In this Chapter we have presented an exploration of the relative merits of AGN selection
in the X-ray with Chandra and in the infrared with Spitzer, along the path toward the
ultimate goal of composing a complete, unbiased census of supermassive black hole growth and evolution in the Universe.
While many of the X-ray-selected AGN are also selected by the Stern et al. (2005a)
Spitzer AGN selection criteria, we find that a large fraction of the low-Lx AGN (Lx <
1043.5erg s−1) identified by Chandra will be missed by Spitzer. In addition, we find that
by Chandra, many of the optically faint (R >22) wedge sources are X-ray undetected. To explore the mean X-ray properties of the X-ray undetected wedge AGN, we stacked the corresponding X-ray data. The stacked wedge sources show significant X-ray signals in the full, soft, and hard X-ray bands.
The hardness ratio of the wedge-selected stack is consistent with moderate intrinsic ob- scuration, but is not suggestive of a highly obscured, Compton-thick source population. It is possible that the stack is missing a population of sources, such that the stacked counts are dominated by a subset of the wedge-selected sources. For instance, perhaps the stack is dominated by fewer than half of the sources, which fall below the SEXSI detection threshold, but have a much higher flux than the other half of the X-ray-undetected sources. In that case the average X-ray fluxes and hardness ratios will not be indicative of the overall source
population, and may miss flux from truly highly obscured sources with NH∼> 1025 cm−2.
Another possibility is that the simple assumption of an intrinsic Γ = 1.9 power-law compo-
nent plus photoelectric absorption is too simple, so that the estimated NH values are not
representative of the sample. The soft X-ray emission may originate in a different location
than the hard, power-law component, skewing theNH estimates to lower values. In reality,
it is likely some combination of these and other effects.
In the short term, we plan to study the distribution of counts in our stack to determine if we can suggest whether the stack is dominated by a subset of the stacked sources. In
addition we plan to calculate the implied contribution of the Spitzer-selected AGN to the
X-ray background using the SEXSI data combined with public data from the deep, pencil-
beam fields included in the Great Observatories Origins Deep Survey (GOODS).4
In the long-term, an X-ray mission sensitive at higher energies will be important to study the Spitzer-selected AGN and their contribution to the X-ray background. For example,
the Nuclear Spectroscopic Telescope Array (NuSTAR)5 is a proposed focusing telescope
designed to image the hard (∼<80 keV) X-ray sky. In addition, future X-ray survey missions
at E ∼< 10 keV that have orders-of-magnitude gains in effective area and non-dispersive
spectroscopic resolution will allow detailed spectral studies of sources that are only faint
detections with Chandra.
4
GOODS: http://www.stsci.edu/science/goods/ 5
Part II
Microwave Kinetic Inductance
Detectors for X-ray Astrophysics
Chapter 7
Focal-Plane Arrays for
Next-Generation Missions
7.1
Scientific Motivation for Next-Generation X-ray Survey
Missions
7.1.1 Overview
Large-FOV imagers with high spectral resolution will enable advances in many areas of astrophysics and cosmology. Here I introduce two key areas: the accretion history of the Universe, through X-ray surveys of active galactic nuclei, and the nature of dark energy and dark matter, via X-ray surveys of galaxy clusters.
Many square degrees of sky have been surveyed with the current generation Chandra
and XMM-Newton X-ray telescopes, studying the intermediate-redshift (z ∼< 2) 0.5 – 10 keV X-ray Universe with unprecedented depth. However, these are CCD-grade low spectral resolution surveys with limited photon counts. There is great scientific potential if these surveys can extend a decade in sensitivity and sample tens of square degrees with a factor
of ∼30 improvement in spectroscopic resolution.
7.1.2 Next-Generation AGN Surveys
The accretion history of the Universe provides a fundamental key to understanding the formation and evolution of our Universe. Supermassive black holes reside at the center of every galaxy, and the formation and growth of each central black hole and host galaxy are
select AGN, and current X-ray telescopes have pinpointed statistical samples of accreting extragalactic sources, probing AGN demographics across a diverse array of AGN types to
z∼1. For example, Chapter5discusses the identified sample of AGN from the SEXSI sur-
vey and compares to otherChandraandXMM-Newtonsurveys. Our current understanding
begins to fall short at higher redshifts; one primary goal for future surveys is to push to higher redshifts to find the first black holes as they are forming. Future surveys should also allow detailed X-ray spectral modeling of large samples of AGN at cosmic distances; this is currently unavailable for all but the brightest sources in the X-ray sky. The current surveys rely heavily on optical spectroscopic followup or many-band optical and IR photometry to determine source redshifts. This severely limits the surveys to lower redshift sources or to sources with intrinsically bright or unobscured optical counterparts. High resolution X-ray
spectral information will not only allow redshift determination via the Fe Kα line at 6.4
keV in many cases but allow studies of the detailed X-ray spectral shape of faint sources that currently only have rough estimates of spectral shape from broad band hardness ratios. These future survey goals require large, multiplexed detector arrays to ensure sufficient sur-
vey area to find the rare, high-z sources, and non-dispersive spectrometers with excellent
energy resolution and high efficiency for spectral study.