ASCA Observations of the Broad-Line Radio Galaxy 3C 445

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ABSTRACT. We present the пrst high-resolution X-ray observation of the nearby (z \ 0.057) broad-line radio galaxy 3C 445 obtained with ASCA in 0.6х10 keV.
THE ASTROPHYSICAL JOURNAL, 495 : 749È756, 1998 March 10 ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

ASCA OBSERVATIONS OF THE BROAD-LINE RADIO GALAXY 3C 445 RITA M. SAMBRUNA1 NRC, NASA/GSFC, Code 662, Greenbelt, MD 20771 ; sambruna=attilla.gsfc.nasa.gov

I. M. GEORGE USRA, NASA/GSFC, Code 662, Greenbelt, MD 20771

R. F. MUSHOTZKY NASA/GSFC, Code 662, Greenbelt, MD 20771

K. NANDRA NRC, NASA/GSFC, Code 662, Greenbelt, MD 20771

AND T. J. TURNER USRA, NASA/GSFC, Code 662, Greenbelt, MD 20771 Received 1997 June 24 ; accepted 1997 October 14

ABSTRACT We present the Ðrst high-resolution X-ray observation of the nearby (z \ 0.057) broad-line radio galaxy 3C 445 obtained with ASCA in 0.6È10 keV. The earlier detection of the Fe Ka line is conÐrmed. The ASCA data are best Ðtted by a model involving complex absorption (dual absorber), where an intrinsically Ñat (! D 1.3) continuum is seen through two distinct layers of absorption of cold gas with column densities N1 D 1023 cm~2 and N2 D 6 ] 1022 cm~2, both larger than the column derived from H Fits with this model H to the nonsimultaneous ROSAT and ASCA data put tighter the optical reddening. constraints on the continuum parameters. With the continuum described by a dual absorber the Fe Ka line has an EW D 270 eV and a width p D 0.2 keV, implying an origin in neutral gas moving with velocities [35,000 km s~1. However, the presence of complex absorption in the nucleus of this type 1 active galactic nucleus (AGN) is unexpected in the context of current uniÐcation models. A statistically good Ðt to the ASCA data is also provided by a power-law plus Compton reÑection model ; however, this model is physically implausible because of the weak (EW D 80 eV) Fe line and unrealistically high (D1046 ergs s~1) luminosity inferred. Among BLRGs observed with ASCA, 3C 445 sticks out as a moderately luminous source with an unusually complex spectrum at soft X-rays. Subject headings : galaxies : active È galaxies : individual (3C 445) È X-rays : galaxies 1.

INTRODUCTION

steepens dramatically at UV energies (Crenshaw et al. 1988). It has been suggested that the large IR emission is the result of the reprocessing of the optical and UV emission by circumnuclear dust (Elvis et al. 1984). Indeed, the polarization of the continuum (Brindle et al. 1990), with a trend of decreasing polarization degree with increasing wavelength (Rudy et al. 1983 ; but see Antonucci 1984b), provide evidence for the presence of dust in 3C 445. The broad Ha line is also polarized (Kay et al. 1996). The amount of reddening, derived from the large Balmer decrement (Ha/Hb D 8 ; Crenshaw et al. 1988 ; Osterbrock, Koski, & Phillips 1976) and by the large Paa/Hb ratio (D5.6 ; Rudy & Tokunaga 1982), is E(B[V ) D 1 mag. For a standard dust-to-gas conversion ratio, E(B[V ) \ N /5.2 ] 1021 cm~2 mag~1 H (Shull & Van Steenberg 1985), an intrinsic absorbing column density N D 5 ] 1021 cm~2 is derived. This is 1 order of magnitudeH larger than the Galactic column density in the direction to the source, NGal \ 5.33 ] 1020 cm~2, H derived from 21 cm measurements (Murphy et al. 1996). Thus, 3C 445 appears to host an obscured AGN. Further support to this conclusion is provided by the luminosity of the broad and narrow emission lines. After the reddening correction, the luminosity of the broad Ha line is L D 4 Ha ] 1043 ergs s~1, while the narrow [O III] j5007 emission has L D 2.6 ] 1042 ergs s~1 (Tadhunter et al. 1993). O III luminosities are the trademark of a powerful Such high

The nearby (z \ 0.057 ; Clements 1983) radio galaxy 3C 445 has an FR II radio morphology (Kronberg, Wielebinski, & Graham 1986), with a linear extension up to 10@ (Schilizzi & McAdam 1975). With a steep radio spectrum between 2.7 and 4.8 GHz (a4.8 \ 0.7) and a core-to-lobe 2.7 intensity ratio R \ 0.039 (Morganti, Killeen, & Tadhunter 1993), the source is clearly lobe dominated. Because of the broad, single-peaked Balmer lines emitted in the optical spectra (Eracleous & Halpern 1994 ; Crenshaw, Peterson, & Wagner 1988 ; Antonucci 1984a), 3C 445 has been classiÐed as a broad-line radio galaxy (BLRG). Optical and IR observations of 3C 445 show that the emission at these wavelengths comes from a very compact core, with little contribution from the host galaxy (Zirbel 1996 ; Kotilainen & Ward 1994 ; Yee & Oke 1978). The IR-to-optical spectrum is steep, a D 2.3È2.6 (Kotilainen, Ward, & Williger 1992 ; Elvis et al. 1984), and the IR colors are indicative of an active galactic nucleus (AGN) (Spinoglio et al. 1995) with a luminosity L D 2 ] 1044 ergs s~1 (Hes, Barthel, & Hoekstra 1995). 60k The spectrum 1 Current address : Pennsylvania State University, Department of Astronomy and Astrophysics, 525 Davey Lab, University Park, PA 16802 ; rms=latte.astro.psu.edu.

