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Scattering and Absorption in Soft X-ray Selected AGN: An Optical Polarization Survey

We have surveyed the optical linear polarization of a completely identified sample of 43 bright soft-X-ray-selected ROSAT AGN. Most (40) of these AGN show low polarization (<1%), and no clear optical reddening. This supports the suggestion from rapid X-ray variability, disk-like spectral energy distributions, and lack of cold X-ray absorption, that we are viewinga bare AGN disk. IRAS F12397+3333 and IRAS 13349+2438 show high polarization increasing to the UV -- clear evidence for scattering. As well as steep, soft-X-ray spectra, they show optical reddening and rapid X-ray variability, but almost no cold X-ray absorption -- a combination that suggests dusty ionized gas along the line-of-sight. Brandt et al. suggested and found these `warm absorbers' for IRAS 13349+2438. IRAS F12397+3333 is a new candidate. Combining our data with the optical and X-ray spectra of the high polarization narrow-line Seyfert 1 nuclei (NLSy1s) investigated by Goodrich reveals strong correlations among optical reddening indicators (such as the optical continuum slope and Balmer decrement), [OIII]/H-beta, and cold intrinsic X-ray absorption. Optical reddening underpredicts the cold X-ray absorption, suggesting dusty warm absorbers in all the highly polarized AGN. The existence of these scattering-polarized and reddened NLSy1s suggests an orientation Unified Scheme within the class of NLSy1s, analogous to that linking Seyfert 1 and Seyfert 2 nuclei. For some highly polarized and optically selected AGN we present new analysis of archival X-ray data, and for the highly polarized AGN new optical spectroscopy is presented in an appendix.


Introduction
A popular idea among astronomers is that all AGN are powered by the same kind of engine: accretion of matter through an accretion disk onto a super-massive black hole. This idea receives strong support from high, Keplerian velocities of gas in the centers of active galaxies, and alignment of the axes of these gaseous disks with powerful radio jets. Understanding of the basic fueling mechanism remains elusive, but there may be new clues, related to the classic energy-budget problem and the so-called First Principal Component, both linking the ionizing continuum and the emission-line regions: (a) The energy budget problem is that there appears to be insufficient ionizing continuum photons to power the broad emission lines, especially the strong, blended Fe II lines (e.g. Netzer 1985;Collin-Souffrin et al. 1988). Solutions may be that the Fe II arises from a different region, excited not by photoionization but by mechanical heating of the gas (e.g. Norman & Miley 1984;Joly 1991), or that the continuum incident on the broad emission-line gas is different from that observed.
(b) In AGN's UV-optical spectra there are several clear, related trends: increasing broad-line Fe II emission corresponds to decreasing width (FWHM) of Hβ from the broad-line region (BLR), and to decreasing line emission from lower velocity, lower density gas -[O III] λ5007 emission from the narrow line region (NLR), and UV line emission from lower velocity gas of the BLR (Boroson & Green 1992;Laor et al. 1997;Lawrence et al. 1997;Brotherton et al. 1997, in preparation). ROSAT, with its high sensitivity to soft X-rays, has played an important role in showing that these trends correspond to increasing steepness of the soft X-ray spectra (Boller et al. 1996, Laor et al. 1997. Correlation analyses show that the dominant spectrum-to-spectrum variation in low and high redshift AGN samples can be reduced to a linear combination of these emission line and continuum parameters -the 'First Principal Component'. This First Principal Component points to a single, as yet mysterious, underlying mechanism relating AGN's ionizing continuum (∼ 0.01-0.2 keV), the optical Fe II emission, and emission from the BLR and NLR.
While rarely discovered in optical surveys, narrow-line Seyfert 1 nuclei (NLSy1s) are found in abundance in soft-X-ray surveys. These AGN, having broad Hβ emission lines with FWHM ∼ < 2000 km s −1 , and steep, strong, soft-X-ray spectra, lie at the rich-Fe II extreme of the First Principal Component, and so could give us insight into central engine mechanisms.
One hypothesis proposed to explain their strong, often time-variable, soft X-ray emission, their steep X-ray spectra and narrow BLR Hβ emission lines, is that NLSy1s represent unobscured, low-mass AGN (Boller et al. 1996;Grupe 1996;Grupe et al. 1997Grupe et al. , 1997. Lower mass means lower gas velocities, producing narrower Hβ emission from the BLR. The strong, soft Xray-emission arises from an unobscured view of the inner regions of a smaller, and therefore hotter, accretion disk. This direct, rather than scattered-light, view of the tiny emission region allows very rapid variability. Further support for this hypothesis is an observed anticorrelation between optical and soft-X-ray spectral indices, and, in the most luminous AGN, an optical continuum slope equal to that expected from optically thick, thermal, disk emission. On this hypothesis, NLSy1s should be unpolarized. Another hypothesis suggests that NLSy1s are Seyfert 1s viewed pole-on. Kinematic projection of a disk-like BLR results in a narrower radial velocity distribution, and steep X-ray spectra result from a view of the 'funnel' of a geometrically thick accretion disk (Madau 1988).
Perhaps some fraction of these NLSy1s are like the archetype of highly polarized QSOs, IRAS 13349+2438 (Wills et al. 1992b. The soft X-ray spectrum of this AGN is strong, variable, and steep, with no signs of absorption by neutral gas. In the optical, it shows typical NLSy1 characteristics -narrow Hβ from the BLR, very strong Fe II, and very weak [O III] λ5007. In this AGN there are two views to the central continuum source and BLR -one a direct view that is partially obscured by dusty hot gas in the torus, and the other a less obscured, but indirect, view in which the spectrum is scattering-polarized by electrons or dust in the opening cone of the torus. The unusual combination of UV-optical reddening and lack of cold X-ray absorption was presented by Brandt et al. (1996) as evidence for dusty, ionized gas near the nucleus. Discovery of more such objects would allow further optical-UV-X-ray investigation of this newly discovered constituent of AGN gas.
The above pictures for NLSy1s are not mutually exclusive. In axisymmetric Unified Schemes, a low-mass or thick-disk central engine may be surrounded by a dusty nuclear torus. A low-inclination polar view may reveal an unobscured central engine, while a higher inclination line-of-sight may graze the dusty nuclear torus, passing through ionized gas (e.g. IRAS 13349+2438). As dust fur-ther dims the direct view of the nucleus, the scattered, polarized AGN spectrum becomes more prominent. At even higher inclinations, cold dusty gas blocks the direct view completely, but a few percent of the NLSy1 spectrum could be scattered towards the observer. Even in a high inclination view, low density NLR gas, ionized on kpc scales, may reveal the presence of a buried NLSy1 in total light. The well-known "Seyfert 1 nucleus buried within the prototype Seyfert 2" galaxy NGC 1068, the Rosetta Stone of Unified Schemes (Antonucci & Miller 1985), may be an excellent high-inclination example with the parsecscale AGN seen only in scattered light -a faint, polarized spectrum with UV-optical continuum, strong Fe II blends, and BLR Hβ intrinsic FWHM ∼2900 km s −1 (Miller et al. 1991), together with a very steep, unabsorbed soft X-ray spectrum (α x ∼2.4), showing no variability on several-year time-scales (Elvis & Lawrence 1988;Marshall et al. 1993;Smith et al. 1993).
In this paper we have used broad-band polarimetry and spectroscopy of a ROSAT sample of soft-X-rayselected AGN to relate scattering and absorption in the optical and X-rays, in order to investigate anisotropic emission, kinematics and dusty gas distribution in these special AGN. In Sect. 2 we describe the sample, as well as observations and data reduction for the optical polarimetry, ROSAT X-ray spectroscopy, and optical spectroscopy. Sect. 3 presents the results, including some correlations.
Here we provide more detailed information for the highly polarized sources, and collect together comparable X-ray and optical data for the AGN of Goodrich's (1989) spectropolarimetry study of optically selected NLSy1s. Sect. 4 discusses the absorption and scattering properties of the sample as a whole, and for individual sources, including a comparison with the optically selected NLSy1s. A summary is given in Sect. 5. Appendix A shows our spectra for all the highly polarized sources discussed in this paper.

