Fe II emission. Marianne Vestergaard has accomplished this task, using a technique similar to that of BG92. A narrow optical Fe II template spectrum (of IZw1 ; BG92) is convolved with a Gaussian function and subtracted from the LBQS composites, ïtting iteratively by eye. The ÿux in the iron template between j4434 and j4684 is within D10% of that found via our simple summation technique. Because of a strong iron multiplet with lines at jj4924, 5015, and 5169, however, subtraction of Fe II decreases the apparent W ([O III]) in both the XF and XB j composites. In the XF composite, where iron emission is strong, the measured W ([O III]) at j5007 decreases from j 13.5 to 7.5, and its measured FWHM decreases from 1330 to 650 km s~1. The XF Hb line changes by comparison only slightly after iron subtraction (from a total equivalent width of 53 to 47). In the XB composite, where iron emission is weak, the measured change in equivalent width of both [O III] and Hb after Fe II subtraction are within the errors (*W \ 3 ñ), as are the changes in FWHM. Since the j strength of [O III] in the XF composite is initially low, the

. EMISSION LINE MEASUREMENTS : LBQS

No analysis of continuum slope is presented for the LBQS spectra, since these observations are not fully spectrophotometric. Using the IRAF task SPECFIT (Kriss 1994), we ït simple empirical models to the LBQS composite spectra in four wavelength regions, deïned in Table 2. In each region, we begin by ïtting a power-law (PL) continuum using only comparatively line-free regions (listed for each region below). The resulting continuum ït parameters are slope a and intercept f , such that ÿux f \ 1000 j f (j/1000)a. Since the LBQS spectra are not spectro1000 photometric, the continuum ït results should be used only to derive exact composite equivalent widths from


174

GREEN

Vol. 498

FIG. 3.õFinal normalized composite spectra of the LBQS sample. The left-hand column contains plots of the XB composites, while the right-hand column shows XF composites. Each row shows a portion of a composite constructed from spectra normalized at a dierent rest wavelength continuum point (see Table 2) : j2798 (top row), j3426 (second row), j4102 (third row), and j4863 (bottom row). Individual lines and their parameters measured using SPECFIT (Kriss 1994) are listed in Table 3. Fitted components are shown in each plot. These include the best power-law continuum ït to relatively line-free continuum regions (dashed lines) and Gaussian emission lines (dotted lines). Lines are identiïed by name in the X-ray faint plots.

dierence in the measured [O III]/Hb ratio between XF and XB composites is substantially increased by Fe II subtraction. Much smaller eects are expected on other emission lines treated here. An important extension of this technique to the UV will be described in an upcoming paper (Vestergaard & Green 1998). 3.2. TheHd Region First, we ït a PL continuum through the windows : jj3790 õ3845, 4020 õ 4050, 4150 õ 4270, 4420 õ 4450, and 4710 õ 4780. Gaussian components are ït between 3800 and 4800 ñ to the following lines : [Ne III] j3869, He I j3889, [S II]/Hd j4068/jj(4076 ] 4102), and Hc/[O III] jj(4340 ] 4363). The only signiïcant dierence seen

between emission lines in this region is that the Hc/[O III] blend is stronger (at the 2.7 p level) in the XF composite. 3.3. The [Ne V] and [O II] Region Line-free regions at jj3200 õ3400, 3500 õ3700, and 3800 õ 4000 are used for the PL ït. Then single Gaussian components are ït for [Ne V] j3426 and [O II] j3727. As can be seen from Figure 3 and Table 3, both the [Ne V] and [O II] line ÿuxes are much stronger in the XB composite, consistent with the relative strengths of other unblended narrow lines measured. For the XF sample, the very weak [O II] line could not be successfully ït when leaving both j and c FWHM free to vary, so we ïxed the central wavelength at 3727 ñ. Similarly, the weakness of [Ne V] demanded that its