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compact source of ionizing radiation (e.g., Mulchaey et al. 1994). X-ray observations at medium-hard energies allow in principle the detection of the primary nuclear emission that can penetrate the layer of obscuring material at these wavelengths. An earlier HEAO 1 A-2 detection with a 2È10 keV Ñux of D3 ] 10~11 ergs cm~2 s~1 was reported for the source H2216[027, identiÐed with 3C 445 (Marshall et al. 1979). However, the large error box of the HEAO 1 detection includes another likely counterpart, the nearby (at D0.5 degrees) cluster of galaxies A2440 (z \ 0.094). The Ðrst X-ray spectrum of 3C 445 was obtained with the EXOSAT medium-energy experiment (ME ; 1È10 keV). The ME data are consistent with a poorly constrained continuum photon index (! \ 1.34`0.89), a column density N \ 5.3`6.1 ~1.34 H ~3.7 ] 1022 cm~2, and an intrinsic 2È10 keV luminosity of L D 2 ] 1044 ergs s~1 (Turner & Pounds 1989). 2~10 keV Ginga observations of 3C 445 yielded a better constrained spectrum, although at the low angular resolution of the large area proportional counter (LAC), confusion with the nearby cluster was not negligible, the latter dominating the Ginga data below D3 keV. After accounting for the cluster thermal emission, Pounds (1990) Ðnds for 3C 445 a continuum with slope ! \ 1.68`0.20, absorbed by a column ~0.18 density N \ 1.8`0.5 ] 1023 cm~2 and with L D 3.3 H ~0.4 2h10 by keV Ginga ] 1044 ergs s~1. An iron line was also detected around 6 keV, with equivalent width EW D 100È150 eV and rest-frame energy D6.4 keV, consistent with an origin by Ñuorescence in cold material (Pounds 1990). In this paper we present the Ðrst higher resolution X-ray spectrum of 3C 445 obtained with ASCA in 0.6È10 keV. The better angular resolution of ASCA compared to Ginga allows an unambiguous study of the high-energy continuum of the radio galaxy, free from contamination from the nearby cluster. The ASCA data conÐrm the detection of the Fe Ka line from the AGN and show a complex X-ray spectrum, with a soft excess below D1.5 keV and an observed Ñat continuum. A preliminary account of the ASCA data was given by Yamashita & Inoue (1996), with consistent results. The paper is structured as follows. In ° 2 the data analysis is described, while ° 3 presents the results of the spectral Ðts. Our Ðndings are summarized and discussed in ° 4, with the conclusions following in ° 5. Throughout this paper, H \ 0 75 km s~1 Mpc~1 and q \ 0.5 are assumed. 0 2.

DATA REDUCTION AND ANALYSIS

3C 445 was observed with ASCA on 1995 June 1 for a total of 45 ks. For a description of the ASCA experiment, see Tanaka, Inoue, & Holt (1994). The solid-state imaging spectrometers (SIS0 and SIS1) operated in 1-CCD FAINT mode, and the gas imaging spectrometers (GIS2 and GIS3) were used in pulse-height mode. The SIS data were converted into BRIGHT2 mode applying dark frame and echo corrections (see The ASCA Data Reduction Guide, 1997). The data were reduced by applying a number of standard screening criteria. For both SIS and GIS we rejected the data accumulated during passage of the South Atlantic Anomaly, in intervals of bright Earth, angular deviation, and elevation angles lower than 20¡, 0¡.01, and 10¡, respectively, and when the geomagnetic rigidity was lower than 6 GeV c~1. Only grades 2, 3, and 4 were retained for the analysis. The e†ective exposures and corresponding source