The Soft X-ray Sample
The sample of bright, soft-X-ray AGN  was selected from the ROSAT All-Sky Survey (RASS, Voges et al. 1993Voges et al. , 1997. This deep survey, using the Position Sensitive Proportional Counter (PSPC, Pfeffermann et al. 1986), was the first to extend to soft-X-ray photon energies ∼ 0.1 keV. Our X-ray sample is completely identified, comprising all 95 AGN with RASS PSPC count rates ∼ >0.5 cts s −1 , hardness ratio HR1 ∼ <0.0, and |b| > 20 • . HR1 is defined as the ratio of the difference to the sum of hard and soft counts of the source, where the hard and soft bands are defined between 0.4-2.4 keV and 0.1-0.4 keV. This HR1 limit corresponds to a steep soft X-ray spectrum with α x ∼ > 1.5 for N H ∼ 2 × 10 20 cm −2 . The bright, soft X-ray selection results in a sample with very little or no neutral hydrogen absorption. The X-ray and optical properties of the sample were presented and discussed by Grupe (1996) and Grupe et al. (1997, and1997 in preparation). About half the sample are NLSy1s. The median FWHM for broad Hβ is 2250 km s −1 , with a 90-percentile range from ∼1300 km s −1 to 4200 km s −1 . We restrict the present polarization survey and analysis to the 43 northern sources (δ > 0 • ).

Polarimetry Observations
For the soft X-ray sample, linear polarization was measured using a broad-band polarimeter (Breger 1979) on the Struve 2.1-m telescope at McDonald Observatory. The detector was a cooled (∼ −20C) Hamamatsu R943-02 phototube with extended red sensitivity [GaAs(Cs) photocathode] operated in photon-counting mode. A new interface and data acquisition computer resulted in improved noise characteristics with uncertainties consistent with photon statistics. Otherwise, the filters, observational technique (a few 200-second integrations on-source, interleaved with background integrations), calibrations, and reductions were as described by Wills et al. (1992a). All measurements were made with an aperture of projected diameter 7.4 ′′ . We were interested in searching for scattering polarization, so we aimed for 1σ measurement uncertainties ∼ <0.4%, because interstellar polarization mechanisms may become important at that level, and are then difficult to distinguish from scattering, based on degree of polarization and wavelength-dependence. In some cases where the measurements indicated real polarization we estimated the contribution from Galactic interstellar polarization by measuring the polarization of stars nearby in projection on the sky. In order to probe as long a path length as possible through our galaxy we chose (where possible) bright, blue stars, preferably known to be of early spectral type. Of the 43 northern AGN, 26 were observed by us and data for 17 were taken from the literature.

