Mon. Not. R. Astron. Soc. 311, 485±492 (2000)
A Seyfert-like nucleus concealed in BL Lacertae?
E. A. Corbett,1 A. Robinson,1 D. J. Axon1,2w and J. H. Hough1
1
2
Department of Physical Sciences, University of Hertfordshire, Hatfield, Hertfordshire AL10 9AB
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Accepted 1999 August 16. Received 1999 July 9; in original form 1998 June 8
A B S T R AC T
We present a series of new spectroscopic observations of BL Lacertae. The observations
were obtained over a period of 30 months and sample the violent continuum flare of 1997
June±August. The equivalent width of the recently discovered broad Ha line varies
approximately inversely with the optical continuum flux, implying that the broad-lineemitting gas does not respond significantly to large variations in the beamed synchrotron
continuum which dominates the spectrum at optical wavelengths. The simplest and most
plausible interpretation of this result is that the broad-line region in BL Lacertae is
`unaware' of the synchrotron beam and is photoionized by another source, a likely candidate
being a hot accretion disc. Alternatively, similar behaviour would be observed if the broadline region is indeed photoionized by the synchrotron beam but is predominantly matterbounded so that it absorbs only a small fraction of the incident ionizing photon flux.
Key words: line: formation ± BL Lacertae objects: individual: BL Lacertae ± quasars:
emission lines.
1
INTRODUCTION
BL Lacertae is the prototype of a class of active galactic nuclei
(AGN) associated with compact radio sources and exhibiting
featureless ultraviolet to infrared continua, rapid flux variability
and high, variable polarization (Moore et al. 1982; Bregman et al.
1990; Mead et al. 1990; Carini et al. 1992). BL Lac objects share
these characteristics with optically violent variable (OVV) or
high-polarization (HP) quasars. The extreme continuum properties
of these objects, collectively known as blazars, are widely
believed to result from Doppler beaming of synchrotron emission
from the relativistic jets which power their radio sources. BL Lac
objects are conventionally distinguished from OVV/HP quasars by
their lack of strong emission lines. This distinction has become
somewhat blurred, however, by the detection of broad emission
lines in several sources that were originally classified as BL Lac
objects (Ulrich 1981; Sitko & Junkkarinen 1985; Moles,
Masegosa & Del Olmo 1987). Recently a broad Ha line has
been observed in BL Lacertae itself (Vermeulen et al. 1995),
raising the question of whether the eponymous empirical
classification does indeed reflect a genuine difference in physical
properties (e.g. in the amount of gas present in the nucleus).
Alternative possibilities are that BL Lac objects represent a
distinct evolutionary state (e.g. episodic broad-line emission) or
are merely low-luminosity counterparts of the OVV/HP quasars
(with correspondingly weak ionizing continua).
BL Lacertae was identified as the optical counterpart of the
w
Affiliated with the Astrophysics Division of ESA.
q 2000 RAS
radio source VRO 42.22.01 by Schmitt (1968). It lies within an
elliptical galaxy (Miller, French & Hawley 1978) and weak,
narrow emission lines have been detected, allowing an accurate
redshift to be determined …z ˆ 0:0688 ^ 0:0002: Miller & Hawley
1977). The radio source is highly compact, but on milliarcsecond
scales has structures moving away from a stationary central
component at superluminal speeds (Mutel et al. 1990). In 1995
Vermeulen et al. (1995) discovered broad emission lines at the
redshifted wavelengths of Ha and Hb . The broad Ha line has
Ê ), but its luminosity …LHa ,
a small equivalent width (,6 A
1041 erg s21 † and full width at half-maximum …FWHM <
3700 km s21 † are comparable to those of Seyfert type 1 galaxies
such NGC 4151 (Corbett et al. 1996, hereafter C96).
It is important to establish the origin of the broad-line emission
in BL Lacertae. It is widely accepted that in Seyfert type 1 nuclei
and quasars the broad-line-emitting gas is photoionized by
extreme-ultraviolet (EUV) continuum radiation. According to
unification schemes (e.g. Barthel 1989; Antonucci 1993), the
observed continuum in radio-loud AGN is a combination of highly
beamed non-thermal and comparatively isotropic thermal components. The non-thermal component is attributed to synchrotron
emission from the radio jet. In blazars, our line of sight is closely
aligned with the radio jet, and Doppler-boosted synchrotron
emission therefore dominates the observed continuum from radio
to optical wavelengths. The thermal component is thought to
originate from an accretion disc, and extends from UV
wavelengths to soft X-rays. This component dominates the
optical±UV spectrum of most AGN, both radio-loud and radioquiet, and appears to be present in at least some OVV/HP quasars
486
E. A. Corbett et al.
(e.g. Smith et al. 1986; Koratkar et al. 1998), as is required by
unification schemes. However, there is as yet no evidence for such
a component in BL Lac objects.
Assuming that the broad-line-emitting gas in BL Lacertae is
similarly photoionized, the only apparent source is the beamed
synchrotron continuum emitted by the jet. The broad-line region
(BLR) would then be illuminated by a highly anisotropic radiation
field and, unless BL Lacertae is a special case, this would raise the
possibility that relativistically beamed synchrotron radiation also
influences the broad-line emission in other radio-loud AGN. We
have shown (C96) that the observed Ha equivalent width is
consistent with photoionization of the BLR by beamed synchrotron emission alone for jet Lorentz factors and viewing angles
within the limits implied by the observed superluminal motion of
the radio source. Nevertheless, it can equally well be explained by
thermal radiation from an accretion disc without easily observable
consequences for the optical continuum: e.g. a spectral flattening
or depolarization of the observed continuum towards the blue. If
this were the case, it would be reasonable to suppose that BL
Lacertae and its class are intrinsically similar to quasars in that
both a BLR and an EUV-emitting accretion disc are present in
their nuclei.
