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ROSAT PSPC Observations of the Infrared Quasar IRAS 13349

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ROSAT PSPC Observations of the Infrared Quasar IRAS 13349 Mon Not R Astron So c Printed August ROSAT PSPC observations of the infrared quasar IRAS evidence for a warm absorb er with internal dust WN Brandt AC Fabian and KA Pounds Institute of Astronomy Madingley Road Cambridge CB HA Internet wnbastcamacuk acfastcamacuk Xray Astronomy Group Department of Physics Astronomy University of Leicester University Road Leicester LE RH Internet kapstarleacuk ABSTRACT We present spatial temp oral and sp ectral analyses of ROSAT Position Sensitive Pro p ortional Counter PSPC observations of the infrared loud quasar IRAS IRAS is the archetypal highlyp olarized radioquiet QSO and has an op ticalinfrared luminosity of erg s We detect variability in the ROSAT count rate by a factor of in ab out one year and there is also evidence for p er cent variability within one week We nd no evidence for large intrinsic cold absorption of soft Xrays These two facts have imp ortant consequences for the scatteringplus transmission mo del of this ob ject which was developed to explain its high wavelength dep endent p olarization and other prop erties The soft Xray variability makes electron scattering of most of the soft Xrays dicult without a very p eculiar scattering mirror The lack of signicant intrinsic cold Xray absorption together with the large observed E B V suggests either a very p eculiar system geometry or more probably absorp tion by warm ionized gas with internal dust There is evidence for an ionized oxygen edge in the Xray sp ectrum IRAS has many prop erties that are similar to those of narrowline Seyfert s Key words galaxies individual IRAS Xrays galaxies ment that its ultraviolet continuum slop e may change with INTRODUCTION time see their sect Despite its high p olarization and IRAS is the rst previously unidentied quasar radioquiet nature IRAS has not b een seen to that was selected through its infrared emission and is the have broad absorption lines BAL see Boroson Meyers prototype radioquiet infraredbright quasar Beichmann et al It has a redshift of z and a high p olar ization p ercentage which rises towards the blue from p er cent at m K band to p er cent at m U band In this pap er we present a detailed study of the soft astro-ph/9508057 12 Aug 1995 Wills et al hereafter W W found no variability Xrays from IRAS We use the observed soft of the p olarization or ux density on time scales from days Xray prop erties to explore and test facets of the W to months To explain its p olarization and other prop erties mo del In particular we shall examine soft Xray absorp W have developed an elegant mo del for the inner regions tion and shall compare our measured cold absorption with of the QSO and argue for a bip olar geometry They suggest that suggested by the large reddening of E B V that a direct but attenuated AGN sp ectrum reaches Earth ROSAT allsky survey detections of IRAS were through a dusty inclined disc that is parallel to the plane of briey rep orted in Walter Fink hereafter WF the host galaxy and probably b etween the broad line region and Brinkmann Sieb ert but our temp orally sepa and the narrow line region In addition light escaping in rated and deep er p ointed observations allow us to p erform p olar directions is much less attenuated and is scattered to a much more sophisticated study of this imp ortant ob ject wards Earth This scattered light is hence p olarized parallel It is worth noting that IRAS is one of the p e to the ma jor axis of the host galaxy culiar outlying ob jects in the WF ROSAT allsky survey correlation b etween sp ectral slop e and ultraviolet A Beichmann et al Barvainis and W to keV ux have mo delled the dust emission from IRAS The optical continuum of IRAS exhibits a dis tinct upturn in the blue ab ove ab out m in contrast to A value of the Hubble constant of H km s what is often seen in p owerful infrared quasars see sect Mp c and a cosmological deceleration parameter of q I I Ic of Frogel et al Lanzetta Turnshek Sandoval present IUE sp ectra of IRAS and com have b een assumed throughout 2 WN Brandt AC Fabian KA Pounds Temporal analysis OBSERVATIONS AND DATA REDUCTION Count rates should b e averaged over the s ROSAT wob ROSAT PSPC Trumper Pfeermann et al ble p erio d when used for source ux determination as vari observations were made of IRAS starting on ability on timescales shorter than this can b e inuenced by Jan RP total raw exp osure of s over thick and thin wire shadowing The wobble path can b e geo s in two observation intervals and Dec metrically distorted by digitization uncertainties in the star RP total raw exp osure of s in one obser sensor readout and this can