X-ray data
The RASS data have been used for the statistical investigation of the soft-X-ray sample, except for well-known AGN. Details of the reductions are given by Grupe (1996) and Grupe et al. (1997). For the 19 well-known sources we retrieved pointed observation data from the ROSAT public archive at MPE Garching. The January 1996 version of the EXSAS package (Zimmermann et al. 1994) was used for data reduction and analysis. The X-ray spectral index α x (F ν ∝ ν −αx , the photon index Γ = 1+α x ) was determined in the energy range 0.1 -2.4 keV using single power-law fits, assuming neutral absorption given by Milky Way HI 21cm column densities N H,gal (Dickey & Lockman 1990) with abundances given by Morrison & Mc-Cammon (1983). Uncertainties in α x are typically 0.1 to 0.5, depending on X-ray brightness. Grupe et al. (1997) also made fits to the X-ray spectra with unconstrained neutral hydrogen column-density N H . Although the latter was rarely significantly different from Galactic values, we measured the difference ∆N H = N H − N H,gal as a potential indicator of intrinsic neutral hydrogen absorption. For those objects for which no X-ray spectra were readily available, we used the hardness ratios HR1 and HR2 given in the bright source catalogue (Voges et al. 1997). HR2 is like HR1, but with hard and soft bands defined between 0.9-2.0 keV and 0.5-0.9 keV. The method is to simulate count-rate spectra for a grid of α x and N H values, using the known instrumental response. From these spectra we calculate HR1 and HR2 for each combination of α x and N H . Thus we derive the unique combination α x and N H that reproduce the observed HR1 and HR2. Uncertainties in ∆N H are similar to those in N H determined from the fits with N H unconstrained. The standard deviations for individual values are about 0.8 ×10 20 cm −2 and 1.2 ×10 20 cm −2 as determined from the median and mean of the distribution of standard deviations given from the fitting.
For the highly polarized AGN for which ROSAT pointed observations were available, the data were analyzed in the same way as for the RASS data.
The emission line measurements and their uncertainties are presented and described in detail by Grupe et al. 1997, in preparation). Some representative 1σ errors are given in later tables. The blended optical Fe II emission was measured and subtracted using a I Zw 1 template like that used by Boroson & Green (1992). All other measurements were made from these Fe II-subtracted spectra. To determine the rest-frame equivalent width of Fe II we measured the flux in the template between rest wavelengths 4250Å and 5880Å, after scaling it to match the object spectra, and then divided by the continuum flux density at 5050Å. In the following, Hβ refers to the whole Hβ line. Hβ b refers to the broad component of Hβ, obtained after subtraction of the narrow component by using the [O III]λ5007 velocity profile as a template. This method subtracts as much of the template as possible while still retaining a sensible shape to the peak of Hβ b . For sources where the narrow-line emission dominates we found an [O III]/Hβ narrow intensity ratio of about 10, which is not significantly different from the observed median in Seyfert 1s and Seyfert 1.5s (Koski 1978;Cohen 1983), providing support for our method.
To determine the Hα/Hβ intensity ratio, we used the total flux in the lines, because the broad and narrow components are difficult to separate for the Hα blend. In general, to subtract the fluxes of the [N II] λλ6548,6584 lines from the Hα blend, it was quite adequate to subtract 35% of the [O III]λ5007 line flux from it (Ferland & Osterbrock 1986). For the highly polarized sources, as we see later, reddening of the NLR emission could be important. So for these we used velocity templates derived from [O III] and total Hβ lines to disentangle the Hα-[N II] blend.
Optical continuum spectral indices α opt were measured between rest-frame wavelengths of 4400Å and 7000Å.

The Optically Selected Sample
For comparison with our soft-X-ray-selected sample, we have compiled polarization, spectroscopic and X-ray data for the optically selected sample of NLSy1s that Goodrich (1989) investigated by spectropolarimetry. While the selection of that sample is based on a mixture of spectroscopic criteria (narrow Hβ emission, strong Fe II emission, [O III]λ5007/Hβ < 3, and presence of high-ionization lines) it is unbiased with respect to soft-X-ray properties (α x or ∆N H ). Measurements were made, as far as possible, in the same way as for our sample. Emission-line measurements were derived from literature data, except for the more highly polarized sources for which we obtained new spectroscopic data including both the Hβ and Hα regions (Appendix A).

The ROSAT Soft X-ray AGN Sample
The polarimetry data are given in Table 1, where we list the object's RX J name derived from the RASS J2000 coordinates, other name, the filter used, degree of polarization p%, and the polarization position angle (E vector, measured on the sky plane in degrees from north through east), and a literature reference where relevant. Errors in p% are 1σ, based on photon-counting statistics. The corresponding position-angle error may be estimated from 28.65 • × the fractional error in p%. No corrections have been applied for positive bias in p%. Neither have we made any attempt to correct for unpolarized starlight from the host galaxy. Table 2 gives measurements we made for interstellar polarization. In no case was this large enough to justify a correction to our AGN polarizations.  Three of the 43 objects show p >1%. While we have chosen 1% as a practical cut-off for investigating scattered-light polarization, we note that two other AGN, with detailed results from the literature, show low but significant polarization, above interstellar levels. One is the well-known NLSy1 NGC 4051. The other is the broad-lined NGC 5548, with HR1 ∼0.
Because one motivation of this study was to investigate Unified Schemes for NLSy1s, we ask whether there are systematic differences between high and low polarization AGN. The FWHM(Hβ b ) of the highly polarized AGN are not extremely narrow but typical for the soft X-ray sample. Also, α x ∼1.7 for the highly polarized AGN, so their soft X-ray spectra tend to be flat compared with most of our sample; but steep compared with ROSAT spectra of optically-selected Seyfert 1 nuclei, for which α x ∼ 1.0 -1.7 .
Reddening of the broad lines and continuum is another test for whether we have a direct view to a bare nucleus, with the narrow Hβ b and steep α x being intrinsic properties, or whether soft-X-ray-selected AGN with narrow Hβ b may be viewed through dusty gas near the nucleus, as in orientation Unified Schemes. In Fig. 2 we compare α opt and the Balmer decrement, as indicators of reddening. The low polarization AGN (most of the open circles) have small Hα/Hβ ratios, and flat optical spectra (small α opt ), and show no relation between Hα/Hβ and α opt outside the expected relation resulting from systematic calibration uncertainties, so reddening is unlikely to be important for them. While the low polarization sources do not show significant reddening, the highly polarized sources do. All the polarized AGN (filled symbols) have Balmer decrements ∼ >6 (except CBS 126, which we discuss later). The one low polarization source with Balmer decrement >5.8 is IC 3599, which is thought to be a Seyfert 2,  We can also test how the highly polarized sources might be related to the First Principal Component. In Fig.  3 we show the line ratio [O III]/Hβ b vs. equivalent width of the optical Fe II blend, EW(Fe II). The low polarization sources of the present sample show the well known inverse [O III] -Fe II trend seen for radio-quiet AGN as a whole (Boroson & Green 1992. Our high polarization sources appear to lie at larger [O III]/Hβ b or larger EW(Fe II) than the low polarization sources (see Sect. 4).