In principle, we can distinguish between these two continuum
sources from the way in which the Ha equivalent width, WHa ,
varies with the optical continuum brightness. If an accretion disc
is the primary source of photoionizing radiation for the BLR, the
line emission will be independent of variations in the observed
optical continuum, since this is dominated by synchrotron
emission, and WHa will therefore be anticorrelated with the
continuum brightness. On the other hand, we expect WHa to
remain constant as the optical continuum varies, if the broad-line
emission is powered by radiation from the same source, i.e. the
synchrotron beam. In this paper we present a series of spectroscopic observations of the Ha line in BL Lacertae, obtained over a
period of 30 months, and use these data to determine the nature of
the relationship between the line equivalent width and the optical
continuum flux.
2
O B S E RVAT I O N S
Intermediate-resolution spectra of the Ha region of BL Lacertae
were obtained during a series of observations with the 4.2-m
William Herschel Telescope (WHT) at the Observatorio de la
Roque de los Muchachos on La Palma. The ISIS dual-beam
spectrograph was used with a 1-arcsec slit in all observations. In
total, eight spectra were acquired at irregular intervals during a
Table 1. Log of spectroscopic observations obtained at the WHT
between 1995 June 4 and 1997 December 7.
Date
1995
1995
1995
1996
1997
1997
1997
1997
Jun 4
Jul 7
Aug 24
Aug 18
Jun 27 & 28
Nov 141
Nov 302
Dec 7
No.
obs.
Total
exp. (s)
V-band
mag.
Flux
correction
1
1
2
1
2
3
3
2
2400
1200
2400
1200
2700
3600
3600
2700
15.76 ^ 0.02
0.73
15.2 ^ 0.1
14.8 ^ 0.1
15.2 ^ 0.1
14.8 ^ 0.1
1.261
1.952
1.23
1
Calibration correction determined from observations of a second
standard star.
2
Observation affected by cloud.
period of 30 months (Table 1). Each spectrum has a wavelength
Ê (covering the region 6400±7400 A
Ê in
range of ,1500 A
Ê pixel21; when several spectra
particular) at a dispersion of 1.5 A
were obtained during one or two (consecutive) nights, these have
been combined to form an average spectrum. The total exposure
times for each of the resulting spectra are listed in column (3) of
Table 1.
On a few occasions we were able to obtain near-simultaneous
CCD photometry of BL Lacertae in the V band (Table 1). On 1995
June 4 the V magnitude of the source was found to be 15:76 ^
0:02 mag from simultaneous observations with the 1.0-m Jacobus
Kapteyn Telescope (C96). On 1997 November 30 and December
7, V-band images of a field centred on BL Lacertae were obtained
with the Auxiliary Port Camera of the WHT. This field includes
two photometrically calibrated stars (stars b and c: Bertaud et al.
1969) which have been used over a number of years as reference
stars in monitoring campaigns of BL Lacertae (e.g. Smith et al.
1987). The V magnitudes of BL Lacertae on these nights were
determined to be 15:2 ^ 0:1 and 14:8 ^ 0:1; respectively.
2.1
Flux calibration
The spectra were flux-calibrated in the usual way using
observations of spectrophotometric standard stars. With the
exception of the night of 1997 November 30, observing conditions
were good, but the photometric accuracy of the flux calibration is
nevertheless limited by slit losses and, for some nights, slight
variations in transparency.
At those epochs when near-simultaneous V photometry was
obtained, corrections were applied to the spectroscopic flux
calibrations as follows. The measured V magnitudes were
Ê and the continuum in each
converted to flux densities at 5500 A
spectrum fitted with a power law which was then extrapolated
Ê . The ratio between the flux density as determined
down to 5500 A
from the photometry and that determined from the extrapolated
spectrum was used to scale the line and continuum fluxes
subsequently measured from the spectrum. On 1997 November 14
a wide-slit observation of reference star b was obtained, and its
known R magnitude used in a similar way to estimate the flux
calibration correction. With the exception of the observations
obtained on 1997 November 30, which were affected by cloud, the
corrections applied were approximately 25 per cent of the measured
fluxes (Table 1). We estimate that the error in the corrected fluxes
measured from these spectra is #10 per cent (the accuracy of the
photometry).
We were unable to obtain supplementary flux calibration data
for the remaining four observations. Nevertheless, count rates
measured from the individual frames comprising each observation
are reasonably consistent, suggesting that the flux measurement
errors do not exceed the <25 per cent corrections applied to the
spectra for which we have photometric data. The systematic errors
in the absolute flux calibration do not, of course, affect the
measurements of the line equivalent widths presented below.
Moreover, the 25 per cent uncertainty in the continuum flux is
small compared with the variability amplitude of BL Lacertae: the
optical continuum brightness changed by a factor of 5 during the
course of our observations.
The 1995 July 7 spectrum was obtained without an ordersorting filter to block out second-order diffraction from the blue
end of the spectrum. As a result, when the flux calibration was
performed for BL Lacertae the spectrum was under-corrected at
q 2000 RAS, MNRAS 311, 485±492
A Seyfert-like nucleus in BL Lac?