Vol. 495

count rates after screening are reported in Table 1. The source is detected with a moderate signal-to-noise ratio, D28È40. 2.1. Spatial Analysis 3C 445 was observed with the ROSAT PSPC in 1993 May for 11,344 s. Since the angular resolution of the PSPC is better than ASCA we examined the ROSAT image looking for contaminating sources nearby the radio galaxy. The galaxy 3C 445 was detected with a count rate 0.016 ^ 0.001 counts s~1, at a position o†-axis by D6@. The radial proÐle of the source was extracted in the energy range 0.25È2.0 keV out to 3@ from the centroid of 3C 445 ; the model proÐle was energy- and position-weighted. A signiÐcant excess of counts is present in the observed proÐle at [email protected]. The ROSAT coordinates of this serendipitous source (from the aspect solution and optical astrometry) are R.A. (2000) \ 22h23m44s. 7, decl. (2000) \ [02¡06@38A. 2, with uncertainties ^5A ; no optical counterpart was found in the databases NED and SIMBAD. Its PSPC count rate is (4.2 ^ 1.2) ] 10~3 counts s~1 (from the algorithm SOSTA in XIMAGE), a factor of B4 fainter than 3C 445. When the serendipitous source is excluded from the analysis, the radial proÐle of 3C 445 is consistent with a pointlike source. There is weak (D1 p) evidence for some residual excess Ñux around 1@, which, however, is not signiÐcant. To date, 3C 445 has not been imaged with the ROSAT HRI. We checked for possible contamination in the ASCA data by the serendipitous ROSAT source by accumulating an SIS image in the range of overlap with the PSPC, 0.5È 2.4 keV. A faint source is present at a position consistent with the ROSAT coordinates, with a count rate (1.6 ^ 0.9) ] 10~3 counts s~1 in SIS0, roughly similar to the ROSAT Ñux and a factor of 30 lower than 3C 445. Thus it is unlikely that the serendipitous source will contaminate signiÐcantly the ASCA spectrum of 3C 445. The contaminating ROSAT source is not present in the GIS images. Since the GIS sensitivity peaks at higher energies than the SIS, this result suggests a steep X-ray spectrum for the serendipitous source. 2.2. Extraction of the ASCA L ight Curves and Spectra The ASCA light curves and spectra of 3C 445 were accumulated in circular regions centered on the source position with radii of 3@ for the SIS and 4@ for the GIS, which has a larger intrinsic point-spread function. In the case of the GIS, we evaluated the background in a circle of radius 5@ located about 16@ away from the target, in a region free from contaminating sources. In the case of the SIS, the background was evaluated in a circle of radius [email protected] located 5@

TABLE 1 ASCA OBSERVATIONS OF 3C 445 Instrument

Observation Date

Exposurea (s)

Count Rateb (counts s~1)

SIS0 . . . . . . . SIS1 . . . . . . . GIS2 . . . . . . GIS3 . . . . . .

1995 Jun 1 ... ... ...

37,176 36,517 39,103 39,193

0.055 ^ 0.002 0.041 ^ 0.001 0.048 ^ 0.001 0.058 ^ 0.002

a E†ective exposure time after data screening. b Source count rate after background subtraction.

ASCA OBSERVATIONS OF 3C 445

No. 2, 1998

away from the target on the same chip. The SIS and GIS light curves of 3C 445 were inspected for variability, and none was found. The spectra were thus accumulated on the whole net exposures (Table 1). The SIS and GIS spectra were rebinned in order to have a minimum of 20 counts in each spectral bin to validate the use of the s2 statistic. In order to increase the signal-tonoise ratio we performed joint Ðts to the data from the four detectors, leaving only the normalizations as independent parameters. The 1994 May response matrices were used for the GIS spectra, while for the SIS data we used the matrices generated by the SISRMG program. The SIS and GIS data were Ðtted in the energy ranges 0.6È10 keV and 0.7È10 keV, where the spectral responses are better calibrated. The Ðts were performed within XSPEC v.10. 3.

SPECTRAL ANALYSIS

In all Ðts, cosmic abundances for the Ðtted column density N and the photoelectric cross sections of Morrison H & McCammon (1983) were used. A Ðt was retained acceptable if the corresponding s2 probability was equal or larger than 5%. The improvement in the s2 obtained by adding additional Ðt parameters was estimated with the F-test, with a threshold for a signiÐcant improvement P \ 95%. F Galactic A single power-law model absorbed by the column density (N \ 5.33 ] 1020 cm~2 ; Murphy et al. 1996) along the line Hof sight provides a poor Ðt to the ASCA data of 3C 445, with a reduced s2 of s2 \ 1.42 for 421 degrees of freedom (dof), which is rejectedr at greater than 99% conÐdence. This Ðt yields a negative photon index ! \ [0.15 ^ 0.05. Figure 1 shows the residuals of the single power-law model for the SIS and GIS detectors

FIG. 1.ÈResiduals in the form of ratio of the data to the model for the Ðt to the ASCA data of 3C 445 with a single power law and Galactic column density. The data from the SIS and the GIS detectors are shown separately in the top and bottom panels, respectively. Deviations in the form of a soft excess below 1.5 keV and the Fe line at 6 keV are apparent.