Comparison with an Optically Selected NLSy1 Sample
The data for the optically selected NLSy1 sample are presented in Table 3 where we list the object's common and RX J name, percentage polarization and angle, X-ray count rates and hardness ratio, α x , ∆N H , FWHM(Hβ b ) and [O III]/Hβ b ratio. The polarization data are mean values over the whole observed wavelength range (Goodrich 1989). Because soft-X-ray absorption is more probable in this non-soft-X-ray-selected sample we give the α x determined from an X-ray spectral fit with N H unconstrained. Sources of data are given below the table.
As expected, the optical selection criteria for this sample result in narrower Hβ b and a wider range of HR1 compared with the X-ray-selected sample. Only half of the optically selected sample have steep α x ∼ > 1.5.
Out of 17 objects, several are polarized at a low level, but three show continuum p>1%, even >2%: Mkn 1239, Mkn 766, and IRAS 15091−2107. As for the highly polarized sources of the soft-X-ray sample, these three have FHWM (

The Polarized AGN
Two of the three highly polarized AGN of our sample show high and wavelength-dependent polarization -IRAS F12397+3333 (Was 61, Wasilewski 1983) and IRAS 13349+2438 (Fig. 4). The IRAS 13349+2438 polarization has been the subject of intensive investigation as a result of its membership in a warm IRAS sample (Sect. 1). While it provided motivation for the present survey, its inclusion in our statistical discussion is justified because it was discovered by completely independent techniques and was not appreciated to be a member of our sample until much later. But IRAS F12397+3333 is new. The strong increase of polarization towards shorter wavelengths is a clear indication of the dilution of a scattered light AGN spectrum by a redder unpolarized component -either reddened AGN light or starlight. The other highly polarized source, CBS 126, appears to be in a different category. The polarization is small and not strongly wavelength depen- Table 4. Properties of the highly polarized AGN. Those of our soft-X-ray sample, together with the median and the 90% ranges for the whole soft X-ray sample are to the left. The numbers involved are given in brackets after the median value. The right side of the table gives the values for Goodrich's highly polarized NLSy1s. The following properties are tabulated: optical monochromatic luminosity log νLV in Watts, assuming H0 = 75 km s −1 Mpc −1 and q0 = 0.5; redshift z; p%; FWHM(Hβ b ) in km s −1 , corrected for instrumental resolution; the intensity ratio [O III]/Hβ b ; Fe II/Hβ b ; rest frame equivalent width EW(Fe II) inÅ; the total line flux Hα/Hβ ratio; αopt between 4400Å and 7000Å; αx, the X-ray spectral index with NH unconstrained; ∆NH, the intrinsic soft-X-ray absorption column density in units of 10 20 cm −2 ; ∆NH,opt is the intrinsic H I column density calculated from EB−V (units as for ∆NH  Molendi & Maccacaro (1994) dent if at all. Unlike the other polarized AGN, the broad lines and continuum are not significantly reddened, and the larger FWHM(Hβ b ) is more typical of normal Seyfert 1 nuclei. Possibly the polarization arises from transmission of nuclear light through well-aligned grains. For all these reasons, we exclude this source from detailed discussion. The three optically selected NLSy1s with p>2% also show p% increasing to short wavelengths, which, in these AGN, arises from dust-scattered continuum and broad lines (Goodrich 1989).
Further details of the X-ray and optical properties of the highly polarized AGN are given in Table 4. The AGN are approximately in order of increasing p% and presumed obscuration. For comparison, in the same table, we include the medians and 90 percentile ranges of all measured quantities for the complete soft-X-ray-selected sample. We give α x and ∆N H based upon the best available X-ray data. For all but CBS 126 and IRAS 13349+2438, the X-ray fits with N H fixed at N H,gal were unacceptable, so for these others we have adopted the α x determined from the power-law fits with N H unconstrained.
All scattering-polarized AGN show α opt and Hα/Hβ indicative of reddening by dust (Fig. 2, Table 4), and these two quantities are well correlated. Except for IRAS 13349+2438, the degree of polarization is correlated with reddening, but any such real relation must be complicated by differential reddening of scattered and direct light, different amounts of starlight dilution, and the sen- sitivity of polarization to details of the non-spherical, projected geometry. Greater optical reddening appears to be well correlated with stronger cold absorption in soft-Xrays as indicated by ∆N H (Table 4). IRAS 15091−2107, with nearly 3 magnitudes of absorption in the optical, is nearly completely absorbed in soft X-rays below 0.5 keV. We should be careful here. The true soft-X-ray absorption may be even greater than indicated by ∆N H if a 'soft excess' is present at the lowest photon energies -the kind attributed to a very hot accretion disk (Puchnarewicz et al. 1995b;Grupe et al. 1997).
Except for IRAS 13349+2438, the scattering-polarized AGN all have high [O III]/Hβ b ratios (Fig. 3). Alternatively, one could describe the highly polarized sources in this figure as having high EW(Fe II) for a given [O III]/Hβ b ratio (4.2.1).
Like the X-ray-selected AGN as a whole, the highly polarized AGN show rapid, large-amplitude, time-variability in soft X-rays. IRAS 13349+2438 has shown ∼50% variations over ∼6000 s (Brinkmann et al. 1996. For IRAS F12397+3333 we find ≈50% variations over ≈20 ks. Mkn 766 varied by a factor 2 in 1000 seconds (Leighly et al. 1996). Mkn 1239 varied between the RASS and pointed observations about two years later. The count rate doubled and α x changed from 1.69 during the RASS to 1.94 in the pointed observations, as derived from single power law fits with N H fixed to N H,gal (Rush & Malkan 1996). IRAS 15091−2107 has shown a probable variation from 0.2 cts s −1 to 0.3 cts s −1 over 3 days of ROSAT pointed observations, compared with 0.37 cts s −1 in the RASS 2.5 years earlier.

