22 research outputs found
The Chandra X-Ray Observatory's Radiation Environment and the AP-8/AE-8 Model
The Chandra X-ray Observatory (CXO) was launched on July 23, 1999 and reached
its final orbit on August 7, 1999. The CXO is in a highly elliptical orbit,
approximately 140,000 km x 10,000 km, and has a period of approximately 63.5
hours (~ 2.65 days). It transits the Earth's Van Allen belts once per orbit
during which no science observations can be performed due to the high radiation
environment. The Chandra X-ray Observatory Center (CXC) currently uses the
National Space Science Data Center's ``near Earth'' AP-8/AE-8 radiation belt
model to predict the start and end times of passage through the radiation
belts. However, our scheduling software uses only a simple dipole model of the
Earth's magnetic field. The resulting B, L magnetic coordinates, do not always
give sufficiently accurate predictions of the start and end times of transit of
the Van Allen belts. We show this by comparing to the data from Chandra's
on-board radiation monitor, the EPHIN (Electron, Proton, Helium Instrument
particle detector) instrument. We present evidence that demonstrates this
mis-timing of the outer electron radiation belt as well as data that also
demonstrate the significant variablity of one radiation belt transit to the
next as experienced by the CXO. We also present an explanation for why the
dipole implementation of the AP-8/AE-8 model is not ideally suited for the CXO.
Lastly, we provide a brief discussion of our on-going efforts to identify a
model that accounts for radiation belt variability, geometry, and one that can
be used for observation scheduling purposes.Comment: 12 pgs, 6 figs, for SPIE 4012 (Paper 76
Recommended from our members
Optical Spectroscopy Of X-Ray Sources In The Extended Chandra Deep Field South
We present the first results of our optical spectroscopy program aimed to provide redshifts and identifications for the X-ray sources in the Extended Chandra Deep Field South. A total of 339 sources were targeted using the IMACS spectrograph at the Magellan telescopes and the VIMOS spectrograph at the VLT. We measured redshifts for 186 X-ray sources, including archival data and a literature search. We find that the active galactic nucleus (AGN) host galaxies have on average redder rest-frame optical colors than nonactive galaxies, and that they live mostly in the "green valley." The dependence of the fraction of AGNs that are obscured on both luminosity and redshift is confirmed at high significance and the observed AGN spatial density is compared with the expectations from existing luminosity functions. These AGNs show a significant difference in the mid-IR to X-ray flux ratio for obscured and unobscured AGNs, which can be explained by the effects of dust self-absorption on the former. This difference is larger for lower luminosity sources, which is consistent with the dust opening angle depending on AGN luminosity.National Aeronautics and Space Administration PF8-90055, NAS8-03060NSF AST0407295Spitzer JPL RSA1288440Natural Science and Engineering Research Council of Canada (NSERC)National Academy of SciencesNASA/INTEGRAL NNG05GM79GAstronom
Mid-infrared Properties and Color Selection for X-ray Detected AGN in the MUSYC ECDF-S field
We present the mid-infrared colors of X-ray-detected AGN and explore
mid-infrared selection criteria. Using a statistical matching technique, the
likelihood ratio, over 900 IRAC counterparts were identified with a new MUSYC
X-ray source catalog that includes ~1000 published X-ray sources in the Chandra
Deep Field-South and Extended Chandra Deep Field-South. Most X-ray-selected AGN
have IRAC spectral shapes consistent with power-law slopes, f_{nu} ~
nu^{alpha}, and display a wide range of colors, -2 < alpha < 2. Although X-ray
sources typically fit to redder (more negative alpha) power-laws than non-X-ray
detected galaxies, more than 50% do have flat or blue (galaxy-like) spectral
shapes in the observed 3-8 micron band. Only a quarter of the X-ray selected
AGN detected at 24 micron are well fit by featureless red power laws in the
observed 3.6-24 micron, likely the subset of our sample whose infrared spectra
are dominated by emission from the central AGN region. Most IRAC
color-selection criteria fail to identify the majority of X-ray-selected AGN,
finding only the more luminous AGN, the majority of which have broad emission
lines. In deep surveys, these color-selection criteria select 10-20% of the
entire galaxy population and miss many moderate luminosity AGN.Comment: 29 pages, including 7 figures. Accepted for publication in the
Astrophysical Journa
Black hole growth and host galaxy morphology
We use data from large surveys of the local Universe (SDSS+Galaxy Zoo) to
show that the galaxy-black hole connection is linked to host morphology at a
fundamental level. The fraction of early-type galaxies with actively growing
black holes, and therefore the AGN duty cycle, declines significantly with
increasing black hole mass. Late-type galaxies exhibit the opposite trend: the
fraction of actively growing black holes increases with black hole mass.Comment: 4 pages, 2 figures. Proceedings of the IAU Symposium no. 267,
"Co-Evolution of Central Black Holes and Galaxies: Feeding and Feedback",
eds. B.M. Peterson, R.S. Somerville and T. Storchi-Bergman
The Extended Chandra Deep Field-South Survey: X-ray Point-Source Catalog
The Extended Chandra Deep Field-South (ECDFS) survey consists of 4 Chandra
ACIS-I pointings and covers 1100 square arcminutes ( 0.3
deg) centered on the original CDF-S field to a depth of approximately 228
ks. This is the largest Chandra survey ever conducted at such depth, and only
one XMM-Newton survey reaches a lower flux limit in the hard 2.0--8.0 keV band.