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separately. Deviation from the model in the form of excess Ñux below 1.5 keV and the Fe line at D6 keV are apparent. The modeling of the Fe line depends critically on the parameterization of the underlying continuum. We thus concentrated Ðrst on Ðnding an adequate description of the latter, and restricted our spectral Ðts to the energy range 0.6È10 keV excluding the 5È7 keV region. The Ðts to the continuum are discussed in ° 3.1, with the Fe line Ðts following in ° 3.2. The results of the spectral Ðts are summarized in Table 2, where all quoted uncertainties on the parameters are 90% for one parameter of interest (*s2 \ 2.7). All quantities in Table 2 are in the observerÏs rest-frame. 3.1. Fits to the 0.6È10 keV Continuum 3.1.1. Soft Excess Emission

We Ðrst attempted to Ðt the soft excess in Figure 1 with a thermal model. ROSAT and ASCA observations of radio galaxies are indeed consistent in several cases with the presence of a low-energy thermal component with temperatures around 0.5È1.0 keV, while the AGN emission is described with a power law at harder energies (Worrall et al. 1994). We thus Ðtted the ASCA data of 3C 445 with a model consisting of an absorbed power-law plus a RaymondSmith thermal component. Although the Ðt with this model is formally acceptable (s2 \ 1.09/310), the temperature of the thermal component r to kT D 20 keV and the best-Ðt abundance is zero, goes actually mimicking a steep power law at soft energies. We thus reject a thermal interpretation of the soft component in 3C 445. As a crude parameterization of the data, we Ðtted the ASCA spectrum with two power laws, both absorbed by Galactic N only and with independent slopes and normalH model yields a good Ðt (s2 \ 1.08/308), with a izations. This r soft component ! \ 1.23`0.47, steeper than at hard soft ~0.28 energies (! \ 0.64`0.21), and an intrinsic (absorptionhard ~0.29 corrected) luminosity in 0.5È4.5 keV of L int D4 0.5h4.5We keV can ] 1042 ergs s~1 (from the softer power law only). compare this value with the luminosity expected for a starburst component using equations (1) and (3) of David, Jones, & Forman (1992) and f (60 km) \ 0.3 Jy and f (100 km) [ 1 Jy for 3C 445 (from the on-line IRAS database). The predicted 0.5È4.5 keV starburst luminosity for 3C 445 is [7 ] 1040 ergs s~1, at least 1 order of magnitude lower than observed with ASCA. This conÐrms that the starburst activity in 3C 445 is negligible (e.g., Spinoglio et al. 1995), and shows that the soft excess in the ASCA data must be due to the emission of the AGN. We thus looked for a nonthermal explanation of the observed soft excess. A Ðrst possibility, absorption by ionized gas along the line of sight, is immediately ruled out by the lack of absorption edges due to oxygen in the 0.7È1 keV energy range, the trademark of a warm absorber ; the upper limit to the optical depth of the O VII edge at 0.70 keV (observed energy) is q [ 0.6 at 99% conÐdence. A O VII the nuclear emission leaks ““ patchy ÏÏ absorber, where through a medium of column density N covering a fraction H could be a viable C of the source (e.g., Holt et al. 1980), F alternative. When this model was Ðtted to the ASCA data of 3C 445, an acceptable Ðt was obtained (Table 2A). Alternatively, the soft excess could be due to the scattering into the line of sight of a fraction of the nuclear continuum by some medium close to the nucleus. In prac-

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Vol. 495

TABLE 2 SPECTRAL FITS TO THE ASCA DATAa Model

2 r (dof)

Parameterb

Notes

A. ASCA Data, Excluding 5È7 keV ! \ 0.85`0.19 1.09/312 ~0.14 Ncold \ 7.8`1.4 ] 1022 H ~1.2 C \ 0.78`0.06 F ~0.05 Dual absorber . . . . . . . . . . . . . . . . . . . . . . . ! \ 1.30`0.35 1.08/310 ~0.40 N1 \ 2.4`3.1 ] 1023 H ~1.2 C \ 0.84 ( [ 0.54) F N2 \ 5.5`2.5 ] 1022 H ~2.2 B. ROSAT plus ASCA Data, Excluding 5È7

Partial covering/scattering . . . . . . . . .

Dual absorber . . . . . . . . . . . . . . . . . . . . . . .

Dual absorber plus Gaussian . . . . . .

! \ 1.53`0.33 1.09/318 ~0.23 N1 \ 2.7`1.6 ] 1023 H ~0.8 C \ 0.94 ^ 0.05 F N2 \ 5.2`1.7 ] 1022 H ~1.3 C. ASCA Data, Including 5È7 keV ! \ 1.25`0.29 ~0.27 N1 \ 2.5`8.7 ] 1023 H ~1.3 C \ 0.88`0.13 F ~0.21 N2 \ 5.8`3.2 ] 1022 H ~1.8 E 4 6.05 p \ 0.15`0.15 ~0.12 N \ 5.0`4.0 ] 10~5 line ~2.5 EW \ 268`272 eV ~118

1.05/414

... Column density of cold gas (cm~2) Covering fraction ... Column density of cold gas (cm~2) Covering fraction Column density of cold gas (cm~2) keV ... Column density of cold gas (cm~2) Covering fraction Column density of cold gas (cm~2)

... Column density of cold gas (cm~2) Covering fraction Column density of cold gas (cm~2) Gaussian line (keV), Ðxed Fe Ka line width (keV) Fe Ka line Ñux (ph cm~2 s~1 keV~1) Fe Ka line equivalent width (eV)

a All Ðts include a column density Ðxed to the Galactic value in the direction to 3C 445, N \ 5.33 ] 1020 cm~2, H Murphy et al. 1996. b Errors are 90% conÐdence for one parameter of interest (*s2 \ 2.7).

