Anatomy of a radio to gamma-ray outburst between the black hole and jet core in the active galactic nucleus BL Lacertae
Alan P. Marscher1, Svetlana G. Jorstad1,2, Francesca D. D’Arcangelo1, Paul S. Smith3, G. Grant Williams4, Valeri M. Larionov2,5, Haruki Oh1*, Alice R. Olmstead1, Margo F. Aller6, Hugh D. Aller6, Ian M. McHardy7, Anne Lähteenmäki8, Merja Tornikoski8, Esko Valtaoja9,10, Vladimir A. Hagen-Thorn2,3, Eugenia N. Kopatskaya2, Walter K. Gear11, Gino Tosti12, Omar Kurtanidze13, Maria Nikolashvili13, Lorand Sigua13, H. Richard Miller14 & Wesley T. Ryle14
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 ergosphere surrounding the black hole1-3. The flow velocity increases down the jet in an acceleration and collimation zone (ACZ) containing a tightly wound helical magnetic field4,5. 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 0.99c, corresponding to a Lorentz factor of 184.108.40.206 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 speeds, an illusion caused by their relativistic motion6.
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 double flare during this period. The highly significant detection8 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.
The location of such flares has been controversial, with some observations9,10 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 observations of jet collimation are quite limited. For example, a radio image11 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 either a toroidal magnetic field4 or external confining gas pressure that declines with distance from the black hole12. The flow seen in M87 could include a “sheath” that moves more slowly and is less focused than the “spine.”13 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 jet axis14. 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 angle than that of the magnetic field5. 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 emission15. 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.
Previous authors15,16,17 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 quite similar to one of those proposed in these previous papers15,16, 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 can be quite sharp5, 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 driven by velocity shear across the jet6 downstream of the point where the magnetic and particle energy densities reach rough equipartition4. 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.
The smoothness of the EVPA vs. time curve eliminates the possibility17,18 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 during flares of BL Lac and similar objects15.
Both synchrotron radiation and inverse Compton scattering contribute to the X-ray emission from BL Lac, with synchrotron dominating when electrons are accelerated to energies in the TeV range19. 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 the quasar PKS 042001418, 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 longer at 43 GHz, at which the synchrotron radiation requires lower energy electrons 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 linear distance of ~2 light-years ~ 21018 cm between the end of the ACZ and the core at 7 mm wavelength. The flatness of the radio spectrum down to ~0.7 mm wavelength20 implies that the jet is a self-similar cone with constant Lorentz factor downstream of the point where the jet cross-section becomes ~ 0.1 times its value at the 7 mm core21. Although the cross-sectional radius of the jet at the position of the 7 mm core is too small to resolve on our VLBA images, we can estimate a value ~ 11017 cm from the timescale of radio flux variability and brightness temperature arguments.22 If the site where the jet becomes optically thin at 0.7 mm wavelength is at the outer radius of the ACZ23, the cross-sectional radius at that point ~ 11016 cm, equivalent to ~200 Schwarzschild radii (Rs) for a black-hole mass M ≈ 2108 solar masses, as estimated for BL Lac24. The observed rotation rate of the polarisation, 50° day-1, corresponds to a single full twist of the streamline occurring across an axial distance of ~ 1 light-year ~ 71017 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 vrot ~ 0.06c at the outer edge of the ACZ. At smaller distances from the black hole, down to the Alfvén radius where the flow speed equals the local Alfvén velocity, vrot should vary as 1/r to conserve angular momentum. In this case, the rotational velocity vrot is a fraction x of the speed of light c when the cross-sectional radius ~61014 x1 cm, or ~20x1Rs. Inside the Alfvén radius the flow should co-rotate with the footpoint of the magnetic field in the accretion disk or ergosphere, so that vrot r. In the simulations of Vlahakis and Königl4, x ≈ 0.3. If we set the rotational speed at the footpoint as vrot(rf) = xcrf/(71014 x1 cm) = (GM/r)1/2, we can estimate that the outer magnetic field of the jet is anchored at rf ~ 30 Rs. If instead x ≈ 1, as might be expected for almost pure Poynting flux jets launched from the ergosphere of a rotating black hole25,26 , we arrive at a value rf ~ 6Rs. The uncertainties in both the numerical value of 6 and Rs are of order a factor of 2. Hence, these distances from the black hole are 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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The authors thank Dr. N. Vlahakis for enlightening discussions. The research reported here is based on work at Boston University supported by the U.S. National Science Foundation under grant AST-0406865 and NASA grant NNX06AG86G, at St. Petersburg State University supported by grant no. 05-02-17562 from the Russian Fund for Basic Research, and at the University of Michigan supported by the U.S. National Science Foundation under grant AST-0607523 and a series of NASA grants. P.S.S. acknowledges support from NASA contract 1256424. AL and MT acknowledge support from the Academy of Finland for the Metsähovi observing projects. The VLBA is an instrument of the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The authors declare that they have no competing financial interests.
Correspondence and requests for materials should be addressed to A.P.M. (e-mail: email@example.com).
Figure 1 | Sequence of VLBA images of BL Lac at a wavelength of 7 mm (frequency = 43 GHz). The images are convolved with a circular Gaussian function of FWHM=0.1 milliarcseconds (mas), the resolution of the longest baselines of the array, which corresponds to 0.12 parsecs (pc) at the distance of 291 Mpc derived from the redshift, z=0.069, and the Hubble Law with an assumed slope H0 = 72 km s-1 Mpc-1 (ref. 7). Contours indicate total intensity, starting at 2.30 Jy/beam and increasing by factors of 2. Colour scale represents polarised intensity, with peak (yellow) at 0.215 Jy/beam. The yellow line segments indicate the electric vector position angle (EVPA) of the polarisation, which is uncertain by ±7° in each image. The approximate path of the centroid of the knot is given by the slanted red line, the definition of which includes a point from an image obtained on 12 March 2006 in addition to those shown. The observed proper motion of the knot of 1.2 mas yr-1 is equivalent to an apparent speed of 5.0c after correction for time dilation owing to the cosmological redshift.