Ê . This has been corrected by
wavelengths above about 6700 A
forcing the continuum of the 1995 July 7 observation to have the
same shape as an average spectrum formed from the 1995 June 4
and 1997 November 14 data (the two spectra with similar
continuum flux levels to the 1995 July 7 spectrum).
The Ha region of the spectrum is affected by atmospheric
Ê which
absorption features, notably the oxygen B band at 6870 A
falls close to the blue wing of the line. These features were
corrected by dividing the observed spectra by templates formed
from the spectra of photometric standards obtained during the
same night. The quality of the correction depends on the
photometric conditions during the observations, and hence small
residuals remain in some spectra.
The flux-calibrated spectra are shown in Fig. 1.
we applied corrections for Galactic extinction and contamination
by starlight as described in C96. A value E…B±V† < 0:3 was used
for the differential extinction in the direction of BL Lacertae. The
contribution of host galaxy starlight was estimated by matching a
template (a spectrum of the E0 galaxy NGC 4339) to the TiO
feature in the spectrum obtained on 1995 June 4, when our
photometric data indicate that the source was in its lowest state.
We estimate that starlight accounts for 15 per cent of the measured
Ê in this spectrum. The template spectrum
flux density at 6400 A
was scaled accordingly and subtracted from each spectrum of BL
Lacertae. For comparison, Vermeulen et al. (1995) estimated a
Ê.
Galactic contribution of approximately 12 per cent at 5480 A
3
2.2 Correction for Galactic extinction and contamination by
the host galaxy
Prior to measuring the line and continuum fluxes from the spectra,
487
R E S U LT S
The broad Ha line was clearly detected in all of our spectra except
that obtained from the observations of 1997 June 27 and 28, when
BL Lacertae was particularly bright. At this epoch the continuum
flux was higher by a factor of .2 than it was in its next brightest
state (1997 December 7), and a factor of .5 higher than it was in
the faintest state that we have observed (1995 July 7). Data from
other sources confirm that BL Lacertae underwent a major flare
during 1997 June, July and August at both optical and g -ray
wavelengths (Takalo, private communication; Bloom et al. 1997;
Noble et al. 1997; Nesci et al. 1998). In subsequent observations
obtained in 1997 November and December, after the source had
faded, broad Ha was again detected at a strength comparable to
that seen in the observations prior to 1997 June.
3.1
Measurement of the broad Ha line
The equivalent width and integrated flux of the broad Ha line
were measured from each spectrum by modelling the blended
Ha 1[N ii] feature as the sum of four Gaussian profiles
representing, respectively, the broad wings and narrow core of
Ha and the [N ii]ll 6548, 83 doublet. This procedure is described
in detail in C96. Prior to performing the fit, the local continuum
(under the line) was removed by subtracting either a power-law or
a quadratic fit to continuum windows either side of the emissionline feature. The choice of function was dictated by the particular
atmospheric standard used for each spectrum. In view of the
uncertainty surrounding the reddening correction and, in particular, the strength of the Galactic contribution, we have measured
the emission-line fluxes both before and after these corrections
Ê (F6563), and the flux
were applied. The continuum flux at 6563 A
(FHa ), equivalent width (WHa ) and FWHM of the broad Ha line
are listed for each case in Table 2.
The simulations described in C96 suggest that the minimum
Ê in data of comparable
detectable line equivalent width is 1±2 A
quality to those obtained on 1997 June 27 and 28. We therefore set
Ê on the equivalent width of the
a conservative upper limit of 2 A
broad Ha feature at this epoch.
3.2 Relationship between the Ha line and the local
continuum
Figure 1. The observed spectra of BL Lacertae before correction for
Galactic extinction and contamination by the host galaxy. Note the large
change in continuum flux over the 30-month period.
q 2000 RAS, MNRAS 311, 485±492
The variation of WHa with the local continuum flux is shown for
the corrected data in Fig. 2. It is clear that WHa is anticorrelated
with the continuum flux. Fig. 3 shows these two quantities plotted
in chronological order. There is an anticorrelation in detail, lower
488
E. A. Corbett et al.
Table 2. Continuum flux and broad Ha line properties measured from the spectra before and after correction for extinction and host galaxy
contamination. Flux is measured in units of 10215 erg cm22 s21.
Date
1995
1995
1995
1996
1997
1997
1997
1997
Ê 21)
F6563 (A
Jun 4
Jul 7
Aug 24
Aug 18
Jun 27
Nov 14
Nov 30
Dec 7
2.9 ^ 0.2
2.8 ^ 0.7
4.9 ^ 1.2
5.2 ^ 1.3
15.3 ^ 3.8
3.4 ^ 0.3
5.0 ^ 0.5
7.0 ^ 0.7
Uncorrected data
Ê)
FHa
WHa (A
14.9 ^ 1.0
16.0 ^ 4.1
29.1 ^ 7.5
16.1 ^ 4.3
±
18.4 ^ 2.3
20.1 ^ 2.5
8.7 ^ 1.5
5.2 ^ 0.7
5.7 ^ 0.6
5.9 ^ 0.7
3.1 ^ 0.4
,2
5.5 ^ 0.7
4.0 ^ 0.5
1.2 ^ 0.2
FWHM (km s21)
3720 ^ 220
3900 ^ 120
4080 ^ 160
2950 ^ 150
±
3550 ^ 160
4760 ^ 240
2640 ^ 240
Figure 2. Plot showing the variation of WHa with the continuum flux
under the line for the data after correction for Galactic reddening and the
host galaxy contribution. The upper limit is for 1997 June 27 and 28, based
on our non-detection of the broad line in that spectrum. The solid line is a
least-squares fit of the function W Ha ˆ constant/F 6563 to the data.
values of WHa invariably corresponding to higher continuum
fluxes. Similar trends are apparent in the uncorrected data.