lead to apparent variations in vation interval Hereafter we shall refer to RP as count rate over dierent wobble p erio ds However such ef Pointing or P and RP as Pointing or P fects are generally small and Brinkmann et al state IRAS has and raw background sub that the accuracy of ux determinations in wobble mo de is tracted counts in P and P resp ectively go o d to within p er cent We have used s binning The ROSAT observations were p erformed in the stan for all temp oral analysis work dard wobble mo de to avoid accidental shadowing of We have searched the ROSAT housekeeping data for sources by the coarse wire grid which forms part of the instrumental eects that might pro duce the variability dis PSPC entrance window supp ort structure ROSAT p er cussed b elow The asp ect uncertainty ASPERR is normal forms a slow dithering motion diagonal to the detector axes for all observations We have removed data where the mas with a p erio d of s and an amplitude of arcmin ter veto rate EEMV We have checked for general The PSPC detector at the fo cus of the ROSAT X correlations b etween the count rate of IRAS ray telescop e has a bandpass of keV over a and the master veto rate and no correlations are apparent diameter eld of view Its energy resolution is E E : We have done this for b oth P and P jointly as well as for E FWHM where E is the photon energy just P and just P to prevent the relatively large source measured in keV The spatial resolution for the full bandpass count rate change b etween P and P from confusing our is arcsec FWHM onaxis Hasinger et al The correlation checking We have examined the count rates of PSPC background count rate is very low and is dominated a serendipitous source near the centre of the ROSAT eld by the cosmic ray induced particle background scattered so of view and it shows no signs of correlated variability with lar Xrays and the diuse Xray background The rejection IRAS b etween P and P this source was the eciency for the particle background is excellent and the brightest one in the eld after IRAS The residual count rate due to cosmic rays is only source is to o faint to allow us to sensitively lo ok for cor count s arcmin keV Snowden et al related variability within just p ointing P It is lo cated at Reduction and analysis of the PSPC image data was hms dms It is not listed p erformed with the Starlink ASTERIX Xray data pro cessing in NED and has a count rate of ab out counts s We system have rep eated some of the temp oral analysis of the data us ing the IDL ROSAT analysis software system and obtain ANALYSES the same general results Fig shows the IRAS full ROSAT band Spatial analysis light curves during the P and P ROSAT p ointings The The keV spatial prole of IRAS in b oth weighted mean count rate during P is a factor of times observations is consistent with that of a p oint source con that that during P and during P IRAS has volved with the ROSAT XRT and PSPC spatial resp onses b ecome one of the brighter AGN in the ROSAT sky The Hasinger et al IRAS lies in the cen end of the P p ointing and the start of the P p ointing are tres of the ROSAT elds of view for b oth observations separated by ab out days The Xray centroid of the source we identify with IRAS A comparison of the data from the rst and second ob is hms dms servation intervals of P suggests further variability Exactly during P and its centroid is hms the same source and background regions were used for b oth dms during P These p ositions are consistent observation intervals in making this comparison and b oth with the optical p osition of hms observation intervals were pro cessed together A constant dms and the p ositional error of the ROSAT PSPC mo del is a p o or description of the P data giving of arcsec for seven degrees of freedom we have included p er cent We have extracted source counts for IRAS ux determination errors as p er Brinkmann et al in from circular source cells chosen to b e large enough to en this analysis This can b e rejected at greater than sure that all of the source counts are included given the p er cent condence electronic ghost imaging which widens the p oint spread We have examined ratios of keV count rate to function b elow keV Hasinger et al The radii keV count rate and there is no strong evidence for of the circular source cells were not a function of channel hardness ratio variability either b etween P and P or b e energy Background counts were subtracted from the source tween the two observation intervals of P cells using large nearby circular sourcefree background cells Corrections were included for PSPC dead time vignetting Sp ectral analysis and shadowing by the coarse mesh window supp ort We Counts from the corrected circular source cells were ex have conservatively disregarded data
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