Statistics of Soft-X-ray NLSy1s
We found 40 of 43 soft-X-ray AGN to have p <1%, consistent with no intrinsic polarization. Most objects therefore have a direct, unobscured line-of-sight to the nucleus. Some arguments in favor of this can be summarized: -Lack of cold X-ray absorption along the line-of-sight.
The distribution of ∆N H is basically consistent with the errors in N H derived from spectral fits with N H unconstrained ). -Rapid X-ray variability is common in this sample, suggesting an emission source less than a few light-days in size see also Boller et al. 1993see also Boller et al. , 1996see also Boller et al. , 1997. This argues for a direct view of a tiny powerful nuclear source of X-rays and against an extended source such as is expected for a scattering origin. -The flat optical spectra and small Balmer line ratios (Hα/Hβ) imply no significant dusty gas. -The lack of significant optical linear polarization suggests a dustless line-of-sight.
These arguments for a direct unobscured view support the hypothesis, based on spectral energy distributions, that we view the inner accretion disk. Polarization and reddening are usually associated with cold, dusty gas. What makes our soft-X-ray AGN sample interesting is that these sources have steep X-ray spectra and show no soft X-ray absorption by cold, neutral matter, at least in the RASS spectra . Thus, it is a surprise that we find any polarized sources at all -but we find three -two of which clearly show scattering polarization and signs of UV-optical obscuration. These are not the ones with the narrowest BLR emission lines, or steepest X-ray spectra. Both IRAS F12397+3333 and IRAS 13349+2438 have FWHM(Hβ b ) close to the 'magical' NLSy1 borderline of 2000 km s −1 .

UV-Optical Reddening
Assuming that dust is responsible for the continuum and emission-line reddening in the highly polarized AGN, where is this dust located? The two measures of optical absorptionα opt and Hα/Hβ -are well correlated (Fig.  2). We show on this figure a line representing equal emission line and continuum reddening with respect to typical, essentially unreddened, values for the X-ray-selected sample as a whole. While we have used total Hα and Hβ line strengths to determine Hα/Hβ in this figure, we note that the four most reddened objects have strong narrow line emission. In de-blending the Hα complex, we derived ratios [N II] λλ6548,6584/[O III] λ5007 of 0.4 -0.6. While uncertain, comparison of these with values in Koski (1978) and Cohen (1983) suggests little reddening of this NLR emission. Less reddening of NLR Hα and Hβ would imply even larger Hα b /Hβ b ratios for the broad lines than are shown in the figure. Therefore, while the continuum and emission lines appear about equally reddened, it seems likely that the BLR is actually more reddened than the continuum. If more extensive investigation shows this to be true, then the dust may be more closely associated with the BLR than with a region that obscures both the BLR and the continuum source.
A consistency check of BLR reddening is to use the line and continuum reddening determined from the relation shown in Fig. 2 to correct the Hβ b intensity, and thus the ratio [O III]/Hβ b . If the BLR suffers reddening similar to the continuum, EW(Fe II) may not be much affected by reddening. This correction brings all the highly polarized sources into line with the low polarization AGN in Fig.  3, consistent with the [O III] λ5007 being relatively unreddened, and arising in a more spatially extended region such as an ionization cone. The BLR reddening has another interesting implication. The highly polarized AGN cover a wide range of [O III]/Hβ b and EW(Fe II) and, after applying this correction for absorption of Hβ b they follow the same well known inverse ('First Principal Component') relation between [O III]/Hβ b and EW(Fe II) shown by other AGN (Boroson & Green 1992;Grupe et al. 1997, in preparation). We note that, for AGN in general, nuclear reddening must contribute to the scatter in the strong [O III]/Hβ b -EW(Fe II) relation and must partially obscure it in samples unbiased with respect to reddening.
Reddening introduces important biases when QSOs are selected by optical-UV brightness and color. It is no accident that three IRAS-discovered QSOs are among our sample of highly polarized, reddened AGN. When corrected for reddening IRAS 12397+3333 and IRAS 13349+2438 have log νL ν ∼ 38.0 (watt). Similarly log νL ν for Mkn 1239 and IRAS 15091−2107 becomes 37.2 and 38.1. IRAS 12397+3333, IRAS 13349+2438, and IRAS 15091−2107 are then among the most intrinsically luminous AGN at low redshift.