Table 2 shows that the continuum flux varied by a factor of .5
between the highest and lowest states sampled by our observations. In contrast, five out of seven measurements yield broad Ha
fluxes within about 25 per cent of one another. The range in
flux both before and after correction for reddening and host
galaxy contamination …1:5±2:0 10214 and 2:2±2:9 10214 erg
cm22 s21, respectively) is comparable to the uncertainties on the
individual measurements. The continuum flux varies by a factor
of 2 in the same set of spectra. The two remaining line flux
measurements yield, respectively, the highest and lowest values of
the series. The former has a relatively large uncertainty and its
deviation from the above range is not significant at the 2s level.
The other measurement is nearly a factor of 2 lower than the
mean, and its formal uncertainty suggest that this is significant.
However, the continuum brightness in this observation was the
second highest that we recorded, and we believe that our fitting
procedure may have significantly underestimated the flux in the
broad-line wings which are largely swamped by the continuum.
This is consistent with the fact the broad-line FWHM in this case
is the lowest (Table 2) returned by the model fit to the Ha 1[N ii]
Corrected data
Ê)
FHa
WHa (A
Ê 21)
F6563 (A
3.9 ^ 0.2
3.7 ^ 0.9
7.3 ^ 1.8
7.7 ^ 1.9
25.9 ^ 6.5
4.4. ^ 0.4
7.4 ^ 0.7
10.8 ^ 1.1
22.9 ^ 2.7
26.1 ^ 7.3
48 ^ 12.9
26.6 ^ 7.1
±
28.6 ^ 5.4
28.6 ^ 4.6
17.1 ^ 2.6
5.9 ^ 1.4
7.0 ^ 1.2
6.6 ^ 0.9
3.4 ^ 0.5
,2
6.5 ^ 1.2
3.9 ^ 0.6
1.6 ^ 0.2
FWHM (km s21)
4020 ^ 450
4000 ^ 300
4050 ^ 220
2580 ^ 160
±
5050 ^ 490
4530 ^ 390
2030 ^ 150
Figure 3. A time-series plot of the data showing the continuum flux at Ha
(open circles) and the line equivalent width (filled circles) for the data after
correction for reddening and host galaxy contamination. Again, the
equivalent width for the 1997 June 27 and 28 is shown as an upper limit
based on our non-detection of a line in that spectrum.
blend. Therefore, although the largest change in line flux that can
be admitted by our measurements is approximately 50 per cent,
we believe that any changes that have occurred are probably
somewhat smaller, and indeed there is no compelling evidence in
our data that the broad Ha flux has undergone any significant
variations.
We draw the following conclusions from our data:
(i) the equivalent width of the broad Ha line is inversely
proportional to the optical continuum flux;
(ii) consistent with the above, the line flux did not vary
significantly in response to the large changes which occurred in
the optical continuum between 1995 June and 1997 December.
Therefore the broad Ha emission does not show a significant
response to large variations in the beamed synchrotron continuum,
implying that the line-emitting gas is effectively `unaware' of such
variations.
4
DISCUSSION
In C96 it was shown that the Ha equivalent width observed in
1995 June could be explained by photoionization of the BLR by
q 2000 RAS, MNRAS 311, 485±492
A Seyfert-like nucleus in BL Lac?
the beamed synchrotron continuum alone for jet parameters
consistent with the observed superluminal motion of the radio
source. Here we consider whether this picture can still be
supported in the light of the subsequent variations in the line
equivalent width.
4.1 Photoionization by Doppler-boosted synchrotron
continuum
If the Doppler-boosted synchrotron continuum emitted by the
relativistic jet is the primary source of ionizing radiation seen by
the BLR, we might naively expect the Ha equivalent width to
remain constant as the featureless optical continuum varies, since
the latter is emitted by the same source. This assumes: (i) that the
photoionizing flux varies simultaneously with, and with at least
the same amplitude as, the optical continuum; (ii) that light
traveltime delays within the illuminated region of the BLR are
negligible; and (iii) that the line-emitting clouds are radiationbounded in the sense that all ionizing photons impinging on them
are absorbed.
The first two of these assumptions are reasonable. In BL Lac
objects the optical and UV continuum fluxes appear to be closely
correlated, although variation amplitudes tend to increase to
shorter wavelengths. For example, the recent intensive monitoring
of PKS 2155±304 (Urry et al. 1997, and references therein)
showed a close correspondence between the optical and UV
fluxes, with variations in these bands following somewhat larger
changes in the EUV after a lag of ,2 d. The variability behaviour
of blazars differs widely from object to object (and even
individual objects may exhibit different behaviour at different
epochs), so we cannot necessarily assume that BL Lacertae
exhibits similar characteristics. Nevertheless, it is worth noting
that, during the 1997 outburst in BL Lacertae, variations in the
optical bands were closely correlated (Nesci et al. 1998) and the
g -ray flare was apparently followed within hours by a flare in
the optical continuum (Bloom et al. 1997). We therefore have
reasonable grounds for assuming that the EUV continuum is
approximately proportional to the optical continuum. We also
expect light traveltime delays within the BLR to be negligible,
since relativistic beaming of the photoionizing continuum results
in most of the line flux being emitted from clouds close to our line
of sight. Even in Seyfert galaxies, where a high degree of beaming
is unlikely, the lag between the continuum and line variations is
typically less than, or comparable to, the intervals between our
observations (e.g. Peterson 1993). For example, in NGC 4151,
which has an Ha luminosity comparable to that of BL Lacertae,
the lag is only ,1 week for the Balmer lines (Maoz et al. 1991).
