A&A 442, L53-L56 (2005)
DOI: 10.1051/0004-6361:200500189
E. Piconcelli - M. Guainazzi
European Space Astronomy Center (ESA), Apartado 50727, 28080 Madrid, Spain
Received 18 May 2005 / Accepted 14 September 2005
Abstract
We present the analysis and the results of a 20 ks
XMM-Newton observation of the extremely X-ray loud (
erg s-1) flat-spectrum radio quasar RBS 315 at a
redshift of 2.69. This EPIC observation has allowed us to strongly
constrain the slope of the continuum (
)
as
well as to discover the presence of a sharp drop below
2 keV in its spectrum.
Such a flat photon index and the huge luminosity suggest that the
X-ray emission is due to the low energy tail of the Comptonized
spectrum, produced from plasma in a relativistic jet oriented close
to our line of sight. Even though the hypothesis of a break in the
continuum cannot be completely discarded as an explanation of the soft
X-ray cutoff, the presence of intrinsic absorption appears more plausible.
Spectral fits with cold (
=
1.62
+0.09-0.09
1022 cm-2) and lukewarm (
= 2.2
+0.9-0.3
1022 cm-2;
= 15
+38-12 erg cm-2 s-1) absorbers are
statistically indistinguishable. Remarkably, our results are very similar to
those reported so far for other absorbed high-z Blazars observed by
XMM-Newton. The existence of this "homogeneous'' class of jet-dominated
superluminous obscured QSOs at high z therefore could be
important in the context of the formation and cosmological evolution
of radio-loud objects.
Key words: galaxies: active - quasar: general - X-ray: individual: RBS 315
According to Active Galactic Nuclei (AGNs) Unified models (Urry &
Padovani 1995) a relativistic jet emitting a beamed
non-thermal continuum is present in radio-loud quasars (QSOs): if the
jet points along our line of view, the QSO is seen as a Blazar. Due
to strong amplification and collimation in the observer's frame
Blazars appear as the most luminous objects in the sky from radio
through gamma-rays. As pointed out by Fossati et al. (1998) the
X-ray spectrum of Blazars hardens with increasing luminosity: in fact
flat-spectrum radio-loud QSOs (FSRQs), which represent the most
luminous class of Blazars, typically show photon indices
in the 2-10 keV band (e.g. Boller et al. 2000;
Donato et al. 2005; Ferrero & Brinkmann 2003, F03 hereafter)
i.e. flatter than usually observed in "normal'' radio-loud
(
;
Sambruna et al. 1999) or
radio-quiet QSOs (
;
Piconcelli et al. 2005a).
The discovery of a low-energy cutoff due to heavy cold absorption
(10
22-23 cm-2) in the X-ray spectra of some luminous
high-z FSRQs (Elvis et al. 1994; Cappi et al. 1997; Boller et al. 2000; Yuan et al. 2000) was therefore unexpected since our line of sight should not
intercept the obscuring torus. An explanation in terms of obscuration
associated with intervening systems was promptly ruled out by
O'Flaherty & Jakobsen (1997) and Fiore et al. (1998) on the basis of
statistical arguments, supporting therefore the hypothesis that the
absorber is physically associated to the QSO. However, the properties
of the absorbing matter are still poorly known and the observational constraints about its
nature are very sparse. Unfortunately the low sensitivity and
limited soft X-rays bandpass of ASCA and BeppoSAX data as
well as the usual high z of the "obscured'' FSRQs prevented
deeper investigations about the origin of the absorption features.
Thanks to its unprecedented large collecting area, XMM-Newton has allowed
the investigations in this field to be re-opened. Worsley et al. (2004a, W04 hereafter) claimed for a warm absorber in the
spectrum of the Blazar PMN J0525-3343, while F03 found that both a
warm and a cold absorber can fit the data of the Blazar PKS 2126-158.
On the other hand, some recent works based on XMM-Newton observations (F03;
Grupe et al. 2004; Piconcelli et al. 2005b) do not confirm earlier
ASCA measurement of a strong absorption toward three high-zQSOs.
In this letter we present the analysis of the XMM-Newton observation of
RBS 315 that has allowed to explore in detail the 0.3-12 keV spectrum
of this source for the first time. RBS 315 was discovered as a powerful
flat-spectrum radio source (
mJy;
)
by Lawrence et al. (1983) and subsequently identified
as a QSO at z = 2.69 with a radio-loudness parameter
in the ROSAT Bright Survey (RBS; Schwope et al. 2000). RBS 315 was
detected in X-rays by ROSAT and RXTE during the All-Sky
Slew Survey (XSS; Sazonov et al. 2004). Both X-ray measurements imply
a huge luminosity of
erg s-1 and a very flat
hardness ratio, which is a clear indication of a convex spectrum.
