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Mem. S.A.It. Vol. 76, 61
c
# SAIt 2005 Memorie della
On the Nature of BL Lac Objects
M.T.Whiting
School of Physics, University of New South Wales, Sydney 2052, NSW, Australia e­mail:
mwhiting@phys.unsw.edu.au
Abstract. The BL Lacertae objects, or BL Lacs, have multi­wavelength continua dom­
inated by non­thermal emission from a relativistic jet, and are characterised by a lack of
optical emission lines. We find, through observations and modelling that the dominant fac­
tor in a source being called a BL Lac is the intrinsic lack of flux from the broad­line region
-- likely connected with a weak accretion disc -- rather than an overwhelmingly powerful jet.
This has important implications for the nature of the accretion flow in these objects.
Key words. BL Lacertae objects: general -- quasars: general -- accretion
1. Introduction
BL Lacert objects, or BL Lacs, are a class
of AGN that are defined on the basis of their
optical spectra. A BL Lac exhibits no emis­
sion line with a rest frame equivalent width
W # > 5 (Stocke et al., 1991) (although this
criterion is sometimes weakened in the case of
low­luminosity objects where the host galaxy
is important in the optical spectrum (March”a &
Browne, 1995)). BL Lacs are generally found
via surveys at radio or X­ray wavelengths and
confirmed as BL Lacs through follow­up opti­
cal spectroscopy.
In this paper, we discuss the reason for
the lack of prominent emission lines in BL
Lacs. Most theories (Blandford & Rees, 1978;
Urry & Padovani, 1995) invoked to explain
this lack involve the presence of an extra con­
tinuum component that masks emission lines.
From the shape of the radio--IR--optical spec­
tral energy distribution (SED) (e.g. Impey &
Neugebauer, 1988), as well as high optical and
radio polarisation, this extra component is un­
derstood to be synchrotron emission, which
comes from the relativistic jet. In this picture,
the Doppler boosting from the jet, which is
viewed at a small angle to the direction of its
propagation, strongly increases the flux of this
synchrotron component, swamping the emis­
sion line flux.
2. Modelling the PHFS
We are able to examine this model for
BL Lacs by modelling the optical emission
from sources in the Parkes Half­Jansky Flat­
spectrum Sample (PHFS, Drinkwater et al.,
1997). This is a sample of 323 radio­bright flat
spectrum radio sources, consisting mostly of
flat­spectrum radio quasars, FSRQs, with some
BL Lacs and radio galaxies.
Optical--near­infrared spectral energy
distributions (SEDs), covering the filters
BVRIJHK, were obtained through quasi­
simultaneous observations by Francis et al.
(2000) for a subset of the PHFS. In Whiting
et al. (2001), we fit models to this broad­band
data for approximately a third of the PHFS,
concentrating on the unresolved ``stellar''

62 M.T.Whiting: On the Nature of BL Lac Objects
Fig. 1. Example fits from Whiting et al. (2001). From left to right: accretion disc dominated;
equal amounts of accretion disc and synchrotron; synchrotron dominated.
sources where the host galaxy is not an
important contributor to the optical emission.
The two models are: a blue power law repre­
senting accretion disc emission, with a slope
f # # # -1.7 (the median slope of optically­
selected LBQS quasars (Francis, 1996)); a
synchrotron component comprising a power
law at long wavelengths with an exponential
cuto# parametrised by some ``peak'' wave­
length # p , representing emission from the jet.
Some examples of the fits are shown in Fig. 1.
Approximately a third of these sources are
dominated by the accretion disc component
with no sign of optical synchrotron. These
sources do have synchrotron emission from the
jet present (as it is responsible for the flat­
spectrum radio emission), but that component
most likely turns over at longer wavelengths
than those probed by our optical/NIR data. A
similar number of sources, however, do show
good evidence for optical synchrotron emis­
sion. In many cases, this emission dominates
the continuum, and there is no evidence for any
accretion disc component.
All the BL Lac objects in this sample fall
in the latter group, as they all have SEDs domi­
nated by the synchrotron component. However,
their properties are indistinguishable from the
rest of the optical synchrotron sources. Their
distributions of # p and ratio of synchrotron
to accretion disc flux are statistically indis­
tinguishable (although all the BL Lacs have
a high ratio). The main di#erence lies in the
emission line equivalent widths, with all the
BL Lacs at the low end of the distribution.
BL Lacs, then, seem to have similar broad­
band spectral properties to quasars with optical
synchrotron emission. The defining di#erence
appears to be with the strength of the emission
lines, where BL Lacs have intrinsically weaker
lines. This is in accord with previous studies of
BL Lacs and quasars, such as Scarpa & Falomo
(1997).
3. Clues from other surveys
What support is there from the literature for
such a picture of BL Lacs? Firstly, we can
look at ultraviolet spectroscopy of BL Lacs and
FSRQs, such as that of Kinney et al. (1991).
If there was an accretion disc and broad­line
region (BLR) present but swamped by a syn­
chrotron component, their emission would be
relatively more prominent in the UV, since the
disc flux increases for shorter wavelengths and
stronger lines are seen at shorter wavelengths,
while the synchrotron components tend to de­
crease (due to the cuto#s seen in the PHFS
sources). However, the BL Lac objects in
Kinney et al. (1991) show either very weak
lines or an absence of lines, suggesting an in­
trinsically weak BLR flux in these objects.
In the objects we have looked at thus far,
the synchrotron component has only extended
to optical (or UV) energies. However, there
are high­frequency­peaking BL Lacs (HBLs)

