H. Richard Miller

The inner jet of an active galactic nucleus as revealed by a radio-to-γ-ray outburst

Blazars are the most extreme active galactic nuclei. They possess oppositely directed plasma jets emanating at near light speeds from accreting supermassive black holes. According to theoretical models, such jets are propelled by magnetic fields twisted by differential rotation of the black hole’s accretion disk or inertial-frame-dragging ergosphere. The flow velocity increases outward along the jet in an acceleration and collimation zone containing a coiled magnetic field. Detailed observations of outbursts of electromagnetic radiation, for which blazars are famous, can potentially probe the zone. It has hitherto not been possible to either specify the location of the outbursts or verify the general picture of jet formation. Here we report sequences of high-resolution radio images and optical polarization measurements of the blazar BL Lacertae. The data reveal a bright feature in the jet that causes a double flare of radiation from optical frequencies to TeV γ-ray energies, as well as a delayed outburst at radio wavelengths. We conclude that the event starts in a region with a helical magnetic field that we identify with the acceleration and collimation zone predicted by the theories. The feature brightens again when it crosses a standing shock wave corresponding to the bright ‘core’ seen on the images.

Disk-Jet Connection in the Radio Galaxy 3C 120

We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 120 between 2002 and 2007 at X-ray (2-10 keV), optical (R and V bands), and radio (14.5 and 37 GHz) wave bands, as well as imaging with the Very Long Baseline Array (VLBA) at 43 GHz. Over the 5 yr of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. Consistent with this, the X-ray flux and 37 GHz flux are anti-correlated with X-ray leading the radio variations. Furthermore, the total radiative output of a radio flare is related to the equivalent width of the corresponding X-ray dip. This implies that, in this radio galaxy, the radiative state of accretion disk plus corona system, where the X-rays are produced, has a direct effect on the events in the jet, where the radio emission originates. The X-ray power spectral density of 3C 120 shows a break, with steeper slope at shorter timescale and the break timescale is commensurate with the mass of the central black hole (BH) based on observations of Seyfert galaxies and black hole X-ray binaries (BHXRBs). These findings provide support for the paradigm that BHXRBs and both radio-loud and radio-quiet active galactic nuclei are fundamentally similar systems, with characteristic time and size scales linearly proportional to the mass of the central BH. The X-ray and optical variations are strongly correlated in 3C 120, which implies that the optical emission in this object arises from the same general region as the X-rays, i.e., in the accretion disk-corona system. We numerically model multi-wavelength light curves of 3C 120 from such a system with the optical-UV emission produced in the disk and the X-rays generated by scattering of thermal photons by hot electrons in the corona. From the comparison of the temporal properties of the model light curves to that of the observed variability, we constrain the physical size of the corona and the distances of the emitting regions from the central BH. In addition, we discuss physical scenarios for the disk-jet connection that are consistent with our observations.

Correlated X-Ray and Optical Variability in Markarian 509

We present the results of a 3 year monitoring campaign of the Seyfert 1 galaxy Markarian 509, using X-ray data from the Rossi X-Ray Timing Explorer and optical data taken by the SMARTS consortium. Both light curves show significant variations and are strongly correlated with the optical flux leading the X-ray flux by 15 days. The X-ray power spectrum shows a steep high-frequency slope of –2.0, breaking to a slope of –1.0 at a timescale of 34 days. The lag from optical to X-ray emission is most likely caused by variations in the accretion disk propagating inward.

NEW MULTIWAVELENGTH OBSERVATIONS OF PKS 2155-304 AND IMPLICATIONS FOR THE COORDINATED VARIABILITY PATTERNS OF BLAZARS

The TeV blazar PKS 2155-304 was the subject of an intensive 2 week optical and near-infrared observing campaign in 2004 August with the CTIO 0.9 m telescope. During this time, simultaneous X-ray data from RXTE were also obtained. We compare the results of our observations to the results from two previous simultaneous multiwavelength campaigns on PKS 2155-304. We conclude that the correlation between the X-ray and UV/optical variability is strongest and the time lag is shortest (only a few hours) when the object is brightest. As the object becomes fainter, the correlations are weaker and the lags longer, increasing to a few days. Based on the results of four campaigns, we find evidence for a linear relationship between the mean optical brightness and lag time of X-ray and UV/optical events. Furthermore, we assert that this behavior, along with the different multiwavelength flare lag times across different flux states, is consistent with a highly relativistic shock propagating down the jet producing the flares observed during a high state. In a quiescent state, the variability is likely to be due to a number of factors including both the jet and contributions outside of the jet, such as the accretion disk.