tice, we Ðtted the data with a model consisting of two power laws with tied photon indices and independent normalizations, one absorbed by a column density N and the H other by Galactic N only. Note that the scattering and H partial covering models are algebraically equivalent, so we will not distinguish between them in the following. In Table 2A, the Ðt to the ASCA data with the partial covering/ scattering model gives a Ðtted column density N D 8 H of ] 1022 cm~2, a fraction of leaking/scattered continuum D20%, and a very Ñat photon index, ! D 0.9, which implies a negative energy spectrum. This is in contrast with the results from previous Ginga observations of 3C 445, which yielded ! D 1.7 (Pounds 1990). 3.1.2. Overall ASCA Continuum

An inverted energy continuum is unexpected in the context of current theoretical models (e.g., Haardt, Maraschi, & Ghisellini 1994), which predict an intrinsic slope of ! D 1.9, and is in contrast to recent ASCA and Ginga observations of Seyfert 1 galaxies and BLRGs, which showed intrinsic continuum slopes consistent with this predicted value for both classes (Nandra & Pounds 1994 ; Eracleous, Halpern, & Livio 1996 ; Nandra et al. 1997 ; Grandi et al. 1997). In an attempt to reconcile the Ñat ASCA spectrum of 3C 445 with an intrinsically steeper continuum we Ðtted the data with a dual absorber and a power-law plus reÑection model. Dual absorber.ÈThis model parametrizes the case of a nuclear power-law continuum that is seen through two dif-

ferent layers of gas, one of which is only partially covering the source. In practice, we constructed a model with two power laws with tied indices and independent normalizations, one absorbed by a nonuniform column density N1 and covering fraction C , and the other by a uniform (C \H F F 100%) absorber with column density N2 (both in the obserH verÏs frame). Both components were absorbed by the Galactic column density. As shown in Table 2A, the Ðt with the dual absorber is good, although there is no statistical signiÐcant improvement over the partial covering (*s2 \ 5 for two additional free parameters). The Ðtted photon index, ! \ 1.30`0.35, is consistent within the uncer~0.40 tainties with the Ginga value of ! \ 1.7, and the Ðtted column densities, N1 D 2 ] 1023 and N2 D 6 ] 1022 cm~2, indicate substantial Hintrinsic absorption.HFigure 2 shows the best-Ðt folded model and the residuals. The observed Ñuxes in 0.2È2 and 2È10 keV are F D 3 ] 10~13 ergs s~1 and F D 7.7 ] 10~120.2h2 ergskeV s~1. The corresponding 2h10 keV(not corrected for absorption) are L obs luminosities D 2 ] 1042 ergs s~1 and L obs D 5 ] 1043 ergs 0.2h2 s~1, keV while 2h10 keV the intrinsic (absorption-corrected) luminosities are L int D 3 ] 1043 ergs s~1 and L int D 8.5 ] 1043 0.2h2 keV The latter is well within the 2h10 keVexpected from ergs s~1. range the [O III] luminosity (1043È1044 ergs s~1), and close to the IR luminosity, L D 1 ] 1044 ergs s~1. The ASCA data 25k is one of the most luminous AGNs with conÐrm that 3C 445 high intrinsic X-ray absorption columns (Turner & Pounds 1989 ; Pounds 1990). Power-law plus reÑection.ÈCompton scattering of the primary continuum by cold material (e.g., Lightman &

No. 2, 1998

ASCA OBSERVATIONS OF 3C 445

753

1994), and requiring a decrease of the elemental abundances by a factor of 10 to reconcile the observed value with the theoretical predictions (e.g., George & Fabian 1991). We thus discard the power-law plus reÑection model as an explanation of the ASCA data of 3C 445 and explore more closely the solution provided by nonuniform dual absorption. 3.1.3. Constraints to the Dual Absorber Model from ROSAT plus ASCA Joint Fits

FIG. 2.ÈResults from the Ðt to the ASCA data with a dual absorber model. (a) Folded model and (b) residuals, plotted as the ratio of the data to the model. Only the SIS data are shown for clarity, but the Ðt was performed jointly to both the SIS and GIS detectors. The 5È7 keV region, which was excluded from the Ðt, has been added back to the plots to show the proÐle of the Fe Ka line. The latter is signiÐcant at P [ 99% conF Ðdence and has a measured equivalent width of EW D 270 eV.

