Issue |
A&A
Volume 508, Number 3, December IV 2009
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|
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Page(s) | L31 - L34 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/200913566 | |
Published online | 04 December 2009 |
A&A 508, L31-L34 (2009)
LETTER TO THE EDITOR
Power for dry BL Lacertae objects
A. Paggi1 - A. Cavaliere1 - V. Vittorini2 - M. Tavani1,2
1 - Dipartimento di Fisica, Università di Roma
``Tor Vergata'', and INFN Roma Tor Vergata via della Ricerca
Scientifica 1, 00133 Roma, Italy
2 - INAF/IASF-Roma, via Fosso del Cavaliere 1, 00100 Roma, Italy
Received 28 October 2009 / Accepted 16 November 2009
Abstract
Is it significant that the intrinsic outputs of several BL Lacs are observed to level off at values of about 1046 erg s-1? In searching for an answer, we compare -ray observations by the AGILE
satellite of the BL Lac S5 0716+714 with those of
Mrk 421 and Mrk 501; the former are particularly marked by
intense flares up to fluxes of
photons cm-2 s-1
in the 0.1-10 GeV energy range. These ``dry'' BL Lacs show
evidence of neither thermal disk emissions nor emission lines signaling
any accreting or surrounding gas; the spectral distributions of their
pure non-thermal radiations are effectively represented by the
synchrotron self-Compton process. With source parameters
correspondingly derived and tuned with simultaneous multiwavelength
observations, we find for S5 0716+714 a total jet power of about
erg s-1, which makes it one of the brightest dry BL Lacs so far detected in
rays.
We evaluate the mass of the associated Kerr hole to be around
,
implying that the source is significantly gauged in terms of the maximal power around
erg s-1 extractable via the Blandford-Znajek electrodynamical mechanism; other dry BL Lacs observed in
rays remain well below that threshold. These findings and those forthcoming from Fermi-LAT will provide a powerful test of electrodynamics in the surroundings of the hole, that are dominated by GR effects.
Key words: BL Lacertae objects: general - radiation mechanisms: non-thermal - black hole physics - accretion, accretion disks
1 Introduction
Blazars rank among the brightest active galactic nuclei on the basis of
their inferred isotropic luminosities that may attain some
erg s-1.
Actually, these sources radiate from a narrow relativistic jet closely
aligned with the observer's line of sight. The jet emits highly beamed
non-thermal radiations, with observed fluxes enhanced by aberration and
Doppler effects of Special Relativity (Begelman et al. 1984; Königl 1986; Urry & Padovani 1995). So the luminosities
greatly exceed the intrinsic outputs of the jets that easily level off at
.
The BL Lac objects (henceforth BL Lacs) in particular are
blazars that show no or just weak and intermittent emission lines.
Their spectra are represented well as a continuous spectral energy
distribution (SED)
featuring two peaks: one at a lower frequency due to synchrotron
emission by highly relativistic electrons; and a higher frequency
counterpart due to inverse Compton upscattering by the same electron
population of seed photons provided by the synchrotron emission itself
(synchrotron self-Compton, SSC; see Jones et al. 1974; Marscher & Gear 1985; Maraschi et al. 1992), with possible additions from sources external to the beam (external Compton, EC; see Dermer & Schlickeiser 1993; Sikora et al. 1994).
The BL Lacs also exhibit strong variability on timescales of days to minutes with substantial flux variations (flares) particularly at high energies as realized early on (see Setti & Woltjer 1994, and references therein).
Here we focus on ``dry'' BL Lacs, that is, sources with no
evidence of surrounding gas, such as emission lines or a big blue bump
(see Peterson 1997; Kembhavi & Narlikar 1999)
related to current accretion. They provide an appropriate testing
ground for comparing their intrinsic outputs with maximal powers
extractable from rotating supermassive black holes and from the dragged
accretion disks by means of large-scale electromagnetic fields, via the
intriguing, variously debated Blandford-Znajek electrodynamics (BZ
et al. 1977; see also Ghosh & Abramowicz 1997; Krolik 1999; Livio et al. 1999; Cavaliere & D'Elia 2002; McKinney 2005; Nemmen et al. 2007; Tchekhovskoy, et al. 2009).
The bare hole contribution can yield up to
erg s-1, given the hole mass
in units of
and a magnetic field
G threading its horizon.
In the following, we adopt the standard, flat cosmology with H0 = 72 km s-1 Mpc-1 and
(Dunkley et al. 2009).
2 The radiation process
The SEDs of the dry BL Lacs are widely understood in terms of the
simple, homogeneous SSC process. This is based on radiations produced
in a region containing a magnetic field and relativistic electrons
accelerated to high random energies
(with
up to
106-108) that move toward the observer with bulk Lorentz factors
(see Ghisellini et al. 1993).
To begin with, we assume the sources to have an isotropic geometry with a radius R as a single size parameter, and to contain the relativistic electrons and non-relativistic protons with the same ,
at a common density n. Observed (primed) frequencies and fluxes are related to the rest frame (unprimed) quantities by means of
and
(Begelman et al. 1984), where
is the beaming factor related to the angle
between the jet and the line of sight. Small viewing angles
yield
.
Correspondingly, the intrinsic variability and crossing times R/c will be longer than the observed ones
.
On empirical and theoretical grounds, we adopt log-parabolic
shapes for the electron energy distributions. These are obtained from a
Fokker-Planck equation in the presence of systematic and stochastic
acceleration processes as first shown by Kardashev (1962) and computed in detail by Paggi et al. (2009); the acceleration times scale as
,
in terms of the effective electric field E (Cavaliere & D'Elia 2002). We therefore write the electron distribution in the form
![]() |
(1) |
where s is the constant contribution to the slope, r is the ``curvature'', and