All the spectral fits were performed with the XSPEC package (version 11.3.0). The models presented in this section include an
absorption component due to the line-of-sight Galactic column of
= 1021 cm-2 (Dickey & Lockman 1990). We initially fitted
the hard (2-10 keV, corresponding to 7.4-37 keV in the frame of
RBS 315) portion of the spectrum with a power law. This fit turned out to
be very good yielding an associated
=1.07 with a resulting
best-fit value of the photon index
.
Visual inspection of the data-to-model ratio residuals did not
suggest the presence of any emission/absorption features. However, the
extrapolation of the power law to energies lower than 2 keV clearly
revealed the presence of a deep deficit in the soft range of the
spectrum (Fig. 1). Fitting the data over the 0.3-12 keV band
with a simple power law (Model PL) yielded a
(d.o.f.) 1821(455). We
accounted for the soft X-ray spectral drop by means of a neutral ("cold'')
absorption component in the fitting model. This fit (model APL
hereafter) resulted in a dramatic statistical improvement i.e. at the
>99.99% confidence level according to an F-test once compared
with the model PL (
), with a
(d.o.f.) = 479(454).
This fit therefore provides an excellent parametrization of the
X-ray spectrum of this Blazar with a best-fit value of
and
=1.62
cm-2 for the spectral index
and the intrinsic column density
of the "cold'' absorber, respectively.
Using Model APL we measured a flux of
=
erg cm-2 s-1 and
=
erg cm-2 s-1 in the low (0.5-2 keV)
and high (2-10 keV) energy bands, respectively. After correction for
both Galactic and intrinsic absorption column densities, these
correspond to luminosities of
=
erg s-1 and
=
erg s-1, respectively.
It is worth to note that if the redshift of the absorber if fixed to z=0,
the resulting column density value decreases to
(z=0) =
cm-2. However this fit, with an associated
(d.o.f.) = 508(454), is statistically worse
than Model APL (i.e.
for the same number of d.o.f.)
and some residuals, as ratio between model and data, are clearly present in the
soft X-ray band. The hypothesis that the absorber could be associated with our Galaxy
appears, therefore, quite unlikely.
![]() |
Figure 1: Continuum power-law fit to the hard band of the PN ( top) and MOS ( bottom) spectra of RBS 315 extrapolated over the 0.3-12(0.8-10) keV band for PN( MOS). The lower panel shows the deviations of the observed data from the model in unit of standard deviations. |
Open with DEXTER |
As W04 suggested the presence of a warm absorber as the most
plausible explanantion of the soft X-ray cutoff in the XMM-Newton spectrum of
the Blazar PMN J0525-3343 at z = 4.4,
we then investigated if it is the case also for RBS 315.
This fit, performed using the ABSORI model in XSPEC, is statistically equivalent to that with the APL model, i.e. associated (d.o.f.) = 478(453). The resulting column density of the warm gas was
= 2.2
+0.9-0.3
1022 cm-2, while the best-fit value ionization parameter
was
erg cm-2 s-1. This value is comparable to what reported by W04.
An alternative explanation for the convex spectrum in Blazar is the
presence of an underlying break in the continuum.
Fossati et al. (1998) have shown that
the spectral energy distribution of Blazars is "double-humped'' with
two main components due to the synchrotron emission and Inverse
Compton (IC) scattering of relativistic electrons off synchrotron or
ambient "seed'' photons, respectively.
In the case of the FSRQs, such as RBS 315, the IC hump peaks at 10-100 MeV: this means that
the spectrum in the XMM-Newton bandpass is dominated by the Comptonized "boosted'' emission from the jet.
A spectral break
can originate through a low-energy cutoff in the energy distribution
of the radiating electrons or by a sharply peaked external "seed'' photon
distribution (e.g. Fabian et al. 2001). We then tried an alternative fit modeling the spectrum
with a broken power law. This parameterization yielded unacceptable fit with an associate
(d.o.f.) = 544(453),
i.e. significantly worse than that derived with model APL.
A model with two (unabsorbed) power laws also turned out in a very poor fit (
3).
![]() |
Figure 2:
Confidence contour plot showing the photon index against the
rest-frame column density (
![]() |
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The EPIC observation presented here provides the first good-quality
X-ray spectrum of RBS 315. It has allowed us to strongly constrain the
slope of the continuum (
)
as well as to
discover the presence of a sharp drop in the observed spectrum below
1.5 keV. Such a value of photon index is consistent with previous
X-ray observations of luminous FSRQs (
,
Sambruna et al. 1997; see also W04 and F03) and
is also in agreement with the trend found by Fossati et al. (1998) for
a large sample of Blazars according to which the more X-ray luminous
the object is, the flatter X-ray spectrum it possesses. The explanation
for this behavior is that the X-ray continuum in FSRQs is believed to
be the low energy tail of the Comptonized spectrum peaking in the
MeV-GeV range and as the peak moves to lower energies with increasing
luminosity, the X-ray spectrum therefore becomes harder.