White 1988 ; Guilbert & Rees 1988) has been often invoked to reconcile the apparently Ñat Ginga and ASCA spectra of Seyfert galaxies with the canonical AGN slope ! \ 1.9 (Nandra & Pounds 1994). We thus Ðtted the ASCA data of 3C 445 with a power law plus a reÑection component, using the PEXRAV model in XSPEC (Magdziarz & Zdziarski 1995) for the latter, assuming face-on geometry (the Ðt turned out to be insensitive to the value of the inclination angle) and solar abundances. The Ðt with this model leaves some weak structure around 1 keV in the form of excess counts ; adding a Raymond-Smith model provides a signiÐcantly improved Ðt (*s2 \ 8 for two additional parameters, P D 95%). The Ðt with the power-law plus reÑection plus F Raymond-Smith model gives s2 \ 1.08/311, temperature r L kT \ 0.9 ^ 0.1 keV, and luminosity D 3.6 ] 1041 0.2h2power-law keV ergs s~1 for the thermal component, slope ! \ 1.85 ^ 0.13, and covering factor for the reÑection component (deÐned as the ratio of the reÑection and power-law normalizations) R \ 756 ^ 530. Although formally acceptable, the reÑection model is physically unappealing for a number of reasons. First of all, the very large value of R, possibly indicating radiation strongly beamed toward the reprocessor (Ghisellini et al. 1991), yields an intrinsic 2È10 keV luminosity L D3 keVthan ] 1046 ergs s~1, at least 2 orders of magnitude 2h10 higher the 2È10 keV luminosity expected from the [O III] luminosity. Moreover, little or no reÑection was observed in the earlier Ginga data, where R D 0 at 1 p (Wozniak et al. 1998). Finally, the observed EW of the Fe line within this model is EW D 80 eV, much weaker than expected for a continuum dominated by reÑection (Matt et al. 1996 ; Fukazawa et al.

Because of the limited bandpass of ASCA and the presence of complex absorption, the parameters of the continuum in the dual absorber model are rather poorly determined. As we show below, tighter constraints can be obtained from Ðts to the joint ROSAT and ASCA data sets. Bearing in mind that cross-calibration uncertainties are still present between the two instruments, we performed Ðts to the 0.3È10 keV data with the dual absorber model. The two data sets are not simultaneous, however the ROSAT Ñux in 0.2È2.0 keV, F D 3 ] 10~13 ergs cm~2 s~1, is similar to ASCA.0.2h2 keV The Ðt to the ROSAT plus ASCA data with the dual absorber is good (s2 \ 1.09/318) and is strongly preferred r model (*s2 \ 30 for two additional over a partial covering parameters, signiÐcant at P [ 99% conÐdence). As apparF ent from Table 2B, the addition of the ROSAT data causes the photon index to steepen (! D 1.5) and the covering fraction to increase (C D 94%) with respect to the ASCA data F alone, and puts more stringent limits on the absorber column densities. The conÐdence contours of ! and C are F plotted in Figure 3 for the ROSAT plus ASCA data (solid line) and for the ASCA data alone (dotted line), these con-

FIG. 3.ÈContours at 68%, 90%, and 99% conÐdence for the photon index and covering fraction from a Ðt with the dual absorber model to the ASCA (dotted line) and ROSAT plus ASCA (solid line) data of 3C 445. Because of the low signal-to-noise ratio, meaningful contours could be produced only by Ðxing the column densities to their best-Ðt values. Note the tighter constraints on the continuum given by the combined ROSAT and ASCA data, illustrating the merits of broadband observations for 3C 445 for measuring the continuum parameters.

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tours illustrate how much better the continuum parameters are when they are determined from the Ðts to the broader band data. (Note that, because of the low signal-to-noise ratio, the contours were produced by Ðxing the column densities to their best-Ðt values.) Figure 3 illustrates the advantage of broadband X-ray observations of 3C 445 to reÐne the measure of the continuum. 3.2. T he Fe L ine We modeled the Fe line by adding a Gaussian line to the Ðts with the dual absorber. At Ðrst, leaving the energy of the Gaussian free to vary, we always Ðnd it to be consistent with the redshifted energy of Fe Ka line, 6.05 keV [6.4/ (1 ] z), with z \ 0.057]. In the following, the Gaussian energy is Ðxed to this value. The line is highly signiÐcant : adding the Gaussian component causes the s2 to drop by *s2 \ 29, a P [ 99% F 2 lists conÐdence improvement. The lower section of Table the Ðtted parameters of the Gaussian. The line is probably broad, with width p \ 0.15`0.15 keV (90% conÐdence errors) ; a line broader than ~0.12 p \ 0.6 keV is excluded at greater than 99% conÐdence. Because of the limited signalto-noise ratio, the line width was held at its best-Ðt value of 0.15 keV in deriving the uncertainties for the other Ðt parameters in Table 2C. The line EW is, however, still poorly constrained, EW \ 268`272 eV. Fixing the contin~118 uum parameters to their best-Ðt values from the Ðt to the ROSAT and ASCA data, we obtain EW \ 252`124 eV. ~81 4.

