Two years of microwave background observations with the Cosmic Background Imager (CBI) have been combined to give a sensitive, high resolution angular power spectrum over the range 400 < ℓ < 3500. This power spectrum has been referenced to a more accurate overall calibration derived from the Wilkinson Microwave Anisotropy Probe. The data cover 90 deg 2 including three pointings targeted for deep observations. The uncertainty on the ℓ > 2000 power previously seen with the CBI is reduced. Under the assumption that any signal in excess of the primary anisotropy is due to a secondary Sunyaev-Zeldovich anisotropy in distant galaxy clusters we use CBI, Arcminute Cosmology Bolometer Array Receiver, and Berkeley-Illinois-Maryland Association array data to place a constraint on the present-day rms mass fluctuation on 8 h −1 Mpc scales, σ 8 . We present the results of a cosmological parameter analysis on the ℓ < 2000 primary anisotropy data which show significant improvements in the parameters as compared to WMAP alone, and we explore the role of the small-scale cosmic microwave background data in breaking parameter degeneracies.
Using the Cosmic Background Imager (CBI), a 13-element interferometer array operating in the 26-36 GHz frequency band, we have observed 40 deg 2 of sky in three pairs of fields, each $145 0 Â 165 0 , using overlapping pointings (mosaicking). We present images and power spectra of the cosmic microwave background radiation in these mosaic fields. We remove ground radiation and other low-level contaminating signals by differencing matched observations of the fields in each pair. The primary foreground contamination is due to point sources (radio galaxies and quasars). We have subtracted the strongest sources from the data using higher resolution measurements, and we have projected out the response to other sources of known position in the power spectrum analysis. The images show features on scales $6 0 -15 0 , corresponding to masses $ð5 80Þ Â 10 14 M at the surface of last scattering, which are likely to be the seeds of clusters of galaxies. The power spectrum estimates have a resolution Dl % 200 and are consistent with earlier results in the multipole range ld1000. The power spectrum is detected with high signal-to-noise ratio in the range 300dld1700. For 1700dld3000 the observations are consistent with the results from more sensitive CBI deep field observations. The results agree with the extrapolation of cosmological models fitted to observations at lower l and show the predicted drop at high l (the '' damping tail '').
We report the discovery of a radio counterpart to GRB 990123. In contrast to previous well-studied radio afterglows which rise to peak flux on a timescale of a week and then decay over several weeks to months, the radio emission from this GRB was clearly detected one day after the burst, after which it rapidly faded away. The simplest interpretation of this ``radio flare'' is that it arises from the reverse shock. In the framework of the afterglow models discussed to date, a forward shock origin for the flare is ruled out by our data. However, at late times, some radio afterglow emission (commensurate with the observed late-time optical emission, the optical afterglow) is expected from the forward shock. The relative faintness of the observed late-time radio emission provides an independent indication for a jet-like geometry in this GRB. We use the same radio observations to constrain two key parameters of the forward shock, peak flux and peak frequency, to within a factor of two. These values are inconsistent with the notion advocated by several authors that the prompt optical emission detected by ROTSE smoothly joins the optical afterglow emission. Finally, with hindsight we now recognize another such radio flare and this suggests that one out of eight GRBs has a detectable radio flare. This abundance coupled with the reverse shock interpretation suggests that the radio flare phenomenon has the potential to shed new light into the physics of reverse shocks in GRBs.Comment: pages including 2 figures. Accepted by the Astrophys. J. (Letters
We report measurements of anisotropy in the cosmic microwave background radiation over the multipole range ℓ ∼ 200 → 3500 with the Cosmic Background Imager based on deep observations of three fields. These results confirm the drop in power with increasing ℓ first reported in earlier measurements with this instrument, and extend the observations of this decline in power out to ℓ ∼ 2000. The decline in power is consistent with the predicted damping of primary anisotropies. At larger multipoles, ℓ = 2000-3500, the power is 3.1σ greater than standard models for intrinsic microwave background anisotropy in this multipole range, and 3.5σ greater than zero. This excess power is not consistent with expected levels of residual radio source contamination but, for σ 8 1, is consistent with predicted levels due to a secondary Sunyaev-Zeldovich anisotropy. Further observations are necessary to confirm the level of this excess and, if confirmed, determine its origin.
We present the first results of observations of the intrinsic anisotropy of the cosmic microwave background radiation with the Cosmic Background Imager from a site at 5080 m altitude in northern Chile. Our observations show a sharp decrease in C l in the range l = 400-1500. The broadband amplitudes we have measured are AET band = 58 7 +7 7 −6 3 K for l = 603 +180 −166 and AET band = 29 7 +4 8 −4 2 K for l = 1190 +261 −224 , where these are half-power widths in l. Such a decrease in power at high l is one of the fundamental predictions of the standard cosmological model, and these are the first observations which cover a broad enough l range to show this decrease in a single experiment. The C l we have measured enables us to place limits on the density parameter, ª tot 0 4 or ª tot 0 7 (90% confidence).
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