Imaging Strong Blazars with Space VLBI

2010

One of the defining characteristics of BL Lacertae objects is their strong variability across the electromagnetic spectrum. PKS 0537-441 is one such object and is one of the most luminous blazars from radio to gamma-ray wavelengths. It was detected as a strong and highly variable source by EGRET and has been reported several times to be in an active state by Fermi . It is one of the brightest gamma-ray blazars detected in the southern sky so far. The TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) program is monitoring PKS 0537-441 at VLBI resolutions. We present 8.4 GHz and 22 GHz images of the milliarcsecond scale structure. We also present our ongoing analysis of the spectral and temporal changes in this object.

Single-zone synchrotron self-Compton and external Compton models are widely used to explain broadband Spectral Energy Distributions (SEDs) of blazars from infrared to gamma-rays. These models bear obvious similarities to the homogeneous synchrotron cloud model which is often applied to explain radio emission from individual components of parsec-scale radio jets. The parsec-scale core, typically the brightest and most compact feature of blazar radio jet, could be the source of high-energy emission. We report on ongoing work to test this hypothesis by deriving the physical properties of parsec-scale radio emitting regions of twenty bright Fermi blazars using dedicated 5-43 GHz VLBA observations and comparing these parameters to results of SED modeling.

Journal of Astrophysics and Astronomy, 2011

Blazars are now well understood as approaching relativistic jets aligned with the line of sight. The long-time uncertainty about the demographics of blazars is starting to become clearer: since the Fermi blazar sample includes a larger fraction of high-frequency peaked blazars (like the typical X-ray-selected blazars in, say, the Einstein Slew Survey sample) than did the higher-flux-limit EGRET blazar sample, these low-luminosity sources must be more common than their higher luminosity, low-frequency-peaked cousins. Blazar spectral energy distributions have a characteristic two-component form, with synchrotron radiation at radio through optical (UV, X-ray) frequencies and gamma-rays from X-ray through GeV (TeV) energies. Multiwavelength monitoring has suggested that gamma-ray flares can result from acceleration of electrons at shocks in the jet, and there appears to be an association between the creation of outflowing superluminal radio components in VLBI maps and the gamma-ray flares. In many cases, the gamma-ray emission is produced by inverse Compton upscattering of ambient optical-UV photons, although the contribution from energetic hadrons cannot be ruled out. The next few years of coordinated gamma-ray, X-ray, UV, optical, infrared and radio monitoring of blazars will be important for characterizing jet content, structure, and total power.

Astronomy & Astrophysics, 2014

Context. In the framework of the unification scheme of radio-loud active galactic nuclei, BL Lac objects and quasars are the beamed end-on counterparts of low-power (FR I) and high-power (FR II) radio galaxies, respectively. Aims. Some BL Lacs have been found to possess the FR II-type large-scale radio morphology, suggesting that the parent population of BL Lacs is a mixture of low-and high-power radio galaxies. This seems to apply only to 'low frequency-peaked' BL Lacs, since all the 'high frequency-peaked' BL Lacs studied so far were shown to host exclusively the FR I-type radio jets. While analyzing the NVSS survey maps of the TeV detected BL Lacs, we have however discovered that the high frequency-peaked object SHBL J001355.9-185406 is associated uniquely with the one-sided, arcmin-scale, and edge-brightened jet/lobe-like feature extending to the south-west from the blazar core. Methods. In order to investigate in detail the large-scale morphology of SHBL J001355.9-185406, we have performed low-frequency and high-resolution observations of the source at 156, 259 and 629 MHz using the Giant Metrewave Radio Telescope. Results. Our analysis indicates that no diffuse arcmin-scale emission is present around the unresolved blazar core, and that the arcmin-scale structure seen on the NVSS map breaks into three distinct features unrelated to the blazar, but instead associated with background AGN. Conclusions. The upper limits for the extended radio halo around the TeV-emitting BL Lac object SHBL J001355.9-185406 read as < 10% -1% at 156 -629 MHz. The fact that the integrated radio spectrum of the unresolved blazar core is flat down to 156 MHz indicates that a self-similar character of the jet in the source holds up to relatively large distances from the jet base.