Warm Absorbers
Another way to investigate the dusty regions is to compare the neutral-hydrogen column densities inferred from the intrinsic optical absorption (∆N H,opt ) and from the soft-X-ray spectral fitting (∆N H ). We derived E B−V from Fig. 2 to calculate ∆N H,opt ∼ k × E B−V 10 20 cm −2 (k = 49, Diplas & Savage 1994). The uncertainties could be ∼ 5 × 10 20 cm −2 , judging by the scatter in Fig. 2. These ∆N H,opt values are given in Table 4. It can be seen that the optical reddening significantly overpredicts ∆N H in all cases except perhaps IRAS 15091−2107, where the very high soft X-ray absorption may be consistent with the optical reddening. If we had used k = 53 (Predehl & Schmitt 1995;Predehl & Klose 1996) or k = 66 (Gorenstein 1975) the discrepancy would have been even greater. It is also probable that less-reddened, scattered light contributes to the optical spectra (Sect. 4.2.3). In this case the true reddening along a direct path to the nucleus, and hence ∆N H,opt , would be even greater.
The relatively low soft X-ray absorption could be because the X-rays arise in a region more spatially extended than the UV-optical. However, the large amplitude and rapid variability (Sect. 3.3) provides evidence that all or most of the X-ray emission arises within regions lightdays to light-months in size. This, together with the high brightness in soft X-rays, argues that we have a direct view to the center in X-rays rather than a view via a parsecscale scattering region. The relatively low soft X-ray absorption could be because the dust-to-gas ratio is high, or, more likely in this nuclear environment, the dusty gas is at least partially ionized and therefore has greater transparency to soft X-ray photons. The dusty gas is probably related to the 'warm absorbers' detected in the O VII and O VIII K-shell edges near 0.7 -0.8 keV. This ionized gas has now been detected in IRAS 13349+2438 (Sect. 4.2.3), and Mkn 766 (Leighly et al. 1996). Warm absorbers provide a natural tie-in for the UV-optical reddening, X-ray variability, and bright, unabsorbed soft X-ray spectrum. In fact, for IRAS F12397+3333, neutral hydrogen absorption plus a single power-law does not provide a good fit to the X-ray data . In the residuals we find a depression near 0.8 keV -an indication of O VII and O VIII absorption edges. The X-ray spectrum and variability will be discussed in more detail together with the results of our spectropolarimetry for this AGN (Wills et al. 1997, in preparation).
Recently, Leighly et al. (1997) show that AGN with warm absorber features in their X-ray spectra tend to be significantly polarized. Related to this, Reynolds (1997) also report that Seyfert 1s with UV absorption also showed X-ray warm absorption. Walter & Fink (1993) should be given credit for pointing out that some reddened, highly polarized NLSy1s showed anomalously low νF ν (1375Å) on a plot of Γ vs. νF ν (1375Å)/νF ν (2 keV) (their Fig. 8). The low UV flux (by ∼30) could be explained on the basis of their observed optical reddening, but they found little or no soft X-ray absorption. All except one are NLSy1s, and now known to show warm X-ray absorption. They are IRAS 13349+2438, Mkn 766, NGC 4051 (Table  1, Komossa & Fink 1997), MCG-6-30-15 (FWHM(Hβ b ) ∼1700 km s −1 , Pineda et al. 1980, Akn 564 (Table 3). The other NLSy1s discussed here -IRAS 12397+3333, IRAS 15091−2107, and Mkn 1239follow the same trend. When corrected for a reddened continuum all these NLSy1s show the strong increase of Γ with νF ν (1375Å)/νF ν (2 keV) that led Walter & Fink to argue for a UV to soft X-ray bump in Seyfert 1 nuclei.

Scattered Light Models
We have some evidence that the scattering geometry for NLSy1 may be axisymmetric, and this is certainly true for many other AGN. The polarization (E-vector) is parallel to the major axis of the galaxy in the case of IRAS 13349+2438, and for Mkn 766 and Mkn 1126 the polarization is perpendicular to the elongated radio structure thought to define the jet direction -or angular momentum of the central engine (Ulvestad et al. 1995). Mkn 766 shows an 'ionization cone' in the jet direction (Wilson 1997). If we can assume axisymmetry we can relate scattered and direct views to the geometry of the central engine -for example, accretion disk geometry and kinematics.
IRAS 13349+2438 is the best studied of the highly polarized AGN (Wills et al. 1992b;Brandt et al. 1996Brandt et al. , 1997Brinkmann et al. 1996;Hines 1994). In this AGN the contribution of host-galaxy starlight is <15%, so the wavelength dependence of polarization (Fig. 4) is explained by dilution of polarization by reddened direct AGN light. Therefore we see the AGN from two directions -a direct view, and a view from the vantage point of the scatterers. The polarization E-vector is parallel to the host galaxy's major axis, suggesting an axisymmetric geometry with obscuring dust in the plane of the galaxy, probably a dusty torus. Hβ b and Hα may be slightly narrower in scattered than in total light (Hines 1994). X-ray emission in the ROSAT band is strong and rapidly variable, and therefore not seen reflected from a large (parsec-scale) scattering region, but is instead nuclear light seen directly. The steep X-ray spectrum, lack of neutral absorption in Xrays, but strong UV-optical reddening, led Brandt et al. (1996Brandt et al. ( , 1997 to suggest that ionized, dusty gas absorbs the nuclear light, thus leading to the first clear detection of warm absorbers in a luminous QSO. Both the fact that the central AGN is still optically visible, and that the absorption is warm, suggest a line-of-sight to the center that grazes the dusty torus: the absorption optical depths are significant but not extremely high; grains are not likely to have formed in the ionized gas, but pre-existing grains could have evaporated from the inner torus on exposure to the nuclear continuum. A similar scattered-light-dusty torus model is a plausible explanation of our second highly polarized object, IRAS F12397+3333. In this case direct sky survey images show that unpolarized host-galaxy starlight is likely to contribute to the wavelength dependence of p% (Fig. 4). Warm absorbers provide a natural explanation for the UVoptical reddening, X-ray variability, and bright, essentially unabsorbed soft X-ray spectrum. Goodrich's (1989) spectropolarimetry suggests a similar scattering geometry for the three highly polarized AGN in his sample of 17 optically selected NLSy1s (Tables 4 and  3), and our investigation of their UV-optical reddening and X-ray spectra suggest both cold and warm absorbers along a direct path to the nucleus.
The prototypical NLSy1, I Zw 1, is an interesting example. Because of its extreme NLSy1 properties, NLSy1 are sometimes called 'I Zw 1' AGN. It was too weak to be included in our X-ray sample because of anomalously high Galactic HI absorption and variability. Its HR1 was +0.49 during the RASS, but −0.25 during later pointed observations (Boller et al. 1996). It shows significant scattering polarization, with polarized continuum and broad lines (1.7%, Smith et al. 1997) variable in position angle and wavelength-dependence on several-year time scalesmaybe shorter -and possibly significant optical reddening, E(B-V) ∼ 0.2. Its steep soft-X-ray spectrum (α x ∼ 2.0) is rapidly variable, with very small intrinsic cold absorption (1.5 ± 0.7 × 10 20 cm −2 ). If reddened, this is likely to be another example of a warm absorber -and on axisymmetric Unified Schemes, would therefore be a candidate for a torus-grazing line-of-sight -a potentially powerful illustration that steep X-ray spectra need not be observed from the same direction as the narrow Hβ b (and other 'I Zw 1' properties that go along with these properties).

