Anatomy of a radio to gamma-ray outburst between the black hole and jet core in the active galactic nucleus BL Lacertae
11,213Alan P. Marscher, Svetlana G. Jorstad, Francesca D. D’Arcangelo, Paul S. Smith,
42,511G. Grant Williams, Valeri M. Larionov, Haruki Oh*, Alice R. Olmstead, Margo F.
66788Aller, Hugh D. Aller, Ian M. McHardy, Anne Lähteenmäki, Merja Tornikoski, Esko
9,102,3211Valtaoja, Vladimir A. Hagen-Thorn, Eugenia N. Kopatskaya, Walter K. Gear,
12131313, Maria Nikolashvili, Lorand Sigua, H. Richard Gino Tosti, Omar Kurtanidze
1414 Miller & Wesley T. Ryle
1. Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
2. Astronomical Institute of St. Petersburg State University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia
3. Steward Observatory, University of Arizona, Tucson, AZ 85721-0065, USA 4. MMT Observatory, University of Arizona, Tucson, AZ 85721-0065, USA 5. Isaac Newton Institute of Chile, St. Petersburg Branch, St. Petersburg State University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia 6. Astronomy Department, University of Michigan, 830 Dennison, Ann Arbor, MI 48109-1090, USA
7. Department of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
8. Metsähovi Radio Observatory, Helsinki University of Technology TKK, Metsähovintie 114, FIN-02540 Kylmälä, Finland
9. Tuorla Observatory Väisäläntie 20, FI-21500 Piikkiö, Finland
10. Department of Physics, University of Turku, FIN-20014 Turku, Finland 11. School of Physics and Astronomy, Cardiff University, Cardiff CF24 3YB, Wales, UK 12. Department of Physics, University of Perugia, Via A. Pascoli, 06123 Perugia, Italy 13. Abastumani Astrophysical Observatory, Mt.Kanobili, Abastumani, Georgia 14. Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, USA
* Present address: Department of Physics, University of California, Berkeley, CA 94720-7300 Active galactic nuclei (AGN) are famous for violent outbursts of radiation across the electromagnetic spectrum. These giant flares are especially prominent in the subclass termed ‘blazars,’ whose most prominent members include the AGN BL Lacertae (BL Lac). However, until now we have been unable to specify the location of the flares in the jet and the physical mechanisms that cause dramatic increases in brightness. Such AGN contain compact jets of highly energetic, magnetized plasma that emanate from accreting supermassive black holes. According to theoretical models, such jets are launched by dynamic magnetic fields twisted by the differential rotation of the accretion disk or inertial-frame-dragging
1-3ergosphere surrounding the black hole. The flow velocity increases down the jet
in an acceleration and collimation zone (ACZ) containing a tightly wound helical
4,5magnetic field. Here we report sequences of images and polarisation
measurements of BL Lac that reveal a bright feature in the jet causing a double flare of radiation from optical frequencies to TeV ？-ray energies, as well as a
delayed outburst at radio wavelengths. Our data indicate that the first flare starts in a region that we identify with the ACZ, where the flow follows spiral streamlines
passing through a magnetic field with a toroidal component. The flux rises due to an increase in the relativistic beaming of the radiation as the feature accelerates. The second flare and radio outburst occur as the feature encounters a standing shock wave corresponding to the bright ‘core’ seen on VLBI images.
Figure 1 displays a sequence of Very Long Baseline Array (VLBA) radio images of the jet of BL Lac. The jet approaches us within 8！ of the line of sight at a flow speed of
6 0.99c, corresponding to a Lorentz factor of 7.7，1.6.Relativistic aberration and the
Doppler effect strongly beam the radiation so that the apparent luminosity is hundreds of times higher than if the emitting plasma were at rest. An essentially identical counterjet is presumably present, but too faint to detect because of beaming in the opposite direction. The stationary core lies at the northern end. Bright knots emerge from the core at a rate of 1-2 per year and move southward at apparent superluminal
6speeds, an illusion caused by their relativistic motion.
Figure 2 presents the radio, optical, and X-ray light curves of BL Lac over a two-year period, with the addition of optical polarisation data during late 2005, when our observations are most intensive. As is evident from the light curves, we observe a
8double flare during this period. The highly significant detection of > 0.2 TeV ？ rays
from 2005.819 to 2005.831 during the first X-ray flare implies that acceleration of sub-TeV-energy electrons was particularly efficient at this time. These same electrons both produce X-rays from synchrotron radiation and scatter the X-ray photons to GeV ？-ray
energies that are boosted to the TeV range by relativistic motion of the jet plasma.
9,10The location of such flares has been controversial, with some observations
suggesting that they occur downstream of the core and theoretical models requiring that they take place well upstream of this region, where the plasma is more compact. As we explain below, our data indicate that the first flare in late 2005 corresponds to a disturbance passing through the upstream zone where the jet flow is still accelerating,
while the second is caused by interaction between the disturbance and a standing shock system in the core.