SUMMARY AND DISCUSSION

We have reported the Ðrst high-resolution, moderate signal-to-noise ratio X-ray observations of the broad-line radio galaxy 3C 445 obtained with ASCA. Our principal results are listed below. 1. The ASCA data conÐrm that a powerful AGN is hosted by 3C 445, with an intrinsic (absorption-corrected) 2È10 keV luminosity L int D 8.5 ] 1043 ergs s~1. keV 2. The Fe Ka line is 2h10 conÐrmed. 3. The observed broadband ASCA continuum in 0.6È10 keV is unusually Ñat (! \ 1), and a soft excess is present below D1.5 keV. 4. The luminosity of the soft excess (L int D4 0.5h4.5 ] 1042 ergs s~1) and its spectral shape are mostkeVlikely inconsistent with an origin in the host galaxy, and are related instead to the AGN activity. 5. The ASCA data are well described by a dual absorber with a Ñat (! D 1.3) intrinsic slope and two absorbers, a nonuniform one with N1 D 2 ] 1023 cm~2 and covering fraction C D 80%, and aHuniform one with N2 D 6 ] 1022 H cm~2. TheFFe line has EW D 270 eV. 6. Joint Ðts with the dual absorber to the nonsimultaneous ROSAT and ASCA data provide tighter constraints for the continuum parameters. The amount of X-ray absorption inferred from the ASCA data is 1È2 orders of magnitude larger than the extinction derived from the IR/optical data (assuming Galactic gas-todust ratios), N D 5 ] 1021 cm~2 (see ° 1). This immediately suggestsH that the X-ray absorbers must lie in the inner region of the AGN, inside the BLRG radius, or the

Vol. 495

latter would be obscured. For example, the nonuniform absorber with N D 1023 cm~2 could be interpreted as a H distribution of clouds in thermal equilibrium at distances less than 1015 cm, as in the model of Ferland & Rees (1988), with a second shell of uniform absorption at D6 ] 1022 cm~2 to account for the second ASCA absorber. Alternatively, the X-ray absorber has a lower than Galactic dust content. The presence of cold gas clouds in the inner regions of 3C 445 is in agreement with the results of Granato, Danese, & Franceschini (1997), who compared the column densities derived from IR and X-ray observations for a sample of Seyfert 2s, and concluded that the amount of X-ray absorption always exceeds that inferred at longer wavelengths by a factor of 2È10, indicating that the gas responsible for the X-ray absorption is located inside the dust sublimation radius. In particular, for column densities 1022È1023 cm~2, they Ðnd that the IR opacity is approximately a factor of 10 lower than at X-ray wavelengths, as in the case of 3C 445. We also note in passing that marginal evidence for column variability is suggested in 3C 445. It is difficult to compare the EXOSAT and Ginga results to those of ASCA because of the di†erent bandpasses (and because of the contamination from the nearby cluster in the Ginga data). However, the EXOSAT ME data were consistent with a column density N D 5 ] 1022 cm~2 (Turner & Pounds 1989), H which is 1 order of magnitude lower than ASCA and Ginga. If conÐrmed, the column density variability in 3C 445 could support a scenario where the absorbing gas is in the form of clouds intercepting the line of sight. With the continuum described as a dual absorber, the Fe line is clearly detected. The observed EW (D270 eV), is suggestive of an origin in material with high column densities, N is approximately a few times 1023 cm~2 (Leahy & H 1993). The line is probably broad, p D 0.2 keV, Creighton implying velocities [ 35,000 km s~1 for the emitting gas. The presence of complex absorption by cold gas in 3C 445 is in conÑict with the uniÐcation scenario (extended to radio-loud objects, e.g., Urry & Padovani 1995), where the nucleus of a type 1 AGN is seen directly, with little or no neutral absorption. A Ðrst possibility is that 3C 445 switched its classiÐcation from type 1 to type 2 at the time of the ASCA and Ginga observations. This is a concrete possibility in the reÐned version of the uniÐcation model proposed by Turner et al. (1998). In order to account for the similar Fe Ka proÐles of a sample of Seyfert 1 and 2 galaxies, and to explain their intrinsically di†erent accretion properties, Turner et al. (1998) proposed that the absorbing material in these objects is fragmented into a uniform and isotropic distribution of clouds at the distance of the putative dusty torus, with density and composition of the gas varying from cloud to cloud. Thus an object could easily appear as a type 1 or 2 depending on whether a cloud of sufficient column density intercepts the line of sight. A change of classiÐcation in 3C 445 would imply strong variability for the broad emission lines on timescales of years due to variable reddening. Indeed, the Ha line proÐle has been reported to vary dramatically in 12 yr (Crenshaw et al. 1988), although in a way that does not suggest immediately a change of classiÐcation for the object : a strong Ha blue wing seen at the time of the observations by Osterbrock et al. (1976) is no longer seen by Crenshaw et al. (1988). Alternatively, taking at face value the uniÐcation model, we would be tempted to conclude that 3C 445 is indeed intrin-

