Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/people/Matthew.Whiting/papers/ASCA_conference_paper.ps.gz Äàòà èçìåíåíèÿ: Thu Mar 23 02:58:50 2006 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 15:52:58 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: dust

Soft X­ray absorption in Parkes Quasars
M.t. Whiting 1 , R.l. Webster 1 and P.j. Francis 2;3
1 Astrophysics group, School of Physics, University of Melbourne, Parkville, Victoria 3052, Australia
2 Department of Physics and Theoretical Physics, Faculty of Science, Australian National University, Canberra,
ACT 0200, Australia
3 Joint Appointment with the Research School of Astronomy and Astrophysics
1. Introduction
Quasars from the Parkes Half­Jansky Flat Spectrum
Sample (PHFS) (Drinkwater et al. 1997) are known to
have a large dispersion in optical (B \Gamma K) colours (Web­
ster et al. 1995, Francis et al. 1999), with the reddest ob­
jects having B \Gamma K ? 7. As such, they are much redder
than optically selected quasars.
There are two different theories to account for this
large dispersion in B \Gamma K colour: excess red emission
(most likely synchrotron) is present that increases the
K flux; or dust absorption takes place, reducing the B
flux.
These two models have been explored by Whiting
et al. (in preparation). We have found that there is an
identifiable minority of quasars that are best fit with
dust extinction, rather than synchrotron reddening. It
is expected that, for these sources, there will be some
degree of absorption of soft X­rays by gas associated
with the dust. This possibility has been explored ex­
tensively by Masci et al. (1999) (M99) for quasars from
the PHFS. We repeat their analysis for a subset of the
PHFS, using the more appropriate values for the optical
extinction gained from our model fitting.
2. Selection of dusty sources
To discriminate between the different types of sources
present in the PHFS, we fitted different models to the
photometry: a synchrotron + big blue bump model, a
dust­reddened big blue bump model, and a pure big blue
bump model. From this fitting we were able to pick out
the sources that were clearly dust­reddened (rather than
synchrotron reddened), by choosing the sources best fit
by the dust model. This yielded a sample of 33 sources,
or 28% of the total number considered.
We then plotted the extinction A V gained from the
dust fits against the optical -- soft X­ray continuum
slope, in the manner of M99. Our X­ray fluxes came
from the ROSAT observations of the whole PHFS by
Siebert et al. (1998), as did those used by M99.
3. Results and discussion
The predictions from M99 for the optical--X­ray slopes,
as a function of A V , are:
ff BX = (1:3 +0:3
\Gamma0:2 ) \Gamma (0:13 +0:03
\Gamma0:02 )A V ;
ff KX = (1:0 +0:3
\Gamma0:2 ) + (0:05 +0:04
\Gamma0:03 )AV :
(See M99 for details). Our results, together with the
predicted relationships, are shown in Fig 1.
Fig. 1. Our results, for both optical-- and NIR--Soft X­ray
slopes. Lines show predictions from M99 (solid = predictions,
dotted = limits). Circles are X­ray detections, and triangles
are X­ray upper limits (lower limits to the index).
The best fit relationships to the data, found using
the survival analysis package ASURV, are:
ff BX = 1:54 \Gamma 0:14A V ; ff KX = 0:82 + 0:07A V
These results are consistent with the simple hypoth­
esis that dust is causing the absorption in the opti­
cal/NIR, and that the associated gas is causing absorp­
tion at soft X­ray energies. The fact that the slope of
the best fit lines are within the error margins of the
predictions indicates that the assumption of Galactic
gas­to­dust ratios is valid for these sources.
The intrinsic values of the indices, given by the zero
points on the graphs, are also consistent with the pre­
dictions. The value from ff KX is at the low end of the
predicted range, which perhaps indicates that radio­loud
flat spectrum quasars have a flatter intrinsic slope than
optically selected quasars (from which the prediction
was taken).
These results give support to the assertion that at
least a subset (and an identifiable subset at that) of the
PHFS are affected by dust. This has important impli­
cations if radio­quiet quasars are similarly affected (see
e.g. Webster et al. 1995). Indeed, our results here pre­
dict that about a quarter of quasars should exhibit signs
of dust reddening, and this can be tested by multicolour
surveys such as the Sloan Digital Sky Survey.
References
Drinkwater et al.: 1997, Mon. Not. R. Astron. Soc. 284, 85
Francis, P.J., Whiting, M.T. & Webster R.L.: Publ. Astron.
Soc. Australia, submitted
Masci, F.J., Drinkwater, M.J. & Webster R.L.: 1999, Astro­
phys. J. 510, 703 (M99)
Siebert J. et al.: 1998, Mon. Not. R. Astron. Soc. , 301, 261
Webster, R.L. et al.: 1995, Nature 375, 469