Therefore, if the BLR is radiation-bounded and is illuminated
solely by the beamed synchrotron continuum, the line flux should
vary with the optical continuum flux, while WHa remains
constant. This case is conclusively ruled out by our data (Figs 2
and 3).
However, it may be unrealistic to suppose that the BLR is
everywhere radiation-bounded. Near its axis the beam could be
sufficiently intense that the BLR in this region is matter-bounded
(i.e. optically thin at the Lyman limit). Some fraction of the
ionizing photon flux will pass through the gas without being
absorbed, and the total line luminosity will then no longer scale
linearly with the continuum. In the limiting case where the entire
BLR is matter-bounded, the Ha luminosity is effectively
independent of the ionizing continuum and therefore WHa will
vary inversely with the optical continuum flux, as observed. It is
q 2000 RAS, MNRAS 311, 485±492
489
not clear, however, that higher observed values of WHa could be
reproduced in that case, since matter-bounded clouds produce Ha
photons at reduced efficiency. In the limiting radiation-bounded
(optically thick) case the production efficiency for Ha photons
(the number of Ha photons emitted per incident ionizing photon)
is CpHa , where C is the BLR covering fraction and pHa the
probability that any recombination event leads to the emission of
an Ha photon. For matter-bounded clouds not all ionizing photons
are absorbed and so the efficiency is reduced by the factor N/Ns,
where Ns is the StroÈmgren column density and N the actual
column density of the cloud. Since N/N s ! 1 for high ionizing
fluxes, the measured WHa may be difficult to obtain if a
significant part of the BLR is matter-bounded.
We investigate the relationship between WHa and the optical
continuum for a partly matter-bounded BLR using a model similar
to that considered by C96. The BLR is represented by a spherical
shell illuminated by beams of ionizing radiation from twin,
antiparallel relativistic jets. We adopt the same values for the
radius …r ˆ 2 1016 cm†; covering fraction …C ˆ 0:1† and density
…n ˆ 1011 cm23 † as C96. The illuminating synchrotron continuum
produced by each beam has a power-law spectrum f n / n2a ;
where a ˆ 2; and is Doppler-boosted by a factor D31a , where
D ˆ ‰g…1±bcos u†Š21 ; g and b have their usual meanings, and u is
polar angle (measured from the jet axis). The surface emissivity in
Ha at an angle u from the beam axis is
1 / Cf…u† min …1; N/N s †;
where f (u ) is the total ionizing photon flux and N is the
BLR column density. Note that, since the StroÈmgren column N s /
f…u†/n; the emissivity in the matter-bounded case …N , N s † is
independent of f (u ).
This model was used to calculate the Ha luminosity and hence
WHa as a function of the local continuum flux density for several
assumed values of the BLR column density, and for Lorentz
factors and jet inclinations, i, consistent with the apparent
superluminal expansion speed b app of the radio source. Four
superluminal components have been identified in BL Lacertae
with apparent speeds in the range (3.16±3.79)c h21, where h ˆ
H 0 /100 km s21 Mpc21 and the values quoted are for q0 ˆ 0:05
(Mutel et al. 1990; Vermeulen & Cohen 1994). We assume h ˆ
0:75 and use the weighted mean, bapp ˆ 4:6 ^ 0:2; of the apparent
speeds of the four components as listed by Vermeulen & Cohen.
Guerra & Daly (1997) use this value of b app, together with
Doppler factors estimated from both the inverse Compton and
equipartition methods, to calculate bulk Lorentz factors and jet
inclinations for BL Lac. From the two methods they obtain
Lorentz factors gIC ˆ 5:0 ^ 1:6 and geq ˆ 4:7 ^ 0:4; respectively,
but the jet inclination is poorly constrained. The standard formula
relating b app to b and i implies that i , 258 for the value of b app
obtained for BL Lac. However, in order to restrict the Lorentz
factor to be roughly consistent with the values estimated by
Guerra & Daly (1997), we impose the somewhat more stringent
limit i # 208 (for which g # 6†:
The model was normalized to the observed values of the Ha
equivalent width and the local continuum flux on 1995 July 7,
when the latter was in its lowest observed state (Table 2). In
practice, this was achieved for each value of N by adjusting i
within its upper limit so as to obtain an approximate match to the
Ê ). Note that, since b app is fixed,
target value of WHa (7 A
increasing (or decreasing) i produces corresponding increases (or
decreases) in g and hence in the Doppler boosting factor, but the
490
E. A. Corbett et al.
radiation will be beamed into a smaller (larger) span in polar
angle, since Du , 1/g:
The predicted variation of WHa with continuum flux is shown
in Fig. 4 for several values of the BLR column density. For N ˆ
1025 cm22 ; the BLR remains largely radiation-bounded and
therefore WHa exhibits only a slight decrease as the continuum
increases to a level comparable to that observed in the high state.