The Nature of NLSy1s
In principle, comparison of scattered and direct views allows a test for the predicted axisymmetry in kinematics, UV-and X-ray emission (Sect.1).
The important conclusion from Goodrich's (1989) spectropolarimetry is that the narrow Hβ in NLSy1 is more highly polarized than [O III], whose polarization is often unmeasureably small; Hβ b is narrow not because it arises in the NLR; it is produced in separate, higher density gas.
Assume that polarized Hβ b represents a scattered light, polar view. Broader polarized Hβ b than that seen in total light could then be interpreted as evidence for scattering by electrons in hot ionized nuclear gas (Goodrich 1989), or for a dust-scattered view of a high-velocity inner BLR and central continuum source that are obscured in a direct high-inclination, view. Narrower polarized Hβ b could be interpreted as arising from a polar view of a flattened BLR.
In general, in the ∼5 cases where a comparison is possible, Goodrich does not find significantly broader Hβ b in polarized light, and therefore favors scattering by cool dusty gas. This assumes that a predominantly direct view of the center is seen in total light; without knowing p% for the scattered light alone, we cannot be sure. Also from the similar widths in polarized and total light, we could argue that a higher-velocity, inner BLR is not hidden by the thicker equatorial regions of a dusty torus, and that there is no evidence for an anisotropic velocity field. Mkn 1239 is an interesting exception. Goodrich finds broad, red wings as an additional feature of the Hβ and Hα broad lines in polarized flux, which he interprets as arising in a separate electron-scattering region.
AGN like IRAS 13349+2438 and IRAS 12391+3333 of our sample, and three of the Goodrich sample, show significant scattered light views of an optical NLSy1 BLR spectrum (p max ∼ 9, 7, 6, 6, 8%), but a direct, higher inclination view of a luminous soft X-ray source with steep 0.2-2 keV spectrum. This rules out the hypothesis in which the narrow Hβ b and steep α x belong exclusively to a poleon view of an optically thick accretion disk with coplanar BLR (e.g., Boller et al. 1996). Mkn 1239 is especially extreme, with α x ∼3 and FWHM(Hβ b ) in polarized and total light of ∼1500 km s −1 and ∼1000 km s −1 .