M.T.Whiting: On the Nature of BL Lac Objects 63
where this component dominates all frequen­
cies up to X­rays. In analogy with the low­
frequency peaking objects, we should ex­
pect sources that have an HBL­like SED but
with quasar­like emission lines in the opti­
cal. Recent deep X­ray and radio surveys,
such as DXRBS and RGB, have found sources
that fit this description (Padovani et al., 2003,
also Landt et al. in these proceedings). These
sources have a jet­dominated continuum, but
with a strong enough accretion disc and BLR to
have significant emission lines present in their
optical spectrum.
Finally, there exists one optically­selected
sample of BL Lacs, the 2dF BL Lac sam­
ple (2BL) of Londish et al. (2002). These are
objects selected on the basis of their optical
colour as targets for the 2dF Quasar Redshift
Survey (Croom et al., 2001), and which were
subsequently found to have featureless spectra.
These objects are generally radio­quiet (most
have S 8.4GHz < 0.2mJy) and weak at X­rays.
A likely interpretation for such objects (dis­
cussed in Londish et al., 2004) is that they are
otherwise normal radio­quiet AGN that have a
weak/absent BLR, and so the optical contin­
uum is dominated by just the accretion disc.
These objects are then at the extreme end of
the distribution of BLR strengths in radio­quiet
quasars.
4. How do you make a BL Lac?
It is clear, then, that the disc, the emission lines
and the jet can span a wide range of relative
strengths. Simply having a dominant jet com­
ponent does not necessarily mean that there
will be no emission line or accretion disc flux
seen. In this picture of BL Lacs, the lack of
emission lines is due to an intrinsically weak
BLR flux. Either there is little or no gas in the
BLR, or the gas that is there is not su#ciently
ionised. The continuum from the relativistic jet
then dominates by default, and is not necessar­
ily very strongly boosted.
There are (at least) two scenarios in which
this could be the case. First, the BLR could be
relatively depleted in gas, with a relatively nor­
mal accretion disc present. This is likely to be
the case for the 2BL sources. The second al­
ternative is that the gas is there, but there are
insu#cient ionising photons to produce the re­
quired emission line flux.
The best candidate for the source of ionis­
ing photons is the accretion disc. The jet could
provide some ionising photons, but it is un­
likely to be the major source: Corbett et al.
(2000) found that the response of the weak H#
line in BL Lac to continuum variations was
best explained by the presence of a hot accre­
tion disc, weak enough not to be detected in op­
tical spectra. Thus, we expect that in BL Lacs
the flux from the accretion disc is weaker than
that seen in quasars. This implies that the ac­
cretion process is somewhat di#erent in the BL
Lacs.
5. Accretion processes in BL Lacs
A possible accretion scenario for BL Lacs is
that the accretion onto the black hole (BH) is
not progressing via a standard quasar­style thin
disc but via a low­radiative­e#ciency process,
such as an advection­dominated accretion flow
(ADAF). Wang et al. (2002) found that both
LBLs and HBLs have highly sub­Eddington
accretion rates, which could be explained by
the presence of an ADAF.
An alternative scenario is that the accretion
in BL Lacs does proceed via a ``normal'' thin
disc, but that the inner, hotter regions of the
disc closest to the BH are disrupted or absent.
For an accretion disc around a typical super­
massive BH, the bulk of the ionising radiation
will come from the innermost parts, and so a
depletion of this region would reduce the ion­
isation of the BLR markedly. A hybrid model
might be possible, where a thin disc dominates
at large disc radii, but an ADAF is present in
the inner regions, reducing the disc output (see
e.g. Cao, 2003).
An instructive comparison may be made
by looking at Galactic black hole X­ray bi­
nary systems, in particular the system GRS
1915+105, which has a similar structure to
AGNs with both an accretion disc and jets.
This system shows evidence of coupling be­
tween the disc and the jet, where disturbances
in the inner disc, seen as spikes and dips in X­
rays, coincide with ejections of matter into the

64 M.T.Whiting: On the Nature of BL Lac Objects
jet, as observed in the radio. A comprehensive
review of this source can be found in Fender &
Belloni (2004).
This situation is qualitatively similar to our
description of BL Lacs, where the jet is dom­
inating, and the disc emission is suppressed.
Note that there is no equivalent of the BLR
in X­ray binaries, so we can only compare the
relative strengths of the emission from the in­
ner disc. This explanation would mean that we
are seeing BL Lacs during an ejection event,
where the disc is relatively depleted, with mat­
ter falling into the BH and a small amount be­
ing injected into the jet.
Such a comparison raises the intriguing
possibility that BL Lacs represent a transient
phase of AGNs, where the jet is dominating
during the suppression of the inner accretion
disc. Inevitably the disc will fill up again, with
the AGN reverting back to a quasar­like state.
If one scales the timescales of XRBs (of the
order of 100 s) by the BH mass, the lifetime of
the BL Lac phase is # 30 - 300 yr. The lower
end of this range is becoming observable for
some sources, and we encourage more spectral
monitoring of BL Lacs to search for spectral
variations.
An important caveat on this is that scaling
by mass is probably too simplistic, as the cool­
ing time and dynamical time do not scale the
same way, so the response of an AGN disc to
disruption will di#er from that of an XRB disc.
6. Summary
We find that BL Lacs do not have substantially
di#erent SEDs from quasars with dominant
optical synchrotron components. The primary
di#erence lies in the strength of the emission
line flux. This, in turn, is caused by a reduced
ionising flux from a less radiatively­e#cient
accretion flow than those found in quasars.
Acknowledgements. This work is supported by a
NewSouth Global Postdoctoral Fellowship from
UNSW.
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