No. 1, 1998

SPECTRA OF X-RAY BRIGHT AND X-RAY FAINT QSOs
TABLE 3 SPECFIT RESULTS FOR LBQS COMPOSITES Component 2000 õ3000 ñ continuum ...... Fe II ............................. Mg II broadb ................... Mg II narrow ................... 3200 õ 4000 ñ continuum ...... Ne Vd ........................... [O II]e .......................... 3800 õ 4800 ñ continuum ...... [Ne III] ......................... He I ............................. [S II] ] Hd ..................... Hc ] [O III] .................... 4150 õ5100 ñ continuum ...... Fe II ............................. Hb broad ....................... Hb narrow ..................... [O III] j5007 ................... Parameter f 1000 a fa f j fc j c FWHM f 1000 a f j c f FWHM f 1000 a f j c FWHM f j c FWHM f j c FWHM f j c FWHM f 1000 a fg f j c FWHM f j c FWHMh f j c FWHM XB Composite 2.44 ^ 0.89 ^ 41.1 ^ 42.0 ^ 2796.9 ^ 15.94 ^ 2798.8 ^ 2650 ^ 31.3 ^ 2.81 ^ 1.89 ^ 3425.6 ^ 2.10 ^ 900 ^ 13.5 ^ 1.82 ^ 2.9 ^ 3870.8 ^ 1330 ^ 1.45 ^ 3894 ^ 2100 ^ 4.06 ^ 4103.0 ^ 1330 ^ 15.3 ^ 4347.5 ^ 3240 ^ 30.0 ^ 2.17 ^ 26.5 ^ 64.0 ^ 4869.3 ^ 5660 ^ 9.1 ^ 4865.6 ^ 976 32.32 ^ 5010.2 ^ 976 ^ 0.07 0.03 0.2 2.5 3.5 1.38 0.8 230 1.2 0.03 0.22 1.2 0.24 120 1.3 0.07 1.2 3.6 240 1.3 15 1500 0.6 1.5 240 1.0 1.5 250 3.3 0.07 0.2 2.6 1.7 290 1.4 1.1 1.11 0.3 43 XF Composite 1.79 ^ 0.04 0.59 ^ 0.02 36.8 ^ 0.1 36.4 ^ 2.1 2801.6c 11.26 ^ 1.11 2801.6 ^ 0.8 2530 ^ 240 29.2 ^ 0.8 2.76 ^ 0.02 1.06 ^ 0.15 3425.5 ^ 1.8 0.65 ^ 0.17 1090 ^ 290 35.6 ^ 3.1 2.50 ^ 0.06 2.3 ^ 0.6 3867.3 ^ 4.1 1200f 1.43 ^ 0.6 3887 ^ 6 1200f 4.10 ^ 1.0 4106.7 ^ 6.1 3440 ^ 780 20.7 ^ 1.7 4348.4 ^ 2.8 5200 ^ 620 47.0 ^ 3.4 2.43 ^ 0.05 31.1 ^ 0.1 42.8 ^ 2.5 4872.5 ^ 2.5 5730 ^ 360 10.7 ^ 1.7 4864.2 ^ 1.3 1330 13.46 ^ 0.95i 5009.9 ^ 0.8 1330 ^ 108 so at j c

175

NOTE.õAll ÿuxes are summed over a locally normalized continuum (see text) and are approximately equivalent widths. The value of the (normalized) continuum ÿux any wavelength j can be obtained via f \ f (j/1000)~a. Emission line centroids are in ñ, while FWHM are shown in kmcs~1.1000 a Fe II summed over PL ït from jj2210 õ2730. b FWHM ïxed at 15,000 km s~1. c j ïxed at narrow Mg II values. c d FWHM ïxed at 1000 km s~1. e j ïxed at 3727 ñ. c f FWHM ïxed at 1200 km s~1. g Fe II summed over PL ït from jj4434 õ 4684. h FWHM ïxed at [O III] j5007 FWHM. i W ([O III]) of XF composite decreases to 7.5 ^ 1 after Fe II subtraction. j

FWHM be ïxed because otherwise SPECFIT seeks to ït neighboring features with an unrealistically broad line. The use of composite spectra enables us here for the ïrst time to show that these narrow lines are about 3 times as strong in the X-ray bright QSOs as in their X-ray faint analogs. 3.4. The Mg II Region Here a PL continuum is ït to jj2195 õ2275, 2645 õ2700, and 3020 õ3100 windows. The spectrum from 2000 to 3200 ñ is then ït with the ïxed PL plus these Gaussian components : narrow Mg II and broad Mg II, ïxed in wavelength to the narrow component. Without ïxing the broad line width, the ït would wander o trying to include nearby blended iron lines. Finally, the blended iron emission above

the PL continuum is summed over the wavelength range 2210 õ2730 ñ. We ïnd here that the XB sample has stronger UV Fe II emission (Table 4), opposite to the trend for optical Fe II. Green et al. (1995) also found that QSOs in the LBQS with strong UV Fe II emission are particularly bright in the soft X-ray bandpass. UV Fe II lines have their principal ionizing and heating continuum above 500 eV ; Fe II lines in the optical are principally caused by continuum above 800 eV.
4

