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Register a new user15 GHz VLBI detection of the HST-1 feature in the M87 jet
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A bright feature 100 pc away from the core in the powerful jet of M 87 shows mysterious properties. Earlier radio, optical and X-ray observations have shown that this feature, labelled HST-1, is superluminal, and is possibly connected with the TeV flare detected by HESS in 2005. To examine the possible blazar-like nature of HST-1, we analyzed 2 cm VLBA data from dedicated full-track observations and the 2 cm survey/MOJAVE VLBI monitoring programs observed from 2000 to 2008. Applying wide-field imaging techniques, the HST-1 region was imaged at milliarcsecond resolutions. Here we present the first 15 GHz VLBI detection of this feature and discuss the connection between our radio findings and the TeV detection.
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A bright feature 80 pc away from the core in the powerful jet of M87 shows highly unusual properties. Earlier radio, optical and X-ray observations have shown that this feature, labeled HST-1, is superluminal, and is possibly connected with the TeV flare detected by HESS in 2005. It has been claimed that this feature might have a blazar nature, due to these properties. To examine the possible blazar-like nature of HST-1, we analyzed lambda 2 cm VLBA archival data from dedicated full-track observations and the 2 cm survey/MOJAVE VLBI monitoring programs obtained between 2000 and 2009. Applying VLBI wide-field imaging techniques, the HST-1 region was imaged at milliarcsecond resolution. Here we present the first 2 cm VLBI detection of this feature in observations from early 2003 to early 2007, and analyze its evolution over this time. Using the detections of HST-1, we find that the projected apparent speed is 0.61 +/- 0.31 c. A comparison of the VLA and VLBA flux densities of this feature indicate that is mostly resolved on molliarcsecond scales. This feature is optically thin between lambda 2 cm and lambda 20 cm. We do not find evidence of a blazar nature for HST-1.
We investigate the total flux density, spectral, polarization, and Faraday rotation variability of HST-1 in the M87 jet during the outburst from 2003 to 2007 through multi-epoch VLA observations at 8, 15, and 22 GHz. Contrary to the general case for blazars, the flux densities of HST-1 rise earlier at lower frequencies from radio to X-ray, and the spectra are softening with the growth of outburst, indicating that the newly emerging subcomponents within HST-1 have relatively steep spectra. In particular, the intrinsic EVPA varies monotonically by $sim90^circ$ at the 3 wavebands during the period, and all but the stationary subcomponent in the eastern end of HST-1 move downstream superluminally deviating divergently from the overall jet direction, with the motion of the outmost subcomponent bending from one side of the jet axis to another. These strongly argue for the presence of helical magnetic fields around HST-1, which is also supported by the fact that the subcomponents might be accelerated in this region. The fractional polarization is relatively low in the rising stage, and in the decaying stage the polarization levels are almost comparable at the 3 wavebands. In view of the quite large RM values, Faraday rotation is expected to occur dominantly external to HST-1 in the decaying stage, which is well supported by the presence of diffuse emission around HST-1, and consistent with the scenario that RM decrease gets slower with time.
The relativistic jet in M87 offers a unique opportunity for understanding the detailed jet structure and emission processes due to its proximity. In particular, the peculiar jet region HST-1 at ~1 arcsecond (or 80 pc, projected) from the nucleus has attracted a great deal of interest in the last decade because of its superluminal motion and broadband radio-to-X-ray outbursts, which may be further connected to the gamma-ray productions up to TeV energies. Over the last five years, we have been doing an intensive monitoring of HST-1 with EVN at 5GHz in order to examine the detailed structural evolution and its possible connection to high-energy activities. While this program already yielded interesting results in terms of the detailed mas-scale structure, proper motion measurements and structural variations, the recent HST-1 brightness is continuously decreasing at this frequency. To counter this, we have shifted our monitoring frequency to 1.7GHz from October 2013. This strategy successfully recovered the fainter emission that was missed in the last 5GHz session. Moreover, we again discovered the sudden emergence of a new component at the upstream edge of HST-1, demonstrating that the use of EVN 1.7GHz is indeed powerful to probe the current weak nature of HST-1. Here we report early results from the 1.7GHz monitoring as well as further progress on the long-term kinematic study.
To obtain a better understanding of the location and mechanisms for the production of the gamma-ray emission in jets of AGN we present a detailed study of the HST-1 structure, 0.8 arcsec downstream the jet of M87, previously identified as a possible candidate for TeV emission. HST-1 shows a very peculiar structure with superluminal as well as possible stationary sub-components, and appears to be located in the transition from a parabolic to a conical jet shape, presumably leading to the formation of a recollimation shock. This scenario is supported by our new RHD simulations in which the interaction of a moving component with a recollimation shock leads to the appearance of a new superluminal component. To discern whether HST-1 is produced by a recollimation shock or some other MHD instability, we present new polarimetric 2.2 and 5 GHz VLBA, as well as 15, 22 and 43 GHz JVLA observations obtained between November 2012 and March 2013.
We present new ultraviolet photometry of the jet in M87 obtained from HST WFPC2 imaging. We combine these ultraviolet data with previously published photometry for the knots of the jet in radio, optical, and X-ray, and fit three theoretical synchrotron models to the full data set. The synchrotron models consistently overpredict the flux in the ultraviolet when fit over the entire dataset. We show that if the fit is restricted to the radio through ultraviolet data, the synchrotron models can provide a good match to the data. The break frequencies of these fits are much lower than previous estimates. The implied synchrotron lifetimes for the bulk of the emitting population are longer than earlier work, but still much shorter than the estimated kinematic lifetimes of the knots. The observed X-ray flux cannot be successfully explained by the simple synchrotron models that fit the ultraviolet and optical fluxes. We discuss the possible implications of these results for the physical properties of the M87 jet. We also observe increased flux for the HST-1 knot that is consistent with previous results for flaring. This observation fills in a significant gap in the time coverage early in the history of the flare, and therefore sets constraints on the initial brightening of the flare.
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