Search

We selected 40 candidate Lyman Alpha Emitting galaxies (LAEs) at z ~=3.1 with observed frame equivalent widths >150A and inferred emission line fluxes >2.5x10^-17 ergs/cm^2/s from deep narrow-band and broad-band MUSYC images of the Extended Chandra Deep Field South. Covering 992 sq. arcmin, this is the largest ``blank field'' surveyed for LAEs at z ~3, allowing an improved estimate of the space density of this population of 3+-1x10^-4 h_70^3/Mpc^3. Spectroscopic follow-up of 23 candidates yielded 18 redshifts, all at z ~=3.1. Over 80% of the LAEs are dimmer in continuum magnitude than the typical Lyman break galaxy spectroscopic limit of R= 25.5 (AB), with a median continuum magnitude R ~=27 and very blue continuum colors, (V-z) ~=0. Over 80% of the LAEs have the right UVR colors to be selected as Lyman break galaxies, but only 10% also have R240A which might imply a non-standard IMF. One candidate is detected by Chandra, implying an AGN fraction of 2+-2% for LAE candidate samples. In summary, LAEs at z ~ 3 have rapid star formation, low stellar mass, little dust obscuration and no evidence for a substantial AGN component., ApJ Letters, in press, very minor revisions to match accepted version, 4 pages with 2 color figures

It was observed early on in the Chandra X-ray Observatory mission that the background rates of the Advanced CCD Imaging Spectrometer (ACIS) were highly variable on both short and long timescales. We present analysis of lightcurves of the high energy (> 15 keV) ACIS background spanning most of the mission lifetime. These high energy events are not produced by astrophysical X-rays, but by the particle background, primarily energetic protons. Temporal characteristics of both the quiescent background and background flares will be discussed as well as correlations with Chandra's orbital position and the solar activity cycle. The strength and frequency of background flares appear cyclic with a quasi-annual period, most likely from the motion of Chandra's orbit through the geomagnetic environment., 7 pages, 5 figures. To appear in ``The High Energy Universe at Sharp Focus'' (St. Paul, Minnesota 2001), eds Schlegel and Vrtilek (ASP Conference Series)

The flight calibration of the spectral response of the Advanced CCD Imaging Spectrometer (ACIS) below 1.5 keV is difficult because of the lack of strong lines in the calibration source on-board the Chandra X-ray Observatory. We have been using 1E0102.2-7219 (the brightest supernova remnant in the SMC) to evaluate the ACIS response matrices, since this remnant has strong lines of O, Ne and Mg below 1.5 keV. The spectrum of 1E0102.2-7219 has been well-characterized using the gratings on Chandra and XMM. We have used the high-resolution spectral data from both gratings instruments to develop a spectral model for the CCD spectra. Fits with this model are sensitive to any problems with the gain calibration and the spectral redistribution model. We have fit the data with the latest response matrix for the S3 device released in August 2001 (available in CALDB 2.7 or higher). We have also applied the charge-transfer inefficiency (CTI) correction software (version 1.37) developed at Penn State to both the Backside-illuminated (BI) and Front-side illuminated (FI) data and fit the data with the associated CTI-corrected response matrices. All fits show a significant improvement in the low energy response with the new matrices. The fits to the FI CCDs demonstrate the utility of the FI CCDs for spectral analysis, in spite of the radiation damage the devices suffered early in the mission., Comment: To appear in ``The High Energy Universe at Sharp Focus'' (St. Paul, Minnesota 2001), eds Schlegel and Vrtilek (ASP Conference Series)

The MIT and CXC ACIS teams have explored a number of measures to ameliorate the effects of radiation damage suffered by the ACIS FI CCDs. One of these measures is a novel CCD read-out method called ``squeegee mode''. A variety of different implementations of the squeegee mode have now been tested on the I0 CCD. Our results for the fitted FWHM at Al-K$\alpha$ and Mn-K$\alpha$ clearly demonstrate that all the squeegee modes provide improved performance in terms of reducing CTI and improving spectral resolution. Our analysis of the detection efficiency shows that the so-called squeegee modes ``Vanilla'' and ``Maximum Observing Efficiency'' provide the same detection efficiency as the standard clocking, once the decay in the intensity of the radioactive source has been taken into account. The squeegee modes which utilize the slow parallel transfer (``Maximum Spectral Resolution'', ``Maximum Angular Resolution'', and ``Sub-Array'') show a significantly lower detection efficiency than the standard clocking. The slow parallel transfer squeegee modes exhibit severe grade migration from flight grade 0 to flight grade 64 and a smaller migration into ASCA g7. The latter effect can explain some of the drop in detection efficiency. (ABRIDGED ABSTRACT), Comment: 11 pages, 5 figures. To appear in ``The High Energy Universe at Sharp Focus'' (St. Paul, Minnesota 2001), eds Schlegel and Vrtilek (ASP Conference Series). Paper also at http://hea-www.harvard.edu/~svirani/paper.html