The emitted synchrotron SED is correspondingly given by (Massaro et al. 2004; Tramacere et al. 2007, and references therein)
with a constant contribution a=(s-3)/2 to the spectral index, a spectral curvature






In these SEDs and distributions, the peaks where most of the energy
resides are not materially affected by radiative cooling active on
timescales
longer than the crossing time R/c;
cooling will rather erode the high energy tails. As an added bonus, the
synchrotron and IC radiations from a log-parabolic electron population
irreversibly broaden under the action of the stochastic acceleration component, following
(Paggi et al. 2009);
thus, a sudden increase in the spectral curvature will mark the
emergence of a new electron population. Last but not least, Eq. (2) closely fits (as we illustrate in Fig. 1) the spectra of the sources that we focus on in view of extended spectral coverage provided by their multiwavelength, simultaneous observations.
![]() |
Figure 1: SEDs of the BL Lacs considered in the text: S5 0716+174 ( upper-left frame), Mrk 501 ( upper-right frame) and Mrk 421 ( lower-left and lower-right frames), each in a low (blue line) and a high (red line) state. In terms of intrinsic luminosity S5 0716+714 is the strongest source (data referenced in Sect. 2). |
Open with DEXTER |
This is the case for S5 0716+714, which has the following data
available: a low state provided by historical AIT and EGRET data (Lin
et al. 1995);
a high state in September 2007, in the -ray range covered by AGILE-GRID where the IC peaks for this ``intermediate'' BL Lac (Padovani & Giommi 1995); optical and radio data taken with GASP-WEBT on September 7-12 (Villata et al. 2008; Vittorini et al. 2009). The multiwavelength variations observed by Giommi et al. (2008) and the increased spectral curvature (see Fig. 1) in 2007 are indicative of the injection of a second component.
The source may be compared with Mrk 501, for which we consider two states with simultaneous BeppoSAX and CAT observations on April 7 and 16, 1997 (Massaro et al. 2006); and with Mrk 421, for which we have low and high states in 2000 from BeppoSAX and HEGRA data (Konopelko et al. 2003), and multiwavelength observations performed with GASP-WEBT, RXTE/ASM, Swift, SuperAGILE, AGILE-GRID, ARGO-YBJ, and VERITAS in June 2008 (Donnarumma et al. 2009; Di Sciascio et al. 2009).
3 The source power
We are interested in the intrinsic outputs referred to the jet frame, rather than in the luminosities
inferred from insisting on an isotropic distribution of the observed flux F, at the luminosity distance DL. We assume one ``cold'' proton per electron satisfying
(with the average
bounded in terms of the electron
and the proton
masses), and follow Celotti & Ghisellini (2008) in writing for the intrinsic radiative luminosity
contributed by both the synchrotron and IC radiations and for the related powers carried by the jet, the expressions
The total jet power is therefore given by





The simultaneous, multiwavelength observations enable extended spectral
fits to determine the five key observables (beside the spectral
curvature b)
from the SSC model, namely: the synchrotron peak frequency and flux,
the IC peak frequency and flux, and the variation time (see Paggi
et al. 2009). These lead to robust evaluations of the five source parameters n, R, B,
,
and
(or
)
entering Eqs. (3)-(5); the main parameters are collected in Table 1.
Table 1: Parameters for the BL Lac sources discussed in the text.
4 The BZ benchmark
As anticipated in Sect. 1,
a natural benchmark for these powers is provided by the BZ mechanism
for electrodynamical energy extraction from a Kerr hole spun up to
maximal rotation by past accretion episodes. A minimal, vestigial disk
is required to hold the poloidal magnetic field threading the horizon;
the disk is kept active by low accretion rates
in Eddington units, loses angular momentum mainly via the large-scale field, and contributes some 3 LK to the total power (Blandford & Znajek 1977; Livio et al. 1999). The two contributions add to yield
We note that the balance



The hole mass is then the key parameter, that we evaluate from its correlation with the absolute red magnitude MR of the host galactic bulge (Ferrarese et al. 2000; Gebhardt et al. 2000; Falomo et al. 2003); for our cosmology this reads
with scatter