We detect 651 unique sources -- 587 using a conservative source detection
threshold and 64 using a lower source detection threshold. These are presented
as two separate catalogs. Of the 651 total sources, 561 are detected in the
full 0.5--8.0 keV band, 529 in the soft 0.5--2.0 keV band, and 335 in the hard
2.0--8.0 keV band. For point sources near the aim point, the limiting fluxes
are approximately and in the 0.5--2.0 keV and 2.0--8.0 keV
bands, respectively. Using simulations, we determine the catalog completeness
as a function of flux and assess uncertainties in the derived fluxes due to
incomplete spectral information. We present the differential and cumulative
flux distributions, which are in good agreement with the number counts from
previous deep X-ray surveys and with the predictions from an AGN population
synthesis model that can explain the X-ray background. In general, fainter
sources have harder X-ray spectra, consistent with the hypothesis that these
sources are mainly obscured AGN.Comment: Replaced original paper with the AJ-accepted version. New version
includes simulations on the uncertainties in the derived fluxes due to
incomplete spectral information and catalog completeness. Full catalog
available at http://www.astro.yale.edu/svirani/ecdfs
Ly Alpha-Emitting Galaxies at z=3.1: L* Progenitors Experiencing Rapid Star Formation
We studied the clustering properties and multiwavelength spectral energy
distributions of a complete sample of 162 Ly Alpha-Emitting (LAE) galaxies at
z=3.1 discovered in deep narrow-band MUSYC imaging of the Extended Chandra Deep
Field South. LAEs were selected to have observed frame equivalent widths >80A
and emission line fluxes >1.5E-17 erg/cm^2/s. Only 1% of our LAE sample appears
to host AGN. The LAEs exhibit a moderate spatial correlation length of
r_0=3.6+0.8-1.0 Mpc, corresponding to a bias factor b=1.7+0.3-0.4, which
implies median dark matter halo masses of log10(M_med) = 10.9+0.5-0.9 M_sun.
Comparing the number density of LAEs, (1.5+-0.3)E-3/Mpc^3, with the number
density of these halos finds a mean halo occupation ~1-10%. The evolution of
galaxy bias with redshift implies that most z=3.1 LAEs evolve into present-day
galaxies with L3 galaxy populations typically evolve
into more massive galaxies. Halo merger trees show that z=0 descendants occupy
halos with a wide range of masses, with a median descendant mass close to that
of L*. Only 30% of LAEs have sufficient stellar mass (>~3E9 M_sun) to yield
detections in deep Spitzer-IRAC imaging. A two-population SED fit to the
stacked UBVRIzJK+[3.6,4.5,5.6,8.0]micron fluxes of the IRAC-undetected objects
finds that the typical LAE has low stellar mass (1.0+0.6-0.4 E9 M_sun),
moderate star formation rate (2+-1 M_sun/yr), a young component age of 20+30-10
Myr, and little dust (A_V<0.2). The best fit model has 20% of the mass in the
young stellar component, but models without evolved stars are also allowed.Comment: ApJ, in press, 7 pages including 4 color figure