For smaller values of N, the BLR becomes matter-bounded over
an increasingly large solid angle around the beam axis, and, as a
result, WHa declines increasingly rapidly with increasing
continuum flux.
However, for column densities N , 1024 cm22 the BLR absorbs
only a small fraction of the incident ionizing photon flux, and
therefore produces Ha photons with very low efficiency. Larger
Doppler-boosting factors can be obtained by increasing i, but this
does not lead to a proportionate increase in Ha because more of
the ionizing photons are then beamed into the matter-bounded
`window' in the BLR. As a result, the model cannot reproduce the
observed low-state value of WHa for i # 208 unless the BLR
covering fraction is significantly greater than the canonical value
of 10 per cent. In reality, the Ha equivalent width is likely to be
even lower than predicted for given values of N and i. The on-axis
ionization parameter is U ˆ f…0†/cn , 100; high enough to
destroy the ions that emit the major collisionally excited cooling
lines (e.g. C iv l 1549, O vi l 1034: Korista et al. 1997). In this
situation, the gas becomes thermally unstable and undergoes
runaway heating (Krolik, McKee & Tarter 1981). In effect, the
beam `burns' a hole through the BLR, further reducing the amount
of Ha emission produced and necessitating yet higher BLR
covering fractions to reproduce the low-state value of WHa . If we
simulate this effect in the model by setting 1 to zero within the
polar angle for which U $ 10; we find that, for N # 1023:5 cm22 ;
covering fractions C $ 0:4 are needed to reproduce the observed
low-state value of WHa .
A reasonably good fit to the observed variation of WHa with
continuum flux density can be obtained with the relationship
W Ha / F 21
6563 ; which corresponds to the limiting case in which the
BLR is fully matter-bounded (Fig. 2). The predicted WHa ±F6563
relationship tends toward a somewhat shallower limiting curve
23:5
…W Ha , F 20:8
cm22 ; since the BLR never
6563 † for N # 10
becomes entirely matter-bounded (for lower values of N, higher
Doppler-boosting factors are required to obtain the target value of
WHa , resulting in a narrower beam). Formally, this curve fits the
data equally well, although only if we use BLR covering fractions
higher than the canonical 10 per cent.
To summarize, the model calculations show that, for canonical
BLR parameters, photoionization by Doppler-boosted ionizing
synchrotron radiation from the relativistic jets cannot explain the
observations. For BLR column densities N $ 1024 cm22 ; the
beaming model does reproduce the observed relationship between
WHa and the optical continuum. On the other hand, for N ,
1024 cm22 the model predicts an WHa ±F6563 relationship similar
to that observed, but the BLR absorbs too few ionizing photons to
produce the observed low-state Ha equivalent width for jet
inclinations and bulk velocities consistent with the observed b app
and inferred Lorentz factor. The actual column density of the BLR
is poorly constrained, but a canonical value of N ˆ 1023 cm22 is
often employed in photoionization models (Korista et al. 1997). If
this value is indeed characteristic of the BLR clouds, they would
have to cover 50 per cent of the sky as seen from the source in
order for photoionization by the blazar beam to produce an Ha
equivalent width comparable to that observed.
4.2 An alternative: photoionization by EUV continuum from
an accretion disc
If the beamed synchrotron continuum cannot adequately account
Figure 4. Comparison of the observed Ha equivalent width (filled circles)
with the predicted variation with optical continuum flux density (relative to
the 1995 June 4 value) for a model in which the BLR is photoionized by
relativistically beamed synchrotron radiation. The solid line shows the 1/
Frel fit to the measured points. The dashed lines represent models
calculated for a BLR covering factor C ˆ 0:1 and various column densities
(each model is identified by the logarithm of the column density, N, in
cm22).The dotted line is a model for N ˆ 1023 cm22 and C ˆ 0:35 (see
text).
Figure 5. The predicted spectral energy distribution for a model in which
the continuum spectrum has contributions from both a synchrotron
component (modelled as a power law of spectral index a ˆ 2† and a hot
accretion disc (characteristic temperature T ˆ 1:5 105 K†: The beam
intensity is 16 that of the disc emission at Ha , which is the smallest ratio
predicted by our model. The combined continuum spectrum (thick line) is
shown, with the synchrotron contribution (dashed line) and the accretion
disc component (thin line).
q 2000 RAS, MNRAS 311, 485±492
A Seyfert-like nucleus in BL Lac?
for the observed behaviour of WHa , the BLR must be illuminated
by another, more isotropic source of ionizing photons. This source
must be sufficiently feeble at UV±optical wavelengths that it is
overwhelmed by the synchrotron continuum, and should not
exhibit rapid, large-amplitude variability (so as to avoid large
variations in the broad Ha flux). Thermal emission from the
surface of a hot accretion disc would meet these requirements.