Our results normalized to the respective
from Eq. (6) are represented in Fig. 2.
During flares, the electron rms energies (and the peak frequencies) are
boosted in all sources, and so are the luminosities; this indicates
that rising flares are directly related to increased acceleration of the emitting electrons.
We emphasize that the powerful source S5 0716+714 is apparently constrained to move sideways, as if to skirt the BZ limit; Mrk 421 in the 2008 states exhibits a similar behaviour, although with lower significance. On the other hand, the weaker source Mrk 421 in 2000 and the yet weaker Mrk 501 remain considerably below the BZ limit and so are expected to be free to move more vertically, as they do.
![]() |
Figure 2: For the named sources the
total jet luminosities normalized to their BZ power are plotted
against the electron rms energy. Bars represent the hole mass
uncertainties reflecting those (at the |
Open with DEXTER |
![]() |
Figure 3:
Bright BL Lacs in their context, adapted from Celotti & Ghisellini (2008) with historical data in terms of LT and the energy
|
Open with DEXTER |
5 Discussion and conclusions
For dry BL Lacs with accretion rates
,
the SSC radiation process provides a robust evaluation of the jet luminosities. Whence we conclude that
provides a significant benchmark
for the output of the BL Lacs discussed here, and indeed an upper
limit to both their quiescent states and flares. In fact, during a
recent flare S5 0716+714 was observed to be constrained by
erg s-1, and a similar behavior was observed in 2008 for Mrk 421. Non-thermal, beamed powers in the range
also provide evidence of an accretion disk that is active mainly in
launching and channeling the jets by means of large-scale fields.
Referring to Fig. 3 and its caption, we note that during flares the sources move in the LT -
plane away from the envelope that is outlined by bright BL Lacs with increasing rates
;
the envelope ends up in the locus of the yet brighter flat spectrum radio quasars (FSRQs) with
.
The flares then move into a region of faster radiative cooling (Celotti & Ghisellini 2008, and references therein). This implies short-lived flares on timescales
day, or requires shorter acceleration times
with higher E, as an alternative to structured sources such as decelerating (Georganopoulos & Kazanas 2003) or spine-sheath jets with inner scale R1 < R (Tavecchio & Ghiselllini 2008). Faster acceleration and deviations from the envelope are consistent with flares caused by electron boost rather than episodes of increased accretion onto the disk.
In this context, we recall (see Blandford 1990; Padovani et al. 2007) that sources lying along the envelope in Fig. 3 at higher L and lower
often exhibit stronger evidence of current accretion up to
,
such as thermal emissions and surrounding gas (big blue bump and broad
emission lines), with a larger contribution from EC. In fact, the
progression from dry BL Lacs to FSRQs is likely to involve an
enhanced and extended disk contribution as described by Blandford &
Payne (1982), starting with ``wet'' BL Lacs with
;
these feature larger EC contributions (Dermer et al. 2008) and looming evidence of gas, including some thermal disk emission and weak or intermittent lines (Celotti et al. 2007).
The last step in this progression is constituted by the powerful FSRQs
with extant broad lines, a big blue bump from disks accreting at full
rates
,
and a dominant or towering EC (Maraschi & Tavecchio 2001).
We add that the outputs of even misaligned BL Lacs may be calorimetrically gauged from their feedback actions on the intra-cluster plasma surrounding their host galaxy when located in a cluster or a group of galaxies, as discussed by McNamara et al. (2009). These authors evaluate average powers around 1046 erg s-1 injected into the cluster MS0735.6+7421, and possibly also in the cluster A2029 and the group AWM 4.
The whole of the above evidence provides observational support to the relevance of the electrodynamical BZ mechanism, and invites extended sampling of other interesting sources (see Fig. 3).
If in dry BL Lacs with
the
limit were found to be substantially exceeded by outputs
LT > 1046 erg s-1, this would require
B > 104 G at the Kerr horizon. These fields imply large dynamical stresses bounded only by
,
associated with particle orbits plunging from the disk toward the hole horizon (Meier 2002) into a region fully controlled by strong gravity effects.
Thus, all such sources will provide powerful tests for the coupling of electrodynamics with General Relativity in full swing, and constitute an exciting arena for AGILE and Fermi-LAT data.
We are grateful to R. Falomo for useful discussions concerningevaluations for BL Lacs, and specifically in S5 0716+714. We acknowledge our referee for useful comments and helpful suggestions.
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All Tables
Table 1: Parameters for the BL Lac sources discussed in the text.
All Figures
![]() |
Figure 1: SEDs of the BL Lacs considered in the text: S5 0716+174 ( upper-left frame), Mrk 501 ( upper-right frame) and Mrk 421 ( lower-left and lower-right frames), each in a low (blue line) and a high (red line) state. In terms of intrinsic luminosity S5 0716+714 is the strongest source (data referenced in Sect. 2). |
Open with DEXTER | |
In the text |
![]() |
Figure 2: For the named sources the
total jet luminosities normalized to their BZ power are plotted
against the electron rms energy. Bars represent the hole mass
uncertainties reflecting those (at the |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Bright BL Lacs in their context, adapted from Celotti & Ghisellini (2008) with historical data in terms of LT and the energy
|
Open with DEXTER | |
In the text |
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