No. 2, 1998

ASCA OBSERVATIONS OF 3C 445

sically a type 2 AGN, and that the broad emission lines are produced outside the compact nuclear environs, such as, e.g., in a jet. The luminosity of the hard X-ray component in the ASCA spectrum, L int D 8.5 ] 1043 ergs s~1, is similar 2h10 keV to the EXOSAT luminosity (Turner & Pounds 1989) and a factor of B2 lower than Ginga (Pounds 1990) (after rescaling to the di†erent value of H used by the latter authors), indicating some variability0 of the hard X-rays. This is in contrast to the 0.2È2.0 keV Ñux, which remained constant between the ROSAT and ASCA observations taken 2 yr apart (° 3.1.3), and consistent with an earlier marginal detection of the Einstein IPC (Wilkes et al. 1994). This could support a scattering origin for the soft X-rays, possibly o† free electrons that could be also responsible for producing the polarized Ha line (Kay et al. 1996). More observations of 3C 445 at X-rays and at longer wavelengths are needed to conÐrm this suggestion and to determine the nature of the scattering mirror. As a Ðnal note we would like to compare brieÑy 3C 445 to other radio galaxies observed with ASCA. Data have been published so far for a handful of objects, including the BLRGs 3C 390.3, 3C 120, 3C 382, 3C 109, and Pictor A (Eracleous et al. 1996 ; Grandi et al. 1997 ; Reynolds 1997 ; Allen et al. 1997 ; Eracleous & Halpern 1998), and the NLRGs Centaurus A and Cygnus A (Arnaud 1996 ; Turner et al. 1997). In both BLRGs and NLRGs, a power-law (! B 2) continuum plus a strong and broad Fe line were detected, with the notable exceptions of Pictor A (Eracleous & Halpern 1998) and of Cyg A (Arnaud 1996). To facilitate the comparison we plot in Figure 4 the ASCA photon index, the intrinsic column density (the di†erence of the Ðtted N and the Galactic value), and the Fe line EW versus H keV luminosity for BLRGs (open circle) and the 2È10 NLRGs ( Ðlled triangles) ; 3C 445 is shown as a Ðlled circle. Despite the low statistics, the photon index distribution of both BLRGs and the two NLRGs is remarkably narrow around ! D 1.9 over more than 3 luminosity decades. This value of the photon index is similar to the intrinsic slope of the Seyfert galaxies observed with ASCA (Nandra et al. 1997), in contrast with earlier claims, based on lower sensitivity data, that radio-loud AGNs are Ñatter in the X-rays than their radio-quiet counterparts (e.g., Wilkes & Elvis 1987). A more detailed discussion awaits larger samples of radio-loud objects observed with ASCA (Sambruna et al. 1998). In Figure 4b, little or no excess N over Galactic H column density is measured for BLRGs, contrary to the two NLRGs that are obscured by N D 1023 cm~2. The galaxy H luminous BLRG with the 3C 445 sticks out as a moderately more heavily absorbed X-ray spectrum, while its Fe line is not particularly weak compared to other BLRGs (Fig. 4c). The only other BLRG where intrinsic X-ray absorption was detected so far is 3C 109, a luminous FR II at z \ 0.306 (Allen et al. 1997 and references therein). In 3C 109, as in 3C 445, the optical continuum and Ha line are intrinsically polarized (although with a higher polarization degree than in 3C 445), and the large Balmer decrement and steep IR-tooptical slope indicate the presence of substantial intrinsic absorption, most likely dust (Elvis et al. 1984). The intrinsic X-ray column density in 3C 109 is D2 ] 1021 cm~2 (Fig. 4b) ; however, in 3C 109 the X-ray column density is in good agreement with the value derived from the optical reddening (Allen et al. 1996), and supports a picture where the line of sight to this BLRG skims the outer edge of the

755

FIG. 4.ÈSpectral properties of radio galaxies with published ASCA data. The following quantities are plotted vs. the intrinsic (absorptioncorrected) 2È10 keV luminosity : (a) Photon index of the continuum, (b) intrinsic column density N , measured as the di†erence of the ASCA H column density and the Galactic value, and (c) equivalent width of the Fe Ka line. Open circles represent broad-line radio galaxies, and Ðlled triangles represent the two narrow-line radio galaxies Centaurus A (at low luminosity) and Cygnus A (at high luminosity). The galaxy 3C 445 is plotted as a Ðlled circle, with constraints from the Ðts to the joint ROSAT and ASCA data (Fig. 3) ; it sticks out as a moderately luminous BLRG with the most heavily absorbed X-ray spectrum. Note in panel (a) the narrow distribution of the photon indices around ! D 1.9, similar to Seyfert galaxies.

molecular torus. This is in contrast to 3C 445, where the more complex absorption properties cannot be easily reconciled within the uniÐed models, again underlining the unique nature of this source among BLRGs. 5.

CONCLUSIONS

To summarize, 3C 445 is a BLRG with unusual X-ray properties that are not easily explained in the current version of the uniÐed models. Its ASCA spectrum is consistent with the presence of a substantial amount of cold matter in the nuclear region, challenging the common notion that the nuclei of type 1 AGNs are seen directly. Clearly, 3C 445 is a peculiar BLRG that deserves further spectroscopic follow-up in the X-rays and at other wavelengths. The source qualiÐes as an excellent target for broadband SAX observations, which will allow us to reÐne the measure of the continuum parameters, as well as assess unambiguously the presence of a Compton hump at high energies and of any scattering component at soft X-rays. R. M. S. and K. N. acknowledge Ðnancial support from the NRC Research Associate program. I. M. G. and T. J. T. were Ðnancially supported through University of Space

756

SAMBRUNA ET AL.

Research Association. An anonymous referee provided constructive criticism that improved the presentation of the paper. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet

Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and NASAÏs Astrophysics Data System Abstract Service.

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