The most important results of our analysis is the discovery of a soft
X-ray spectral flattening in RBS 315. In the previous section we
tested three different models in order to account for this spectral
feature: a power law modified by either a (i) cold or a (ii) warm
absorption component and (iii) a broken power law. The latter
provided the worst description of the XMM-Newton spectrum with an associated
= 1.2. Furthermore, as pointed out by Worsley et al. (2004b),
an explanation of the observed cutoff in terms of a spectral break is
not particularly compelling since such a feature is not observed so
far in any of the nearby FSRQs, for which high-quality X-ray data have
been collected. An additional problem that further weakens this
scenario is the sharpness of the observed break as it takes place
over a energy range of a few keVs in the QSO frame. It would
therefore require an unlikely, nearly monochromatic "seed'' photon
distribution (e.g. Ghisellini 1996, for details) to be generated.
Hence, even though the hypothesis of a break in the continuum cannot be completely discarded, it appears quite unlikely once compared with the possible presence of absorption in this FSRQ.
On the other hand, the spectral fittings with cold and warm absorber
are statistically indistinguishable: using the present X-ray data it
is not possible to constrain the ionization state of the absorbing matter better than
erg cm-2 s-1.
Moreover, the most prominent hallmarks of a warm absorber, typically
the Fe M-shell UTA and the O/Ne absorption edges, are located in the
rest-frame energy interval 0.6-0.9 keV which is, unfortunately, outside the
observed (i.e. 0.3-12 keV) band due to the high z of the source.
Furthermore the paucity of information about the characteristics of
RBS 315 at other wavelengths in the literature does not allow to put any
additional useful constraints on the nature of the absorber.
However, the X-ray constraint on the ionization status
matches well with those reported so far for other warm absorbers
possibly detected in
high-z Blazars (e.g. W04; F03; Worsley et al. 2004b) where the
obscuring matter was found to be nearly neutral or with loosely constrained
erg cm-2 s-1.
The inferred column density of the absorbing material is
a few
1022 cm-2. Remarkably, such a value is very similar to
those reported from XMM-Newton observations of all the other obscured
Blazars at high z for which,
bearing in mind the small number statistics, the
measured column densities of the absorbers seem
to narrowly cluster around
cm-2(e.g. F03; W04; Yuan et al. 2005; Grupe
et al 2005).
values
1022 cm-2 are, however, hard to detect at high
redshifts and, hence, a selection effect is likely present.
Soft-energy cut-offs are more common in radio-loud objects (Fiore et al. 1998). One may therefore speculate that a mechanism of collisional ionization might be at work by multiple shocks along the jet (e.g. Gupta et al. 2005). A similar phenomenon is believed to be present in the NLR of nearby Seyfert galaxies which are spatially associated with their radio jet structure, suggesting that the NLR originates from the compression due to the interaction between the outflowing radio material and the ambient gas in the galaxy (Capetti et al. 1996). As the Blazar X-ray emission is dominated by relativistically beamed components from the jet, it is very likely that the obscuration may be due to jet-linked material and physical processes.
Our result strengthens the evidence for the existence
of a population of superluminous Blazar-like AGNs at z > 2 which show soft X-ray cutoffs due to the likely presence of large amounts
of intrinsic absorbing matter with similar properties. The discovery
of these jet-dominated superluminous (i.e.
erg s-1) obscured QSOs at high z is also important in the
context of the formation and cosmological evolution of radio-loud
objects. Soft X-ray spectral cutoffs seems in fact to be a
prerogative of radio-loud QSOs (Fiore et al. 1998): could the
gaseous environments in powerful Blazars be different from that in
radio-quiet AGNs? Is the absorber intimately linked with the presence
of relativistic jets?
Important clues about the physical state and geometry of the absorber would be given by more sensitive ultra-soft X-ray observations, for instance carried out by the under-study ESA XEUS satellite (e.g. Bleeker & Mendez 2002). Furthermore, more robust constraints on the ionization state of the obscuring matter may be also achieved with optical and UV observations by the detection of spectral features typical of an ionized absorber.
Acknowledgements
We thank the anonymous referee for a valuable report. We are grateful to the staff members of the XMM-Newton Science Operations Center for their support. E.P. thanks P. Rodriguez and J. A. Carter for helpful discussions.