10 research outputs found
The fading of Cassiopeia A, and improved models for the absolute spectrum of primary radio calibration sources
Based on five years of observations with the 40-foot telescope at Green Bank
Observatory (GBO), Reichart & Stephens (2000) found that the radio source
Cassiopeia A had either faded more slowly between the mid-1970s and late 1990s
than Baars et al. (1977) had found it to be fading between the late 1940s and
mid-1970s, or that it had rebrightened and then resumed fading sometime between
the mid-1970s and mid-1990s, in L band (1.4 GHz). Here, we present 15
additional years of observations of Cas A and Cyg A with the 40-foot in L band,
and three and a half additional years of observations of Cas A, Cyg A, Tau A,
and Vir A with GBO's recently refurbished 20-meter telescope in L and X (9 GHz)
bands. We also present a more sophisticated analysis of the 40-foot data, and a
reanalysis of the Baars et al. (1977) data, which reveals small, but
non-negligible differences. We find that overall, between the late 1950s and
late 2010s, Cas A faded at an average rate of %/yr in L band,
consistent with Reichart & Stephens (2000). However, we also find, at the
6.3 credible level, that it did not fade at a constant rate. Rather,
Cas A faded at a faster rate through at least the late 1960s, rebrightened (or
at least faded at a much slower rate), and then resumed fading at a similarly
fast rate by, at most, the late 1990s. Given these differences from the
original Baars et al. (1977) analysis, and given the importance of their fitted
spectral and temporal models for flux-density calibration in radio astronomy,
we update and improve on these models for all four of these radio sources. In
doing so, we additionally find that Tau A is fading at a rate of
%/yr in L band.Comment: 17 pages, 12 figures, accepted to MNRA
The Variable Stars of the Draco Dwarf Spheroidal Galaxy - Revisited
We present a CCD survey of variable stars in the Draco dwarf spheroidal
galaxy. This survey, which has the largest areal coverage since the original
variable star survey by Baade & Swope, includes photometry for 270 RR Lyrae
stars, 9 anomalous Cepheids, 2 eclipsing binaries, and 12 slow, irregular red
variables, as well as 30 background QSOs. Twenty-six probable double-mode RR
Lyrae stars were identified. Observed parameters, including mean V and I
magnitudes, V amplitudes, and periods, have been derived. Photometric
metallicities of the ab-type RR Lyrae stars were calculated according to the
method of Jurcsik & Kovacs, yielding a mean metallicity of = -2.19 +/-
0.03. The well known Oosterhoff intermediate nature of the RR Lyrae stars in
Draco is reconfirmed, although the double-mode RR Lyrae stars with one
exception have properties similar to those found in Oosterhoff type II globular
clusters. The period-luminosity relation of the anomalous Cepheids is
rediscussed with the addition of the new Draco anomalous Cepheids.Comment: Accepted to AJ. 61 pages, 14 figures, 10 table
GRB 120711A: an intense INTEGRAL burst with long-lasting soft gamma-ray emission and a powerful optical flash
A long and intense gamma-ray burst (GRB) was detected by INTEGRAL on July 11
2012 with a duration of ~115s and fluence of 2.8x10^-4 erg cm^-2 in the 20
keV-8 MeV energy range. GRB 120711A was at z~1.405 and produced soft gamma-ray
emission (>20 keV) for at least ~10 ks after the trigger. The GRB was observed
by several ground-based telescopes that detected a powerful optical flash
peaking at an R-band brightness of ~11.5 mag at ~126 s after the trigger. We
present a comprehensive temporal and spectral analysis of the long-lasting soft
gamma-ray emission detected in the 20-200 keV band with INTEGRAL, the Fermi/LAT
post-GRB detection above 100 MeV, the soft X-ray afterglow from XMM-Newton,
Chandra, and Swift and the optical/NIR detections from Watcher, Skynet, GROND,
and REM. We modelled the long-lasting soft gamma-ray emission using the
standard afterglow scenario, which indicates a forward shock origin. The
combination of data extending from the NIR to GeV energies suggest that the
emission is produced by a broken power-law spectrum consistent with synchrotron
radiation. The afterglow is well modelled using a stratified wind-like
environment with a density profile k~1.2, suggesting a massive star progenitor
(i.e. Wolf-Rayet). The analysis of the reverse and forward shock emission
reveals an initial Lorentz factor of ~120-340, a jet half-opening angle of
~2deg-5deg, and a baryon load of ~10^-5-10^-6 Msun consistent with the
expectations of the fireball model when the emission is highly relativistic.
Long-lasting soft gamma-ray emission from other INTEGRAL GRBs with high peak
fluxes, such as GRB 041219A, was not detected, suggesting that a combination of
high Lorentz factor, emission above 100 MeV, and possibly a powerful reverse
shock are required. Similar long-lasting soft gamma-ray emission has recently
been observed from the nearby and extremely bright Fermi/LAT burst GRB 130427A.Comment: 21 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
Colour variations in the GRB 120327A afterglow
Aims. We present a comprehensive temporal and spectral analysis of the long Swift GRB 120327A afterglow data to investigate possible causes of the observed early-time colour variations.Methods. We collected data from various instruments and telescopes in X-ray, ultraviolet, optical, and near-infrared bands, and determined the shapes of the afterglow early-time light curves. We studied the overall temporal behaviour and the spectral energy distributions from early to late times.Results. The ultraviolet, optical, and near-infrared light curves can be modelled with a single power-law component between 200 and 2 × 104 s after the burst event. The X-ray light curve shows a canonical steep-shallow-steep behaviour that is typical of long gamma-ray bursts. At early times a colour variation is observed in the ultraviolet/optical bands, while at very late times a hint of a re-brightening is visible. The observed early-time colour change can be explained as a variation in the intrinsic optical spectral index, rather than an evolution of the optical extinction
Colour variations in the GRB 120327A afterglow
Aims. We present a comprehensive temporal and spectral analysis of the long Swift GRB 120327A afterglow data to investigate possible causes of the observed early-time colour variations. Methods. We collected data from various instruments and telescopes in X-ray, ultraviolet, optical, and near-infrared bands, and determined the shapes of the afterglow early-time light curves. We studied the overall temporal behaviour and the spectral energy distributions from early to late times. Results. The ultraviolet, optical, and near-infrared light curves can be modelled with a single power-law component between 200 and 2 × 10 4 s after the burst event. The X-ray light curve shows a canonical steep-shallow-steep behaviour that is typical of long gamma-ray bursts. At early times a colour variation is observed in the ultraviolet/optical bands, while at very late times a hint of a re-brightening is visible. The observed early-time colour change can be explained as a variation in the intrinsic optical spectral index, rather than an evolution of the optical extinction. © 2017 ESO