This idea was considered by C96, who modelled the optical
continuum as a combination of relativistically beamed synchrotron radiation (described by the same power law as used in the
beamed illumination model) and a constant contribution from a
thermal accretion disc. The continuum spectrum emitted by the
disc is given by f n / n1/3 e2hn/kT ; where n is the frequency and T is
the characteristic temperature of the disc. The optical continuum
is dominated by synchrotron emission but the disc emission peaks
in the EUV (for T . 105 K† and is thus the dominant source of
ionizing photons (Fig. 5). Variability in the optical, even with high
amplitudes, will therefore have little effect on the line flux, which
is tied to the disc continuum. The latter may itself be variable, and
indeed the broad emission lines in Seyfert type 1 nuclei are known
to follow fluctuations in the UV±optical continuum, which is
generally attributed to emission from the accretion disc. However,
these variations are small in amplitude compared with the violent
variability observed in blazars. In the case of BL Lacertae,
therefore, we expect WHa to vary approximately inversely with the
continuum flux. Our observations are consistent with this
prediction. As already noted, a function of the form W Ha ˆ
constant/F 6563 produces a reasonably good fit to the data both
before and after correction for the host galaxy and reddening in
our Galaxy (Fig. 2).
Would an accretion disc continuum sufficiently strong to
explain the observed WHa produce an observable signature, such
as a flattening of the continuum spectrum in the near-UV? In
the model discussed by C96 the characteristic temperature of the
accretion disc was assumed to be T ˆ 1:5 105 K; and the
covering fraction of the BLR to be 10 per cent. With these
assumptions we found that the accretion disc would contribute ,6
per cent of the observed optical continuum on 1995 July 7, when
the source was at its faintest for our series of observations. In Fig. 5
we plot the combined continuum spectrum inferred for a model in
which the synchrotron power law is characterized by a ˆ 2
(typical of the optical continuum of BL Lacertae: C96) and the
disc continuum contributes 6 per cent of the total flux at the
wavelength of Ha . The slope of the combined continuum differs
only slightly …a ˆ 1:75† from the pure synchrotron power law in
Ê , with significant flattening only becoming
the range 8000±4000 A
evident at shorter wavelengths. This model represents an upper
limit, since it corresponds to the smallest measured continuum
flux and the largest values of WHa . It is therefore unlikely that,
even at low optical continuum brightness, an accretion disc
component of this characteristic temperature would be detected in
optical spectra. Cooler discs producing the same ionizing
luminosity will make larger contributions to the optical continuum. We estimate that the characteristic disc temperature must
be T . 1:2 105 K for the thermal component to remain
undetected in the optical spectrum.
It is worth considering whether soft X-ray observations might
constrain the spectral energy distribution of the photoionizing
continuum. The exponential high-energy tail of a hot accretion
disc spectrum might, for example, be seen as a soft X-ray `excess'
above the synchrotron power law. In fact, an analysis of
ROSAT data (Lamer, Brunner & Staubert 1996) shows that the
q 2000 RAS, MNRAS 311, 485±492
491
0.1±2.4 keV continuum can be fitted by a power law of spectral
index a ˆ 2:0 ^ 1:6; which is consistent with the slope of the
optical continuum. However, our model shows that a T ˆ 1:5 105 K disc would contribute negligible flux at energies $100 eV
(Fig. 5), and thus does not conflict with the soft X-ray data. In
general, given the large uncertainty in the X-ray spectral index, the
existing data do not strongly constrain either the temperature or
the strength of the putative disc component.
5 CONCLUSIONS
Measurements of the broad Ha emission in eight spectra of BL
Lacertae obtained over a period of 30 months show that its
equivalent width, WHa , varies approximately inversely with the
optical continuum flux. We conclude from this that the broad Ha
line does not respond to changes in the power of the relativistically
beamed synchrotron emission that dominates the optical continuum. Similar results have been obtained from studies of the
OVV quasars 3C 279 (Koratkar et al. 1998) and 3C 273 (Ulrich,
Courvoisier & Wamsteker 1993). In these cases, the flux of the
Lya line remained constant while the continuum flux varied by as
much as a factor of 50.
An analysis of models in which the BLR in BL Lacertae is
photoionized by the beamed synchrotron continuum shows that,
for canonical parameters, the lack of response in the Ha line
cannot be attributed to the presence of a matter-bounded `hole' in
the BLR. However, with an increased covering fraction, models
with BLR column densities N , 1024 cm22 are consistent with the
observations. The canonical value for the BLR covering fraction
of 10 per cent is based on photon-counting arguments (e.g.
Baldwin & Netzer 1978), and these are supported by the low
incidence of Lyman continuum absorption in quasars (Koratkar,
Kinney & Bohlin 1992). Nevertheless, we cannot exclude the
possibility that the covering fraction differs considerably among
AGN. For example, a recent photoionization model of the type 1
Seyfert NGC 5548 (Goad & Koratkar 1998) indicates a BLR
covering fraction approaching 40 per cent, comparable to what we
require to explain the Ha equivalent width in BL Lacertae.
Therefore we cannot rule out the blazar synchrotron beam as the
ionizing continuum source responsible for the observed Ha
emission.
Having said that, we believe that a more plausible explanation
for the observed relationship between the continuum brightness
and WHa is that the BLR is photoionized by emission from a
second continuum source, which is not directly observed in the
optical band. An obvious candidate is a hot accretion disc. If this
interpretation is correct, the prototype BL Lac object harbours not
only a BLR but also a Seyfert-like EUV continuum source. We
estimate that a disc with a characteristic temperature T . 1:2 105 K would be difficult to detect directly in optical spectra. In
order to confirm the presence of such a source, observations in the
far-UV or soft X-rays (10±100 eV) are required.
The modelling reported in this paper indicates that the current
data cannot be used to distinguish the two alternative hypotheses.