. EMISSION LINE MEASUREMENTS : IUE

For the IUE spectra, convincing model ïts to the composite spectra could be obtained when ïtting a simple PL continuum simultaneously with lines. The following single


176

GREEN
TABLE 4 SPECFIT RESULTS FOR IUE COMPOSITES Component 1000 õ2000 ñ continuum ...... Lyb/O VI ....................... Lya broad ...................... Lya narrow .................... N V ............................. O I .............................. S IV ............................. C IV broad ..................... C IV narrow .................... Absorption ..................... Parameter a f 1000 f j c FWHM f j c FWHM f j c FWHM f j c FWHM f j c FWHM f j c FWHM f j c FWHM f j c FWHM f j c FWHM XB Composite 1.51 0.75 19.6 1033.5 5230 87.3 1218.8 12840 47.3 1214.6 3030 3.9 1240 6000 2.6 1310 6700 8.4 1398 6250 52.9 1546.2 12500 30.6 1549.09 3200 5.8 1487 12900 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 0.02 0.02 1.1 0.4 400 3.9 0.8 410 1.7 0.1 80 2.5 3 1200 0.9 4 3400 1.0 1.7 870 1.3 1.6 2800 6.6 0.2 350 3.9 19 1800 XF Composite 1.57 0.75 10.1 1032.5 4560 78.7 1218.7 12700 36.8 1214.9 3470 1.2 1244 6700 6.8 1312 6400 11.2 1395 6270 39.2 1548.3 12000 14.8 1548.34 3400 8.8 1480 12500 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 0.02 0.03 1.0 0.6 450 2.9 0.5 400 2.2 0.2 130 0.3 1 3200 1.9 4 2100 1.6 2.3 950 9.0 1.6 2500 7.9 0.6 940 1.4 12 2800

Vol. 498

NOTE.õParameters have the same meaning as in Table 3.

Gaussian components are ït to the spectrum between 1000 and 2000 ñ : (1) Lyb/O VI, (2) broad Lya, (3) narrow Lya, (4) N V, (5) O I, (6) S IV, (7) broad C IV, (8) narrow C IV, and (9) C IV absorption. Again, Gaussian components are assumed to be symmetric. The data do not warrant multiple Gaussian ïts to lines as weak or weaker than N V. However, some such lines are included in the ït partly to insure that a reasonable continuum estimate is derived. Results from these ïts, shown in Table 4 and Figure 4, are described below. 1. The PL continuum ïts are similar in slope. At the 2 p level, the XF sample may be marginally steeper. If veriïed in other samples, this could be caused by reddening or absorption by dust. However, the similarity of the slopes suggests that the relative attenuation between the XB and XF spectra is gray. 2. Just as for the LBQS composites, we ïnd for the IUE sample that line emission is signiïcantly stronger in the XB composite. Lyb/O VI, Lya, and C IV, when ït with a single Gaussian component, are each signiïcantly ([3 p) stronger in the XB composite. For Lya, the narrow/broad line ÿux ratios are higher in the XB composite as well. Some marginal evidence (D1.5 p) for the same eect is seen for C IV. From Table 4, it is clear that the Lyb/O VI emission line shows a large contrast. The inverse correlation between Lyb/O VI and a was also remarked recently in a sample of ox individual Hubble Space T elescope (HST ) Faint Object Spectrograph (FOS) spectra (Zheng, Kriss, & Davidsen 1995). Among weaker emission lines, O I j1302 and Si IV/ O IV] show a marginal (\2 p) trend in the opposite direction, with smaller W in the XB composite. There is also a j

FIG. 4.õFinal normalized composite spectra of the IUE/Einstein sample. The top plot is the XB composite, and the bottom shows the XF composite. Both these IUE composites were constructed from spectra normalized at j1549, the rest wavelength of C IV. Individual lines and their measured parameters using SPECFIT (Kriss 1994) are listed in Table 4. The best-ït components are shown in each plot. These include a power-law continuum (dashed lines) ït simultaneously with Gaussian emission lines (dotted lines). Lines are identiïed by name in the X-ray faint plot. Note the apparent continuum deïcit centered near j1480 in the XF composite.


No. 1, 1998

SPECTRA OF X-RAY BRIGHT AND X-RAY FAINT QSOs

177

suggestion that in the XF composite, N V is