The identification of the initial flare with the ACZ is significant, since previous
11observations of jet collimation are quite limited. For example, a radio image at 7 mm
wavelength of the radio galaxy M87 appears to reveal an initially broad outflow that narrows into a nearly cylindrical jet. This is consistent with gradual collimation by
4either a toroidal magnetic field or external confining gas pressure that declines with
12distance from the black hole. The flow seen in M87 could include a ―sheath‖ that
13moves more slowly and is less focused than the ―spine.‖ In the case of BL Lac, the
high apparent superluminal motions of bright knots in the jet and its pronounced variability at all wavelengths suggest that the observed radiation arises exclusively from the spine where special relativistic effects dominate.
The primary observational indicator of magnetic collimation requiring a helical magnetic field in the spine of the jet is the evolution of the polarisation. Synchrotron radiation from a circularly symmetric jet with a helical field observed at an angle to its axis displays a net polarisation oriented either parallel or perpendicular to the projected
14jet axis. Such polarisation can be confused with shock waves or velocity shear, respectively, that can produce the same polarisation patterns. However, in a model where magnetic forces gradually accelerate and focus the jet, the flow velocity is directed along streamlines that follow a helical trajectory with a different, wider pitch
5angle than that of the magnetic field. The rotation of the flow traces back to the
footprint in the orbiting accretion disk or differentially rotating ergosphere where the spin of the black hole drags the inertial frames. A shock wave or other condensation propagating down the jet traces a spiral path along the streamlines that cycles through the orientations of the helical field (see Fig. 3 and ref. 5). This should manifest itself as a rotation of the position angle of linear polarisation as the feature moves outward. The
degree of polarisation should drop to a minimum in the middle of the rotation, when the mean magnetic field in the flaring region is transverse to that of the previously existing
15emission. As Fig. 2 demonstrates, we see both effects.
The optical polarisation shown in Fig. 2 rotates steadily by about 240！ over a 5-
day interval before settling at an EVPA of ~195！. The sequence of images (Fig. 1)
reveals a bright, superluminal knot that first appears upstream of the core. It
subsequently moves past the core and proceeds down the jet along position angle ~190！
and with an EVPA that is parallel to the jet to within the uncertainty. The close
correspondence of the optical and 7 mm EVPAs after 29 October implies that the knot is the emitter of the polarised optical emission during the flare.
15,16,17Previous authors have suggested that rotations of the polarisation vector
occur in BL Lac and the similar object OJ287. These earlier observations were more poorly sampled than ours, which allowed multiple interpretations owing to the ?180º ambiguity of the EVPA. Notwithstanding this last point, the model that we advocate is
15,16quite similar to one of those proposed in these previous papers, with the location of
the emission region and connection with high-energy flares now specified by our sequences of VLBA images and multi-waveband light curves.
We interpret the event in the following manner, as illustrated in Fig. 3. Explosive activity at the inlet of the jet near the black hole injects a surge of energy into the jet across part of its cross-sectional area. This disturbance forms a shock wave that propagates along a subset of streamlines down the acceleration and collimation zone of the jet. The shock front compresses the ambient magnetic field and energizes electrons, while the Doppler beaming increases as the knot accelerates along a spiral path that stretches out with distance down the jet. These effects cause the flux of synchrotron radiation of the knot to rise until it dominates the optical, X-ray, and (via inverse
Compton scattering) ？-ray emission as the knot exits the zone of helical magnetic field. Maximum beaming—and therefore the peak in the light curve of the first flare—occurs
during the last spiral when the Lorentz factor of the jet is near its asymptotic value and the velocity vector of the shock points most closely toward our line of sight. The peak
5can be quite sharp, as observed. When the flare dominates the optical flux, we see the optical polarisation vector rotate before the shock exits the ACZ. The ACZ is opaque at radio wavelengths, hence the first flare is absent in the radio light curves.
Beyond the ACZ, the shock encounters a region of turbulence that is possibly
6driven by velocity shear across the jet downstream of the point where the magnetic and
4particle energy densities reach rough equipartition. The ambient magnetic field in the
jet has a chaotic structure in this region. Since the shock front amplifies only the component of the field that is parallel to the front, the EVPA becomes transverse to this direction and therefore essentially parallel to the velocity vector of the knot along PA~190º. During this phase, the flux declines as the knot proceeds down the broadening jet, where there is a gradient of decreasing magnetic field strength and electron density.
If the model we propose is correct, then the variation of EVPA with time should deviate from a strict linear dependence owing to projection effects, since the circular cross-section has an elliptical shape from our vantage point. We have calculated this effect, including relativistic aberration, and show in Fig. 2 that the optical EVPA data do indeed follow the corresponding curve. The small number of brief excursions of the EVPA from the curve, the deviations from the mean EVPA before and after the rotation, and irregularities in the light curves can all be explained by local flare-ups of emission that momentarily amplify both the polarisation along a particular direction and the flux at various wavebands.
