In an effort to locate the sites of emission at different frequencies and physical processes causing variability in blazar jets, we have obtained high time-resolution observations of BL Lacertae over ...a wide wavelength range: with the Transiting Exoplanet Survey Satellite (TESS) at 6000-10000 with 2 minute cadence; with the Neil Gehrels Swift satellite at optical, UV, and X-ray bands; with the Nuclear Spectroscopic Telescope Array at hard X-ray bands; with the Fermi Large Area Telescope at γ-ray energies; and with the Whole Earth Blazar Telescope for measurement of the optical flux density and polarization. All light curves are correlated, with similar structure on timescales from hours to days. The shortest timescale of variability at optical frequencies observed with TESS is ∼0.5 hr. The most common timescale is 13 1 hr, comparable with the minimum timescale of X-ray variability, 14.5 hr. The multiwavelength variability properties cannot be explained by a change solely in the Doppler factor of the emitting plasma. The polarization behavior implies that there are both ordered and turbulent components to the magnetic field in the jet. Correlation analysis indicates that the X-ray variations lag behind the γ-ray and optical light curves by up to ∼0.4 day. The timescales of variability, cross-frequency lags, and polarization properties can be explained by turbulent plasma that is energized by a shock in the jet and subsequently loses energy to synchrotron and inverse Compton radiation in a magnetic field of strength ∼3 G.
The Galactic Centre is the most active and heavily processed region of
the Milky Way, so it can be used as a stringent test for the abundance of
deuterium (a sensitive indicator of conditions in the ...first 1,000 seconds
in the life of the Universe). As deuterium is destroyed in stellar interiors,
chemical evolution models predict that its Galactic Centre
abundance relative to hydrogen is D/H = 5 × 10-12,
unless there is a continuous source of deuterium from relatively primordial
(low-metallicity) gas. Here we report the detection of deuterium (in the molecule
DCN) in a molecular cloud only 10 parsecs from the Galactic Centre. Our data,
when combined with a model of molecular abundances, indicate that D/H =
(1.7 ± 0.3) × 10-6, five orders of magnitude larger
than the predictions of evolutionary models with no continuous source of deuterium.
The most probable explanation is recent infall of relatively unprocessed metal-poor
gas into the Galactic Centre (at the rate inferred by Wakker).
Our measured D/H is nine times less than the local interstellar value, and
the lowest D/H observed in the Galaxy. We conclude that the observed Galactic
Centre deuterium is cosmological, with an abundance reduced by stellar processing
and mixing, and that there is no significant Galactic source of deuterium.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
BL Lacertae has been the target of four observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we present UBVRI light curves obtained by the WEBT from 1994 to ...2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the similar to 15 600 observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a few-day time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a similar to 0.1 bluer-when-brighter slope in the B - R versus R plane; a steeper slope of similar to 0.4 would distinguish the shorter-term variations. This means that, when considering the long-term trend, the B-band flux level is related to the R-band one according to a power law of index similar to 1.1. Doppler factor variations on a "convex" spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism.
We report the discovery of a similar to 500 kpc H I extension southwest of the Virgo Cluster H I-rich pair NGC 4532/DDO 137, detected as part of the Arecibo Legacy Fast ALFA (ALFALFA) Survey. The ...feature is the longest and most massive H I tail structure so far found in the Virgo Cluster and, at 1.8 Mpc from M87, the most distant from the main concentration of the intracluster medium. The structure is spatially and spectrally separated into two ridges and is defined by diffuse emission and discrete clumps of mass (2.5-6.8) x 10 super(7) M. All emission is blueshifted with respect to the NGC 4532/DDO 137 pair emission. Including diffuse emission, the structure has a total mass of up to 7 x 10 super(8) M, equivalent to similar to 10% of the system's H I mass. Optical R-band imaging finds no counterparts to a level of 26.5 mag arcsec super(-2). The characteristics of the structure appear most consistent with a tidal origin.
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