John A. Biretta

Formation of the radio jet in M87 at 100 Schwarzschild radii from the central black hole

Massive galaxies often are the source of well collimated jets of material that flow outwards for tens to hundreds of kiloparsecs from the regions surrounding the presumed black holes at their centres. The processes by which the jets are formed and collimated have been important problems for many years, and observations have hitherto had insufficient spatial resolution to investigate the length scales associated with these processes. Here we report observations at 43 GHz of the inner regions of the nearby active galaxy M87. The data show a remarkably broad jet having an ‘opening angle’ of ∼60° near the centre, with strong collimation of the jet occurring at ∼30–100 Schwarzschild radii (rS) from the black hole: collimation continues out to ∼1,000 rS. These results are consistent with the hypothesis that jets are formed by an accretion disk around the central black hole, which is threaded by a magnetic field.

HST Snapshot Survey of 3CR Radio Source Counterparts. I. Intermediate Redshifts

We have obtained images of 267 3CR radio galaxies and quasars by conducting a snapshot survey with HSTs WFPC2 through a broadband red (F702W) filter. This is the first in a series of papers resulting from this survey, describing and presenting the basic data. Here we focus on the 77 radio galaxies within the redshift range 0.1 < z < 0.5 (91% of the total number of 3CR radio galaxies within this redshift range). The images show that on the 01 scale of the HST observations there is a wide variety of structure in radio galaxy morphology. At least 30% of the galaxies show dust, either well-defined dust lanes or chaotically distributed throughout the galaxy. We find disturbed morphologies, such as multiple nuclei and tails of emission, that could indicate merging systems. There is an obvious optical synchrotron jet in 3C 346, and another eight jet candidates are present.

Optical and Radio Polarimetry of the M87 Jet at 02 Resolution

We discuss optical (HST/WFPC2 F555W) and radio (15 GHz VLA) polarimetry observations of the M87 jet taken during 1994-1995. The angular resolution of both of these observations is ∼ 0.2 ′′ , which at the distance of M87 corresponds to 15 pc. Many knot regions are very highly polarized (∼ 40 −50%, approaching the theoretical maximum for optically thin synchrotron radiation), suggesting highly ordered magnetic fields. High degrees of polarization are also observed in interknot regions. The optical and radio polarization maps share many similarities, and in both, the magnetic field is largely parallel to the jet, except in the “shock-like” knot regions (parts of HST-1, A, and C), where it becomes perpendicular to the jet. We do observe significant differences between the radio and optical polarized structures, particularly for bright knots in the inner jet, giving us important insight into the radial structure of the jet. Unlike in the radio, the optical magnetic field position angle becomes perpendicular to the jet at the upstream ends of knots HST-1, D, E and F. Moreover, the optical polarization appears to decrease markedly at the position of the flux maxima in these knots. In contrast, the magnetic field position angle observed in the radio remains parallel to the jet in most of these regions, and the decreases in radio polarization are smaller. More minor differences are seen in other jet regions. Many of the differences between optical and radio polarimetry results can be explained in terms of a model whereby shocks occur in the jet interior, where higher-energy electrons are concentrated and dominate both polarized and unpolarized emissions in the optical, while the radio maps show strong contributions from lower-energy electrons in regions with B parallel, near the jet surface.

The Jet of M87 at Tenth-Arcsecond Resolution: Optical, Ultraviolet, and Radio Observations

The European Space Agencys Faint Object Camera (FOC) on the Hubble Space Telescope (HST) was used to obtain optical and UV imaging of the jet of M87 prior to the HST refurbishment. We present a detailed comparison to new VLA radio observations at similar spatial resolution and flux information for the radio, optical, and UV bands.While the radio and optical images present a remarkable degree of similarity, there are nevertheless significant differences. The optical/UV images show intrinsically higher contrast than the radio, with compact regions of emission localized within the knots. The jet is narrower in the optical/UV; the emission is more concentrated to the jets center in the optical/UV than in the radio band. The radio-to-optical spectral index of the interknot regions is steeper than that of the knots themselves. There are also differences in the detailed knot structure of the optical emission compared to the radio, and there is a weak overall spectral steepening with distance from the nucleus beyond knot A.The jet does not show pronounced limb brightening in the optical/UV. This indicates that the emission occurs within the jet and not in a thin boundary around the jet, as in some jet models.We explore an idealized synchrotron model of jet emission and derive volume-deprojected physical parameters for the model. While the knots themselves are overpressured with respect to the surroundings, the pressure of the interknot regions in this observationally based model drops to the ambient external pressure, as in theories that invoke jet recollimation at shocks. Alternatively, internal shocks may be triggered by boundary instabilities or time-dependent power output from the nucleus. Downstream from knot A, the situation is less ordered, although in situ acceleration near the jets center line also seems to be required, as the optical jet remains narrower than the radio jet there.

