1. A Survey of EGRET GeV Sources
Chuan Chen
University of California, Irvine
EGRET GeV Catalog
Trials Factor Simulation
Milagro U.L. and EGRET Flux

2. Milagro Collaboration Meeting, 04/20/2007
EGRET GeV Catalog
# of EGRET gamma-ray sources detected > 1 GeV is 57 (Lamb & Macomb, ApJ 488:872, 1997)
46 are high (>5.0 σ),
6 are low (4.5 σ – 5.0 σ),
5 are very low significance sources (4.0 σ – 4.5 σ).
Milagro field of view (-5o < d < 70o) contains 32
25 high, 5 low and 2 very low significance GeV sources
14 are in the Galactic plane ( -10o < l < 10o)
Milagro data is searched for brightest location within 1.0 degree radius of the GeV sources
Chuan Chen
Milagro Collaboration Meeting, 04/20/2007

3. Searching Results for Some Sources
Source Name
1 Hottest spot Location (l, b)
Pre-trials Sigma
2 Post-trials Probability
Identification
GEV J
184.6, -5.6
15.54
2.92E-52
2EG J , Crab
GEV J
75.1, 0.1
10.92
1.45E-25
2EG J
GEV J
40.7, -1.1
8.55
1.93E-15
GEV J
80.5, 1.1
6.89
7.29E-10
GEV J
179.0, 65.1
6.66
3.30E-09
2EG J , Mkn 421
GEV J
80.2, 1.3
6.55
6.66E-09
GEV J
194.9, 4.5
4.99
3.86E-05
2EG J , Geminga
GEV J
78.5, 1.3
4.48
2EG J , possibly γ Cygni SNR
GEV J
106.1, 2.8
2EG J
GEV J
33.5, -0.1
4.26
2EG J , possible W44 SNR
GEV J
204.8, -0.3
3.17
0.04
Possibly Monoceros SNR 3
1 Position with maximum sigma in each searching region 2 Trials factor are determined by Monte Calor
3 They are one source in Milagro
Chuan Chen
Milagro Collaboration Meeting, 04/20/2007

4. Simulation of Trials Factor Using Bin Smoothed Maps
Bins are correlated since we use Gaussian smoothed maps.
Using the total number of bins in the searching region will overestimate the trials factor.
Simulate number of trials factor for different significant sources
Trials Factor
Significance
Nm is the number of maps with the maximum significance greater than n sigmas in the searching region.
Nt is the total maps (1 billion)
Pn is the probability greater than n sigmas. (e.g. P3 = )
NTrials is the number of trials
Chuan Chen
Milagro Collaboration Meeting, 04/20/2007

5. Trials Factor using Gaussian Smoothed Maps
From Crab fit we get psf is / I used 0.6 degrees as the sigma of psf and simulated 20 million maps.
Sigma of psf equals 0.6 is roughly the same as bin size equals 1.7 degrees.
I didn’t find the reason of the disagreement
Trials Factor
Significance
Chuan Chen
Milagro Collaboration Meeting, 04/20/2007

6. Trials Factor for the Galactic Plane Sources
Simulated 200,000 galactic planes (l[0°,180°], b[-5°, 5°])
Using two different bin sizes (1.7 and 2.9 degrees)
Can be scaled to the real galactic plane region we used
Total number of bins in the simulated galactic plane (180,000)
Trials Factor
Significance
Chuan Chen
Milagro Collaboration Meeting, 04/20/2007

7. Milagro Collaboration Meeting, 04/20/2007
Milagro Distribution of Post-trials Probabilities’ for 32 EGRET GeV Sources
1.5 degree binning size is roughly the equivalent binning size for Gaussian smoothed method we used.
Fitting the plot on previous page (2-5 sigmas and extrapolate to higher significance) to get the trials factors for different significant sources.
# of EGRET GeV Sources
post-trial probability cut for the table
Post-trial Probability
Chuan Chen
Milagro Collaboration Meeting, 04/20/2007

8. Milagro u.l. vs EGRET GeV Flux
E2 20 TeV Milagro measurements and upper limit (ergs/cm2 sec)
Red is Crab
Green is Geminga
Blues are in the galactic plane paper
Purples are u.l.
The differential photon spectra of all EGRET GeV sources between 1 GeV and 20 TeV is steeper than E-2
Equal energy flux
E2 1 GeV EGRET measurements (ergs/cm2 sec)
Chuan Chen
Milagro Collaboration Meeting, 04/20/2007

9. Milagro u.l. vs EGRET extrapolation to 20 TeV
E2 20 TeV Milagro measurements and upper limit (ergs/cm2 sec)
Equal energy flux
Red is Crab
Green is Geminga
Blues are in the galactic plane paper
Purples are u.l.
Spectra are taken from the 3rd EGRET catalog of high-energy gamma-ray sources
The differential photon spectra of many EGRET GeV sources steepens above 1 GeV
E2 20 TeV EGRET extrapolation (ergs/cm2 sec)
Chuan Chen
Milagro Collaboration Meeting, 04/20/2007

10. Milagro Collaboration Meeting, 04/20/2007
Conclusion
32 of 57 EGRET gamma-ray sources detected > 1 GeV are in Milagro field of view
14 are in the Galactic plane
11 (6) of them with pre-trial significance > 3 (5) sigma and post-trial significance > 2 (4) sigmas
Upper limits are placed on the TeV flux of EGRET GeV sources
I will calculate the flux for the GeV sources with pre-trial significance greater than 3 sigmas
Chuan Chen
Milagro Collaboration Meeting, 04/20/2007