73 research outputs found
Another look at the BL Lacertae flux and spectral variability
The GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope
(WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical
frequencies. During this period, high-energy observations were performed by
XMM-Newton, Swift, and Fermi. We analyse these data with particular attention
to the calibration of Swift UV data, and apply a helical jet model to interpret
the source broad-band variability. The GASP-WEBT observations show an optical
flare in 2008 February-March, and oscillations of several tenths of mag on a
few-day time scale afterwards. The radio flux is only mildly variable. The UV
data from both XMM-Newton and Swift seem to confirm a UV excess that is likely
caused by thermal emission from the accretion disc. The X-ray data from
XMM-Newton indicate a strongly concave spectrum, as well as moderate flux
variability on an hour time scale. The Swift X-ray data reveal fast (interday)
flux changes, not correlated with those observed at lower energies. We compare
the spectral energy distribution (SED) corresponding to the 2008 low-brightness
state, which was characterised by a synchrotron dominance, to the 1997 outburst
state, where the inverse-Compton emission was prevailing. A fit with an
inhomogeneous helical jet model suggests that two synchrotron components are at
work with their self inverse-Compton emission. Most likely, they represent the
radiation from two distinct emitting regions in the jet. We show that the
difference between the source SEDs in 2008 and 1997 can be explained in terms
of pure geometrical variations. The outburst state occurred when the
jet-emitting regions were better aligned with the line of sight, producing an
increase of the Doppler beaming factor. Our analysis demonstrates that the jet
geometry can play an extremely important role in the BL Lacertae flux and
spectral variability.Comment: 12 pages, 10 figures, accepted for publication in A&
The first Fermi multifrequency campaign on BL Lacertae: characterizing the low-activity state of the eponymous blazar
We report on observations of BL Lacertae during the first 18 months of
Fermi-LAT science operations and present results from a 48-day multifrequency
coordinated campaign from 2008 August 19 to 2008 October 7. The radio to
gamma-ray behavior of BL Lac is unveiled during a low activity state thanks to
the coordinated observations of radio-band (Metsahovi and VLBA),
near-IR/optical (Tuorla, Steward, OAGH and MDM) and X-ray (RXTE and Swift)
observatories. No variability was resolved in gamma-rays during the campaign,
and the brightness level was 15 times lower than the level of the 1997 EGRET
outburst. Moderate and uncorrelated variability has been detected in UV and
X-rays. The X-ray spectrum is found to be concave indicating the transition
region between the low and high energy component of the spectral energy
distribution (SED). VLBA observation detected a synchrotron spectrum
self-absorption turnover in the innermost part of the radio jet appearing to be
elongated and inhomogeneous, and constrained the average magnetic field there
to be less than 3 G. Over the following months BL Lac appeared variable in
gamma-rays, showing flares (in 2009 April and 2010 January). There is no
evidence for correlation of the gamma-rays with the optical flux monitored from
the ground in 18 months. The SED may be described by a single zone or two zone
synchrotron self-Compton (SSC) model, but a hybrid SSC plus external radiation
Compton (ERC) model seems preferred based on the observed variability and the
fact that it provides a fit closest to equipartition.Comment: 14 pages, apj-style emulated, 10 figures. In press by The
Astrophysical Journa
Multi-wavelength observations of blazar AO 0235+164 in the 2008-2009 flaring state
The blazar AO 0235+164 (z = 0.94) has been one of the most active objects observed by Fermi Large Area Telescope (LAT) since its launch in Summer 2008. In addition to the continuous coverage by Fermi, contemporaneous observations were carried out from the radio to γ-ray bands between 2008 September and 2009 February. In this paper, we summarize the rich multi-wavelength data collected during the campaign (including F-GAMMA, GASP-WEBT, Kanata, OVRO, RXTE, SMARTS, Swift, and other instruments), examine the cross-correlation between the light curves measured in the different energy bands, and interpret the resulting spectral energy distributions in the context of well-known blazar emission models. We find that the γ-ray activity is well correlated with a series of near-IR/optical flares, accompanied by an increase in the optical polarization degree. On the other hand, the X-ray light curve shows a distinct 20 day high state of unusually soft spectrum, which does not match the extrapolation of the optical/UV synchrotron spectrum. We tentatively interpret this feature as the bulk Compton emission by cold electrons contained in the jet, which requires an accretion disk corona with an effective covering factor of 19% at a distance of 100 R g. We model the broadband spectra with a leptonic model with external radiation dominated by the infrared emission from the dusty torus. © 2012. The American Astronomical Society. All rights reserved
A tidal disruption event coincident with a high-energy neutrino
Cosmic neutrinos provide a unique window into the otherwise hidden mechanism of particle acceleration in astrophysical objects. The IceCube Collaboration recently reported the likely association of one high-energy neutrino with a flare from the relativistic jet of an active galaxy pointed towards the Earth. However a combined analysis of many similar active galaxies revealed no excess from the broader population, leaving the vast majority of the cosmic neutrino flux unexplained. Here we present the likely association of a radio-emitting tidal disruption event, AT2019dsg, with a second high-energy neutrino. AT2019dsg was identified as part of our systematic search for optical counterparts to high-energy neutrinos with the Zwicky Transient Facility. The probability of finding any coincident radio-emitting tidal disruption event by chance is 0.5%, while the probability of finding one as bright in bolometric energy flux as AT2019dsg is 0.2%. Our electromagnetic observations can be explained through a multizone model, with radio analysis revealing a central engine, embedded in a UV photosphere, that powers an extended synchrotron-emitting outflow. This provides an ideal site for petaelectronvolt neutrino production. Assuming that the association is genuine, our observations suggest that tidal disruption events with mildly relativistic outflows contribute to the cosmic neutrino flux
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Stakeholder Study: Resource Management of Wairarapa Moana
This project aided the Greater Wellington Regional Council (GWRC) in collecting opinions from five stakeholder groups concerning management of the naturally flooding Lake Wairarapa in New Zealand. Our team conducted a total of twenty-nine interviews with the RangitÄne (MÄori), Department of Conservation, South Wairarapa District Council, landowners, and recreational water users regarding the Blundell Barrage Gates which play a vital role in flood management. Interview discussions indicate that water quality, sources of pollution, water levels, flood management, and future operation of the barrage gates are controversial. We identified communication, collaboration, and education as three areas of focus for the GWRC as they continue to improve their management of the region
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