SEXSI X-Ray Analysis and
6.5 Mean X-ray Properties of Infrared Selected Active Galax- iesies
6.5 Mean X-ray Properties of Infrared Selected Active Galax-
total area in the source extraction region to that of the background region.
Source counts and scaled background counts were tabulated for each source in several energy bands: the standard 0.5−8 keV (full band), 2−8 keV (hard band), and 0.5−2 keV (soft band), as well as 2−4 keV, 4−6 keV, and 6−8 keV. Estimates of individual source significance are calculated for each band. Following Laird et al. (2005) we define detection significance asS/√
B and signal-to-noise ratio asS/√
S+B, whereS and B are the net source counts and background counts, respectively. The average exposure per pixel (in each energy band) is calculated for each source by averaging the ARF over the given energy band and then over the source extraction cell. In addition to the IRAC sources, we also followed this procedure using a catalog with randomly shifted source positions.
After calculating individual source statistics for all of the sources we are able to stack the sources. The average signal in each band is calculated by summing the net counts. To calculate an average flux we divide the summed signal by the sum of the average exposure per pixel in each extraction cell to convert from counts to ph cm−2 s−1. To convert from ph cm−2 s−1 to erg cm−2 s−1 we assume a power-law spectrum with photon index Γ = 1.5.
To ensure that the stacks are not dominated by a handful of sources with significant X-ray counts (sub-SEXSI-threshold X-ray sources) we eliminate all sources with an individual source significance of 10 or greater. The choice of sig=10 is slightly arbitrary; the exact significance cut does not affect the stack, it just acts to eliminate the ∼ 10 sources with significant (yet sub-SEXSI) X-ray detections.
6.5.2 Stacking Results
Stacking the emission from the undetected IRAC 4-band detected sources produced sig- nificant detections.3 We stacked three basic samples: all X-ray undetected sources (1788 sources), the wedge-selected sources (288 sources), and those that fall outside of the wedge (1500 sources). Table 6.5 presents a summary of the results of these stacks, where N is the number of sources in the particular stack, Cts provides the number of net counts in the stack, Sig provides an estimate of the source significance, andfx is the X-ray flux in units of 10−17erg cm−2 s−1. The stacks of the sources created by the randomly-shifted source catalog showed no significant detection in any band.
3Only 5 of the 6 fields are included in this preliminary stacking analysis. The sources from field CL 0848+44 will be added at a later date
Table 6.5. Average properties of X-ray undetected IRAC sources
Sample N 0.5 – 8 keV 0.5 – 2 keV 2 – 8 keV
Cts Sig fx Cts Sig fx Cts Sig fx
[10−17] [10−17] [10−17]
All 1788 683 16.8 10.0 473.8 20 2.3 209.5 6.4 6.9
Inside Wedge 288 257 14.3 21.2 135.7 13 3.8 121.0 8.3 22.7
Outside Wedge 1500 427 11.7 7.6 338.0 16 2.0 88.5 3.0 3.6
Inside Extended Wedge 475 379 16.4 18.6 210.5 16 3.5 168.7 9.0 18.9
Outside Extended Wedge 1313 304 9.0 6.3 263.2 13 1.8 40.7 1.5 1.9
For the IRAC sources as a whole, the stacked emission produces a total of 683 net counts with a detection significance of 16.8 in the full band, 473.8 net counts in the soft band with a source significance of 20, and a hard-band detection of 209.5 net counts with a source significance of 6.4. These detections correspond to mean fluxes of 1×10−16erg cm−2 s−1 (full band), 2×10−17erg cm−2 s−1 (soft band), and 7×10−17erg cm−2 s−1 (hard band).
Stacking only the wedge-selected sources, we again find significant detections in all three X-ray bands. The wedge-selected sample contains over six times fewer sources than the full sample, while the stacked signals from the wedge contribute∼1/3−1/2 of the total counts of the full sample. This trend is most significant in the hard band, where the average flux of the wedge-selected sample is over three times that of the total sample. The stack of sources outside the wedge show only a marginal detection in the hard band, with detection significance of 3, and source flux over six times lower than that of the wedge-sources. These findings are consistent with the idea that AGN preferentially lie inside the wedge.
To allow for errors in the IRAC photometry, we also stacked based on an extended wedge defined by:
([5.8]−[8.0])>0.5 ∧ ([3.6]−[4.5])>0.2·([5.8]−[8.0]) + 0.078
∧ ([3.6]−[4.5])>2.5·([5.8]−[8.0])−3.77 ,
(6.1)
where ∧ is the logical AND operator. This extended wedge is constructed by taking the originalStern et al.(2005a) wedge and adding a swath of width 0.1 at each edge. The stacks for inside and outside the extended wedge are also presented in Table 6.5; the extended wedge swath includes 187 sources. Removing these 187 sources from the 1500 sources not
Table 6.6. Wedge stack: impliedNH
z log(NH) 0.0 8.5×1021 0.5 2.3×1022 1.0 4.8×1022 1.5 8.9×1022 2.0 1.5×1023 3.0 3.2×1023
selected by the original wedge removes about half of the counts in the hard-band stack, leaving a source significance of only 1.5 in the extended-wedge hard-band stack. This suggests that the hard-X-ray radiation that is easily produced by AGN but less so by star formation comes from sources in or just outside the wedge, at our detection sensitivity.
Although the wedge-selected sources are harder than those outside the wedge, the col- umn density implied by the hardness ratio of is not extreme, as compared to Chandra- detected samples. Table 6.5provides estimated NH values based on the HR of the wedge- selected stack for assumed source redshifts ranging from 0.0 to 3.0. TheNH values are calcu- lated assuming an underlying power-law spectrum with Γ = 1.9 and a Galactic column den- sity of 1020cm−2. The calculatedNH estimates range from∼9×1021cm−2−3×1023cm−2, suggesting that the sources are intrinsically obscured, but are not, on average, Compton thick.