17,18The smoothness of the EVPA vs. time curve eliminates the possibility that the
rotation is actually a random walk of the polarisation vector owing to a chaotic magnetic field. In this case, our numerical simulations (see ref. 18) indicate that the curve should be much more jagged than observed when the degree of polarisation ~ 5%. In the simulations, this level of polarisation corresponds to synchrotron emission from ~200 independent cells, each with randomly oriented magnetic field. In such a model, apparent rotations by ~240º are very rare in the simulations, whereas they are common
15during flares of BL Lac and similar objects.
Both synchrotron radiation and inverse Compton scattering contribute to the X-ray emission from BL Lac, with synchrotron dominating when electrons are accelerated
19to energies in the TeV range. This generally causes the X-ray flux density (F) vs. ？
;；frequency (？) spectrum to steepen such that the spectral index ； > 1, where F ； ？. ？
Such X-ray spectral steepening occurs during the first flare. In contrast, the X-ray spectrum becomes harder (； < 1) during the second flare, as expected if the X-rays are generated by inverse Compton scattering of optical and infrared photons.
The second flare, which starts at 2005.89, is simultaneous with the passage of the knot through the core seen on the VLBA images. If the core is a standing conical shock, as determined from simultaneous radio and optical polarisation variability in the case of
18the quasar PKS 0420;014, the emission would increase as the knot undergoes
compression by the shock front. The flare dies at optical and X-ray frequencies as the knot propagates away from the core down the expanding jet. However, it lives much
er at 43 GHz, at which the synchrotron radiation requires lower energy electrons long
that have longer energetic lifetimes than those emitting at higher frequencies.
The observed time delay of 18 days between the end of the optical EVPA rotation and the coincidence of the knot with the core in the VLBA images corresponds to a
18linear distance of ~2 light-years ~ 2(10 cm between the end of the ACZ and the core
20at 7 mm wavelength. The flatness of the radio spectrum down to ~0.7 mm wavelength
implies that the jet is a self-similar cone with constant Lorentz factor downstream of the
21point where the jet cross-section becomes ~ 0.1 times its value at the 7 mm core.
Although the cross-sectional radius of the jet at the position of the 7 mm core is too
17small to resolve on our VLBA images, we can estimate a value ~ 1(10 cm from the
22timescale of radio flux variability and brightness temperature arguments. If the site
where the jet becomes optically thin at 0.7 mm wavelength is at the outer radius of the
2316ACZ, the cross-sectional radius at that point ~ 1(10 cm, equivalent to ~200
8Schwarzschild radii (R) for a black-hole mass M ? 2(10 solar masses, as estimated for s
24-1BL Lac. The observed rotation rate of the polarisation, 50? day, corresponds to a
single full twist of the streamline occurring across an axial distance of ~ 1 light-year ~
177(10 cm, since the apparent speed in the plane of the sky is 5.0c and the jet is
foreshortened by a factor of 8 as projected on the sky. The rotational velocity of the streamlines is then v ~ 0.06c at the outer edge of the ACZ. At smaller distances from rot
the black hole, down to the Alfvén radius where the flow speed equals the local Alfvén velocity, v should vary as 1/r to conserve angular momentum. In this case, the rot
rotational velocity v is a fraction x of the speed of light c when the cross-sectional rot
14;1;1radius ~6(10 x cm, or ~20xR. Inside the Alfvén radius the flow should co-rotate s
with the footpoint of the magnetic field in the accretion disk or ergosphere, so that v ； rot
4r. In the simulations of Vlahakis and Königl, x ? 0.3. If we set the rotational speed at
14;11/2the footpoint as v(r) = xcr/(7(10 x cm) = (GM/r), we can estimate that the outer rotff
magnetic field of the jet is anchored at r ~ 30 R. If instead x ? 1, as might be expected fs
for almost pure Poynting flux jets launched from the ergosphere of a rotating black
25,26hole , we arrive at a value r ~ 6R. The uncertainties in both the numerical value of fs
6 and R are of order a factor of 2. Hence, these distances from the black hole are s
consistent with the expectations of models in which the jet is driven by twisting magnetic fields from the vicinity of an accreting black hole.
We constructed the images at 7 mm wavelength from data obtained with the Very Long Baseline Array (VLBA). The optical polarimetric data resulted from observations at Steward Observatory and the Crimean Astrophysical Observatory. The optical flux density points are from photometry at these two sites plus Lowell Observatory, Perugia University Astronomical Observatory, and Abustumani Astrophysical Observatory. All of the optical telescopes are equipped with CCD cameras. We measured the X-ray flux and continuum spectrum via a monitoring program with the Rossi X-ray Timing
Explorer (RXTE), a NASA space observatory. We obtained the radio flux density measurements at the University of Michigan Radio Astronomy Observatory (UMRAO) and the Metsähovi Radio Observatory. Descriptions of the telescopes and data analysis are available in the online supplementary information.
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