Hubble Space Telescope Observations of Globular Clusters in M87 and an Estimate of H0

Hubble Space Telescope observations of over 1000 globular clusters in the central region of M87 have been made using the Wide Field and Planetary Camera 2. The limiting magnitude in V is 26 mag, more than 2 mag beyond the turnover of the luminosity function. The distribution is well fitted by a Gaussian profile with a mean of m -->V0 = 23.72 ± 0.06 mag and a width of 1.40 ± 0.06 mag. Assuming a value of M -->V0 = -7.4 ± 0.25 mag for the globular cluster luminosity function results in a distance modulus of (m - M) = 31.12 mag to the Virgo Cluster. Using a value of ΔComa-Virgo(m - M) = 3.71 ± 0.10 mag and adopting a velocity for the Coma Cluster of 7188 km s-1 result in a value for the Hubble constant of H0 = 78 ± 11 km s-1 Mpc-1. The V - I color distribution is bimodal, with peaks at V - I = 0.95 mag and 1.20 mag. The mean size of the clusters is Reff = 3 pc, with a scatter of 1 pc.

VARIABILITY TIMESCALES IN THE M87 JET: SIGNATURES OF E 2 LOSSES, DISCOVERY OF A QUASI PERIOD IN HST-1, AND THE SITE OF TeV FLARING

We investigate the variability timescales in the jet of M87 with two goals. The first is to use the rise times and decay times in the radio, ultraviolet, and X-ray light curves of HST-1 to constrain the source size and the energy loss mechanisms affecting the relativistic electron distributions. HST-1 is the first jet knot clearly resolved from the nuclear emission by Chandra and is the site of the huge flare of 2005. We find clear evidence for a frequency-dependent decrease in the synchrotron flux being consistent with E 2 energy losses. Assuming that this behavior is predominantly caused by synchrotron cooling, we estimate a value of 0.6 mG for the average magnetic field strength of the HST-1 emission region, a value consistent with previous estimates of the equipartition field. In the process of analyzing the first derivative of the X-ray light curve of HST-1, we discovered a quasi-periodic oscillation which was most obvious in 2003 and 2004 prior to the major flare in 2005. The four cycles observed have a period of order six months. The second goal is to search for evidence of differences between the X-ray variability timescales of HST-1 and the unresolved nuclear region (diameter <06). These features, separated by more than 60 pc, are the two chief contenders for the origin of the TeV variable emissions observed by H.E.S.S. in 2005 and by MAGIC and VERITAS in 2008. The X-ray variability of the nucleus appears to be at least twice as rapid as that of the HST-1 knot. However, the shortest nuclear variability timescale we can measure from the Chandra data (≤20 days) is still significantly longer than the shortest TeV variability of M87 reported by the H.E.S.S. and MAGIC telescopes (1-2 days).

Hubble Space Telescope imaging of compact steep spectrum radio sources

We present Hubble Space Telescope WFPC2 images taken through a broad red filter (F702W) of 30 Third Cambridge Catalog compact steep spectrum (CSS) radio sources. We have overlaid radio maps taken from the literature on the optical images to determine the radio-optical alignment and to study detailed correspondence. All CSS sources for which the relative orientation between the optical and radio can be measured display good alignment between the optical and radio emission down to the lowest redshift in the sample, z similar to 0.1. The alignment effect does not occur at this relatively low redshift for the large-scale 3CR radio sources, which tend to show a significant alignment only at z > 0.6, as shown by McCarthy et al., Chambers et al., and de Koff et al. We find candidates for optical synchrotron hot spots in 3C 213.1 and 3C 380 and an optical jet in 3C 346.

Flaring X-Ray Emission from HST-1, a Knot in the M87 Jet

We present Chandra X-ray monitoring of the M87 jet in 2002, which shows that the intensity of HST-1, an optical knot 08 from the core, increased by a factor of 2 in 116 days and a factor of 4 in 2 yr. There was also a significant flux decrease over 2 months, with suggestive evidence for a softening of the spectrum. From this variability behavior, we argue that the bulk of the X-ray emission of HST-1 comes from synchrotron emission. None of the other conceivable emission processes can match the range of observed characteristics. By estimating synchrotron model parameters for various bulk relativistic velocities, we demonstrate that a model with a Doppler factor δ in the range 2-5 fits our preliminary estimates of light-travel time and synchrotron loss timescales.

Deep 10 micron imaging of M87

We analyze a 10.8 ?m image of M87, obtained with the Gemini 8 m telescope + OSCIR. The image has better than 05 resolution and represents 7 hr of observing time, making it the deepest high-resolution mid-IR image ever. The core is marginally resolved, and we also detect five optically bright knots in the jet. The spectral energy distributions of these features are entirely consistent with synchrotron radiation; we find little evidence of thermal emission from a dusty torus. Four faint jet regions are below the noise level of these observations. We also find evidence for diffuse galactic emission.

Month - timescale optical variability in the M87 jet

A previously inconspicuous knot in the M87 jet has undergone a dramatic outburst and now exceeds the nucleus in optical and X-ray luminosity. Monitoring of M87 with the Hubble Space Telescope and Chandra X-Ray Observatory during 2002-2003, has found month-timescale optical variability in both the nucleus and HST-1, a knot in the jet 082 from the nucleus. We discuss the behavior of the variability timescales as well as spectral energy distribution of both components. In the nucleus, we see nearly energy-independent variability behavior. Knot HST-1, however, displays weak energy dependence in both X-ray and optical bands but with nearly comparable rise/decay timescales at 220 nm and 0.5 keV. The flaring region of HST-1 appears stationary over 8 months of monitoring. We consider various emission models to explain the variability of both components. The flares we see are similar to those seen in blazars, albeit on longer timescales, and so could, if viewed at smaller angles, explain the extreme variability properties of those objects.