Figure 2 | Flux density at various wavebands and optical polarisation of BL Lac as a function of time. Left: Dependence on time of the flux of radiation from BL Lac over a two-year interval at the indicated wavebands. Error bars represent ±1 uncertainties in the values plotted. The exponent of the power-law dependence of X-ray flux density on frequency is x. Right: Blow-up of the 0.25-year time interval marked by vertical dotted lines in the lefthand panels, with optical R-band EVPA and degree of polarisation replacing X-ray spectral index and radio flux density. The time range of highly significant detections8 at photon energies > 0.2 TeV is indicated by the width of the head of the upward arrow in the top panel. The rotation in optical R-band EVPA near the time of the peak of the first optical and X-ray flare is apparent. Since there is an ambiguity of ±180º in the value of the EVPA, we have selected the quadrant of each value that provides a consistent overall trend of rotation between 2005.81 and 2005.83. As is discussed in the main text, the solid curve in the EVPA panel corresponds to the pattern expected according to the model shown in Figure 3 when relativistic aberration is included. The vertical arrow (with error bar) in the bottom panel denotes the time when the superluminal knot is coincident with the stationary core seen in the images displayed in Fig. 1.
Figure 3 | Sketch of the inner jet of BL Lac according to the scenario we propose to explain the behavior during the last quarter of 2005. A shock propagates down the jet along a spiral streamline. The first flare occurs during the last 240 twist of the streamline before the flow straightens and becomes turbulent. The passage of the feature through the mm-wave core stimulates the second flare. A logarithmic scale of distance from the black hole, shown in the bottom panel in terms of Schwarzschild radii, is employed to illustrate phenomena on various scales.
We observed BL Lac at 7 mm wavelength (frequency of 43 GHz) with the Very Long Baseline Array (VLBA) at the epochs displayed in Fig. 1 as well as the following dates: 24 and 28 October 2005, 2 November 2005, and 12 March 2006. After correlation at the Array Operations Center of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, we passed the data through the VLBI routines of the Astronomical Image Processing System (AIPS) software supplied by NRAO for initial calibration and followed the procedures described in ref. 6 to create and analyze the resultant images. We refer the EVPA measurements to a stable feature in the VLBA polarised intensity image of the quasar CTA102. There is good agreement between the thus-calibrated EVPAs of several objects with archival data and with contemporaneous Very Large Array measurements (available at http://www.vla.nrao.edu/astro/calib/polar/). In our analysis, we subtract 16 from the polarisation angle in the pseudocore of BL Lac to compensate for Faraday rotation27.
We collected optical polarimetric data at Steward Observatory and the Crimean Astrophysical Observatory and photometry at these two sites plus three others. At the Steward Observatory 1.55 m Kuiper telescope at Mt. Bigelow, Arizona, USA, we used the SPOL spectropolarimeter28, making a total of 34 measurements, each covering
400-800 nm during 12 contiguous nights from 2005 October 23 to November 3. Details of the reduction procedure can be found in, e.g., ref. 29. At the Crimean Astrophysical Observatory in Crimea, Ukraine, we performed R-band photometry and polarimetry on the AZT-8 70 cm telescope, with a prime-focus photometer-polarimeter equipped with two Savart plates.
We also measured the optical R-band magnitude of BL Lac with the following telescopes: the 1.83 m Perkins telescope at Lowell Observatory in Flagstaff, Arizona, USA, the 0.4 m Automatic Imaging Telescope at Perugia University Astronomical Observatory in Perugia, Italy, and the 70 cm meniscus telescope30 of Abastumani Astrophysical Observatory in Abastumani, Georgia. All telescopes are equipped with CCD cameras. For all flux measurements, we determined the magnitude of BL Lac relative to field stars calibrated by Smith et al.31.
We observed BL Lac an average of 3 times per week for ~2000 s per pointing with the Rossi X-ray Timing Explorer (RXTE). For each X-ray observation, we determined the X-ray flux in photon counts per second over the energy range 2 to 10 keV by subtracting an X-ray background model (supplied by the RXTE Guest Observer Facility) from the raw spectrum, using the standard X-ray data analysis software package FTOOLS. We fit the photon spectrum (observed flux Fobs versus photon energy E from 2.4 to 10 keV) with a model consisting of a power-law intrinsic spectrum with spectral index , F = E(+1), plus photoelectric absorption along the line of sight using the program XSPEC. For the latter, we adopted a hydrogen column density of 2.71021 atoms cm2 (see ref. 32).
Radio flux density measurements.
We determined the flux density of BL Lac at a wavelength of 2 cm (frequency of 14.5 GHz) with the 26-m diameter antenna of the University of Michigan Radio Astronomy Observatory (UMRAO) near Ann Arbor, Michigan, USA and at 8 mm (37 GHz) with the 14 m antenna of the Metsähovi Radio Observatory in Kylmälä, Finland. The data displayed in Fig. 2 are nightly averages. Detailed description of the instrumentation and reduction procedures can be found from Aller et al.33 for UMRAO and Teräsranta, H. et al.34 for Metsähovi.
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