The Astrophysical Journal, 2010

The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed as part of the VLBA Imaging and Polarimetry Survey (VIPS). This large, flux-limited sample of active galactic nuclei (AGN) provides insights into the mechanism that produces strong γ-ray emission. At lower flux levels, radio flux density does not directly correlate with γ-ray flux. We find that the LAT-detected BL Lacs tend to be similar to the non-LAT BL Lacs, but that the LAT-detected FSRQs are often significantly different from the non-LAT FSRQs. The differences between the γ-ray loud and quiet FSRQs can be explained by Doppler boosting; these objects appear to require larger Doppler factors than those of the BL Lacs. It is possible that the γ-ray loud FSRQs are fundamentally different from the γ-ray quiet FSRQs. Strong polarization at the base of the jet appears to be a signature for γ-ray loud AGN.

We present observations of the high redshift (z = 3.03) blazar J08053+6144 at both 5 GHz and 15 GHz using the VLBA. We obtained high dynamic range Stokes I maps of J08053+6144 at 5 and 15 GHz. Polarization maps plotting source intensity (Stokes I) and polarization angle are included with a rotation measure map. Using data obtained from our VLBA observations we provide calculations for the spectral index α that characterizes the source emission ( ν ∝ ν α ). We also calculate the brightness temperature of the source at 5 and 15 GHz. The magnetic field strength is also calculated using a method mentioned by [1] under the conditions that we are solving for a minimum energy condition ume = ( 7 3 )( Bme 2 8π ). We also compare our observations with observations of this source at 5 GHz in 1998 (Taylor, et al ) [2] in order to look for bulk motion within the visible jet component of the source.

Nature Astronomy

Monthly Notices of the Royal Astronomical Society

It is well known that the γ-ray emission in blazars originates in the relativistic jet pointed at the observers. However, it is not clear whether the exact location of the GeV emission is less than a parsec (pc) from the central engine, such that it may receive sufficient amount of photons from the broad-line region (BLR) or farther out at 1–100 pc range. The former assumption has been successfully used to model the spectral energy distribution of many blazars. However, simultaneous detection of TeV γ-rays along with GeV outbursts in some cases indicate that the emission region must be outside the BLR. In addition, GeV outbursts have sometimes been observed to be simultaneous with the passing of a disturbance through the so-called ‘very long baseline interferometry (VLBI) core’, which is located tens of pc away from the central engine. Hence, the exact location of γ-ray emission remains ambiguous. Here we present a method that we have developed to constrain the location of the emiss...

Astrophysical Journal, 2007

We present new Chandra and HST data for four gamma-ray blazars selected on the basis of radio morphology with the aim of revealing X-ray and optical emission from their jets at large scales. All the sources have been detected. Spectral energy distributions (SEDs) of the large-scale jets are obtained as well as new X-ray spectra for the blazar cores. Modeling for each object the core (subparsec scale) and large-scale (k100 kpc) jet SEDs, we derive the properties of the same jet at the two scales. The comparison of speeds and powers at different scales supports a simple scenario for the dynamics and propagation of high-power relativistic jets.

Astrophysical Journal, 2002

We present Very Long Baseline Array images of 42 -ray bright blazars, including 36 with polarization vectors, obtained during the course of a multiepoch monitoring program. Each object was observed at either 43 or 22 GHz, with some objects observed at both frequencies and/or at 15 or 8.4 GHz. The morphologies are varied, with some of the blazars displaying long, thin jets, others short, broad jets, and still others containing cores with only very weak features that are probably knots in faint jets. The polarization of the cores ranges from less than 1% to 8.6%, with electric vector position angles (EVPAs) that are split between nearly parallel and nearly transverse to the jet axis. The polarization of knots in the jets covers a much broader range, from less than 2% to tens of percent. The EVPA of the brightest compact feature in each jet ranges from 0 to 80 from the jet position angle, with roughly half measuring less than 20 . The distribution is consistent with intrinsically oblique magnetic fields whose observed directions are altered by relativistic aberration.