MULTIWAVELENGTH VARIABILITY OF THE BROAD LINE RADIO GALAXY 3C 120

We present results from a multiyear monitoring campaign of the broad-line radio galaxy 3C 120, using the Rossi X-ray Timing Explorer for nearly five years of observations. Additionally, we present coincident optical monitoring using data from several ground-based observatories. Both the X-ray and optical emission are highly variable and appear to be strongly correlated, with the X-ray emission leading the optical by 28 days. The X-ray power density spectrum is best fit by a broken power law, with a low-frequency slope of –1.2, breaking to a high-frequency slope of –2.1, and a break frequency of log νb = –5.75 Hz, or 6.5 days. This value agrees well with the value expected based on 3C 120s mass and accretion rate. We find no evidence for a second break in the power spectrum. Combined with a moderately soft X-ray spectrum (Γ = 1.8) and a moderately high accretion rate (), this indicates that 3C 120 fits in well with the high/soft variability state found in most other active galactic nuclei. Previous studies have shown that the spectrum has a strong Fe Kα line, which may be relativistically broadened. The presence of this line, combined with a power spectrum similar to that seen in Seyfert galaxies, suggests that the majority of the X-ray emission in this object arises in or near the disk, and not in the jet.

The Timescales of the Optical Variability of Blazars. V. 3C 371

Optical photometric observations of the blazar 3C 371 are presented as part of an ongoing study of variations in blazars occurring on timescales of minutes to hours, which has been termed microvariability. The observations presented here provide further evidence of the classwide nature of this phenomenon. Timescales as short as these provide important clues about the nature of blazars, and a discussion of the types of possible mechanisms responsible for producing these variations is given.

THE EXTREME OPTICAL VARIABILITY OF J0948+0022

We report on observations of the optical variability of the radio-loud, narrow-line Seyfert 1 galaxy J0948+0022 on timescales ranging from minutes to years. Implications regarding recent suggestions that the object may constitute a prototype for an emerging class of blazar-like objects similar to FSRQs are discussed. The optical microvariability observed for J0948+0022 is found to be similar to that found for a typical low energy peaked BL Lac object. Based on observations of J0948+0022 in a flaring state and a significantly lower state, one can demonstrate that these rapid variations most likely originate in the relativistic jet and not in the accretion disk.

Simultaneous Multiwavelength and Optical Microvariability Observations of CTA 102 (PKS J2232+1143)

We present analysis of both the short-term optical and long-term multiwavelength variability of CTA 102. In 2004, this object was observed in an intense optical flaring state. Extensive R-band microvariability observations were carried out during this high state. In 2005, we obtained several weeks of contemporaneous radio, optical, and X-ray observations of CTA 102. These observations recorded distinct flaring activity in all three wavebands. Subsequent analysis revealed that this object may appear redder when in a brighter optical state, and that the X-ray, optical, and radio activity do not appear to be correlated. The shape of the observed spectral energy distributions suggests that both synchrotron-related and external inverse Compton processes may contribute to the X-ray emission. Our results are also compared to other results on this object and archival microvariability observations. It appears that more rapid, dramatic microvariability events occur when CTA 102 is in an elevated optical flux state.

Multiwavelength observations of the extreme X-ray-selected BL Lacertae object PG 1553+11 (1ES 1553+113)

PG 1553+11 was the target of a coordinated 3 week multiwavelength campaign during 2003 April and May. A significant X-ray flare was observed during the second half of this campaign. Although no optical flare was recorded during the X-ray campaign, optical observations obtained immediately prior to the campaign displayed a flux higher than that recorded during the campaign. An optical flare was observed a few days after the end of the X-ray campaign and may be related to the X-ray flare. Radio observations were made at three frequencies, with no significant changes in flux detected near the times of the optical and X-ray flares. The spectral energy distributions and flux ratios in different wave bands observed for this object are compared to other X-ray-selected blazars to demonstrate that PG 1553+11 is an extreme member of this group.

THE VARIABLE OPTICAL POLARIZATION AND FERMI OBSERVATIONS OF PMN J0948+0022

We report on observations of the {\gamma}-ray and optical photopolarimetric behavior of the radio-loud, narrow line type-1 Seyfert galaxy PMN J0948+0022 over a twenty seven month period. As this object has recently been suggested to represent a prototype of an emerging class of blazar-like objects, the observed properties are compared to those of blazars. We extract doubling timescales of roughly 4 hours for the optical and {\gamma}-ray bands. The rapid microvariability in the optical/NIR, significant and variable optical polarization, and strong yet rapidly-variable {\gamma}-ray emission we observe for PMN J0948+0022 are all classical observational characteristics associated with blazars. However, since these observations do not show a clear correlation between the {\gamma}-ray and optical behavior, they do not offer conclusive proof that the emissive behavior of PMN J0948+0022 is due to a relativistic jet oriented close to our line of sight.