However, in order to explain the observations in terms of illumination by the blazar beam, a significant fraction of the BLR must
be optically thin at the Lyman limit. In contrast, the strong
correlations that hold in Seyfert 1 galaxies between the Balmer
line and continuum fluxes imply that most of the emitting gas is
optically thick (Peterson 1993). Consonant with this, the broadline intensities of these objects are fairly well reproduced by
492
E. A. Corbett et al.
photoionization models in which the emitting clouds are optically
thick (e.g. Kwan & Krolik 1981; Baldwin et al. 1995). Hence, if it
were indeed the case that a major part of the BLR in BL Lacertae
is optically thin, we would expect the relative intensities of its
broad emission lines to differ substantially from those in type 1
Seyfert nuclei. In particular, high-ionization lines such as Lya ,
C iv l 1550 and the He ii ll 1640, 4686 lines should be considerably stronger (cf. Shields, Ferland & Peterson 1995) relative to
low-ionization lines such as Mg ii l 2800 and the hydrogen Balmer
lines in BL Lacertae than in a typical type 1 Seyfert. It follows that
detailed spectroscopy, preferably extending to the ultraviolet, will
be required finally to resolve the question of whether BL Lacertae
can be understood as a pure blazar beam, or whether an additional
source, namely a Seyfert-like nucleus, is required.
AC K N O W L E D G M E N T S
The authors thank the staff at the WHT for their expert support,
and in particular the staff astronomers who conducted the service
observations. The William Herschel Telescope is operated on the
island of La Palma by the Royal Greenwich Observatory in the
Spanish Observatorio del Roque de los Muchachos of the Instituto
de AstrofõÂsica de Canarias. The work reported in this paper was
partly carried out using facilities and software provided by the UK
Starlink Project. We are grateful for Leo Takalo of Tuorla
Observatory for sharing his photometry data on BL Lacertae
with us, and A. Turnbull for his assistance in the reduction of the
V-band images. EAC acknowledges a PPARC studentship and AR
thanks the Royal Society for financial support.
REFERENCES
Antonucci R., 1993, ARA&A, 31, 473
Baldwin J. A., Netzer H., 1978, ApJ, 236, 388
Baldwin J. A., Ferland G., Korista K., Verner D., 1995, ApJ, 455, L119
Barthel P., 1989, ApJ, 336, 606
Bertaud C., Dumortier B., Veron P., Wlerick G., Adam G., Bigay J.,
Garier R., Dury M., 1969, A&A, 3, 436
Bloom S. D. et al., 1997, ApJ, 490, L145
Bregman J. D. et al., 1990, ApJ, 352, 574
Carini M. T., Miller H. R., Noble J. C., Goodrich B. D., 1992, AJ, 104, 15
Corbett E. A., Robinson A., Axon D. J., Hough J. H., Jeffries R. D.,
Thurston M. R., Young S., 1996, MNRAS, 281, 737 (C96)
Goad M., Koratkar A., 1998, ApJ, 495, 718
Guerra E. J., Daly H. A., 1997, ApJ, 491, 483
Koratkar A., Kinney A. L., Bohlin R. C., 1992, ApJ, 400, 435
Koratkar A., Pian E., Urry C. M., Pesce J. E., 1998, ApJ, 492, 173
Korista K., Baldwin J., Ferland G., Verner D., 1997, ApJS, 108, 401
Krolik J. H., McKee C. F., Tarter C. B., 1981, ApJ, 249, 422
Kwan J., Krolik J. H., 1981, ApJ, 250, 478
Lamer G., Brunner H., Staubert R., 1996, A&A, 311, 384
Maoz D. et al., 1991, ApJ, 367, 493
Mead A. R. G., Ballard K. R., Brand P. W. J. L., Hough J. H., Brindle C.,
Bailey J. A., 1990, A&AS, 83, 183
Miller J. S., Hawley S. A., 1977, ApJ, 212, L47
Miller J. S., French H. B., Hawley S. A., 1978, ApJ, 219, L85
Moles M., Masegosa J., Del Olmo A., 1987, AJ, 94, 1143
Moore R. L. et al., 1982, ApJ, 260, 415
Mutel R. L., Phillips R. B., Su B., Bucciferro R. R., 1990, ApJ, 352, 81
Nesci R., Maesano M., Massaro E., Montagni F., Tosti G., Fiorucci M.,
1998, A&A, 332, L1
Noble J., Carini M. T., Miller H. R., Balonek T. J., Whitman K., Davis
S. M., 1997, IAU Circ. 6693
Peterson B. M., 1993, PASP, 105, 247
Schmitt J., 1968, Nat, 218, 663
Shields J. C., Ferland G. J., Peterson B. M., 1995, ApJ, 441, 507
Sitko M.-L., Junkkarinen V. T., 1985, PASP, 97, 1158
Smith P. S., Balonek T. J., Heckert P. A., Elston R., 1986, ApJ, 305, 484
Smith P. S., Balonek T. J., Elston R., Heckert P. A., 1987, ApJS, 64, 459
Ulrich M.-H., 1981, A&A, 103, L1
Ulrich M.-H., Courvoisier T. J.-L., Wamsteker W., 1993, ApJ, 411, 125
Urry C. M. et al., 1997, ApJ, 486, 799
Vermeulen R. C., Cohen M. H., 1994, ApJ, 430, 467
Vermeulen R. C., Ogle P. M., Tran H. D., Browne I. W. A., Cohen M. H.,
Readhead A. C. S., Taylor G. B., Goodrich R. W., 1995, ApJ, 452, L5
q 2000 RAS, MNRAS 311, 485±492