Unified Schemes
The similarity of the strong, variable, steep X-ray spectra, and narrow Hβ b in both highly polarized and low polarization AGN support a Unified Scheme in which the low radial BLR velocity dispersion and steep X-ray spectra are intrinsic properties that are not highly anisotropic; the differences are all attributable to orientation-dependent scattering and obscuration. When corrected for optical reddening the highly polarized objects fit the inverse correlation between Fe II and [O III] strengths seen for the unreddened AGN, and even the optically-selected NLSy1s show steep X-ray spectra after correction for cold absorption. The highly polarized AGN lead to a picture of partial obscuration of the central continuum and BLR, with polarized light reaching us from scatterers that have a lessobscured view of the center. While the case for an axisymmetric Unified Scheme is based on polarization alignment in only a few AGN, the partial obscuration of a direct view of the AGN by dusty, ionized gas suggests a line-of-sight grazing the dusty torus. All the above leads us to consider a unified picture in which all NLSy1s have dusty tori and scatterers, with some lines-of-sight unobscured.
Our sample of 43 comprises all AGN in a ROSAT X-ray source sample, complete to a given X-ray count rate and hardness ratio. All the AGN (except IC 3599) have prominent AGN-like optical continua and broad Hβ. There are no classical Seyfert 2s in the sample at all, demonstrating that our view towards their nuclei is opaque in soft X-rays. Our sample includes only two out of 43 AGN that are clearly scattering-polarized, demonstrating that warm, dusty absorbers are rarely found along sight-lines without accompanying cold absorbing gas. In fact, the soft-X-ray absorption seen in the highly polarized AGN of the X-ray and optically selected samples, while small, does correlate well with optical reddening, but we suggest that an additional N H of 1.5 -2 ×10 21 cm −2 is accounted for by warm X-ray absorbers.
Assuming axisymmetry, the low polarization AGN represent unobscured views from within an ionizationscattering cone. Several sources in both samples show observed white light p <1%. Goodrich (1989) found that some lower polarization objects show p% increasing to shorter wavelengths with significant continuum and broad-line polarization, indicating that a number of sources in both samples show similar scattering geometry. Their smaller polarizations may arise from a smaller scattering angles, and greater dilution by unpolarized AGN and host-galaxy light. At higher inclination angles, partially ionized dusty gas absorbs the UV, soft X-rays and ∼0.7 keV X-rays. It doesn't take much dusty gas, like that of our interstellar medium, to completely block the UV and soft-X-rays, so it is probably not surprising that only 5% of our sample are highly polarized warm absorbers. As the inclination increases the thicker dusty torus blocks all but the harder X-rays (5-15 keV, e.g. Comastri et al. 1995). At higher inclinations, IRAS 12397+3333, Mkn 766, Mkn 1239, and IRAS 1509−2107 would appear as Seyfert 2s (like NGC 1068, discussed below). Strong-Fe II, weak-[O III] AGN like IRAS 13349+2438 would appear as galaxies with weak emission lines, detectable in hard X-rays and warm mid-infrared surveys. Deep X-ray spectral surveys of complete infrared-selected AGN are needed to understand the covering by cold and warm X-ray absorbers.
The archetype buried Seyfert 1 nucleus, revealed in the polarized light spectrum of the classical Seyfert 2, NGC 1068, may simply be a more edge-on view of a NLSy1, with the UV-optical and soft X-rays seen only in scattered light. In this case, a spatially resolved dustscatterer sees a low inclination view with FWHM(Hβ b ) ∼ 2900 km s −1 , but electron scatterers within a few parsecs of the nucleus produce FWHM(Hβ b ) ∼ 4480 km s −1 presumably the result of thermal broadening at ∼ 10 6 K (Miller et al. 1991). Electron-scattering is wavelength independent, so an intrinsic, polar view is seen in soft X-rays, with α x ∼ 2 (Marshall et al. 1993;Pier et al. 1994). Unlike the result for most NLSy1s with purported low-inclination views, the X-ray spectrum shows no timevariability over several years (Smith et al. 1993), consistent with scattering of X-rays from a parsec-scale region.

IC 3599
IC 3599 is the only AGN of the soft X-ray selected sample to be classified as a Seyfert 2 galaxy (Grupe et al. 1995). Unified Schemes suggest that scattered light polarization might be detectable. We find no significant polarization, but some optical absorption may be present (Fig. 2), a result that may favor the starburst classification suggested by Bade (1993). This object would not usually fit the criteria for inclusion in the soft X-ray sample; it just happened to be in an X-ray outburst at the time of the RASS (Brandt et al. 1995;Grupe et al. 1995).

Summary
We have surveyed the optical linear polarization of the 43 northern objects of our completely-identified sample of bright soft-X-ray-selected ROSAT AGN. All except one are Seyfert 1 nuclei, with median FWHM(Hβ b ) of 2250 km s −1 . Most (40) are of low polarization with p <0.5% -1%, and have flat blue continua and small Balmer decrements. This supports the suggestion from rapid Xray variability, disk-like spectral energy distribution, and lack of cold X-ray absorption, that we are viewing a bare AGN disk .
Three AGN show significant polarization, with no preference for the narrowest Hβ b or steepest α x . One, CBS 126, has p ∼1.3% with no clear wavelength dependence. Two, IRAS 13349+2438 and IRAS 12397+3333, show high degrees of polarization increasing to short wavelengths, and significant optical reddening. In IRAS 13349+2438 a dusty torus dims the central luminous AGN, revealing a less-reddened polar view of the continuum and BLR in scattered light (Wills et al. 1992b). This partial dust covering forms the basis for Unified Schemes -accounting for very different obscured or unobscured views of the same central engine. Brandt et al. (1996) explained its rapidly-variable and bright X-ray spectrum, lack of soft-X-ray absorption, but reddened UV-optical spectrum, by dusty, ionized gas (warm absorbers) along the line-of-sight to the center. The optical polarization and reddening, together with its rapidly-variable and steep Xray spectrum, suggest the same scattering-warm-absorber picture for IRAS 12397+3333.
We have compared the polarization, spectroscopic and X-ray properties of our two highly-polarized sources with three discovered by Goodrich (1989) among a heterogeneous sample of 17 optically-selected NLSy1s. Only about half the optically-selected sample have steep α x , ∼ > 1.5. There appear to be correlations among indicators of dust and cold X-ray absorption: p%, α opt , Hα/Hβ, and ∆N H . When the Hβ b strength is corrected for reddening, the highly polarized objects have similar [O III]/Hβ b to the low polarization AGN, and show the same 'First Principal Component' [O III]/Hβ b -EW(Fe II) anticorrelation. However the neutral soft X-ray absorption is overpredicted by the optical reddening by ∼5 ×10 20 cm −2 . So, in line with the warm absorbers seen in IRAS 1334+2438 and Mkn 766, dusty partially ionized gas is almost certainly absorbing nuclear light.
The intrinsic properties of the high and low polarization AGN are similar, including narrow Hβ b and steep α x ; the differences are all attributable to orientationdependent scattering and obscuration, and this is supported by the periscopic geometry deduced for individual polarized AGN. Thus an orientation Unified Scheme may link all the AGN of our sample, and NLSy1s in general.