The Fermi LAT First
Source Catalog
Jean BalletSAp, CEA Saclay
on behalf of theFermi Large Area TelescopeCollaboration

The First LAT Catalog (1FGL)• 11 months of data 100 MeV to 100 GeV, 23.3 Ms livetime• 10.6 M events over the whole sky after selection• Improved diffuse model (distributed with the data) andcalibration (see poster by Riccardo Rando) with respect to 0FGL
• Very uniform exposure (factor 1.25 between north and south)• Detection based on integrated data (not on flares)• Improved localization

Contents of the LAT source catalog• Source coordinates and error ellipse at 95% confidence• Source significance and overall spectral index• Flux in 5 energy bands 0.1 – 0.3 – 1 – 3 – 10 – 100 GeV• Flux per month, variability index• Extension flag (poster by J. Lande)• Quality flag: sensitivity to diffuse model, confusion,error ellipse not well defined• Associations with known sources in external catalogs

Other LAT catalogs• The AGN catalog, drawn from sources above 10° latitude,excluding known non AGN. Goes with the full source catalog,
which adopts the AGN associations. Similar to LBAS (ApJ 700,597) vs BSL (ApJS 183, 46). Hear Benoit Lott and Marco Ajello,look at Steve Healey’s poster.
• The pulsar catalog, drawn from all pulsed detections of both
radio and γ-ray pulsars. First version (6 months of data)submitted to ApJ (arXiv:0910.1608). Hear Lucas Guillemot.
• The gamma-ray burst catalog, drawn from all GRB detections.
Completely separate (bright GRBs are actually excluded from thedata for the LAT source catalog). Available in quasi real time atFermi SSC. Hear Nicola Omodei.

Source detection•

Difficulty is that point spread function improves enormously from 5°
at 100 MeV to nearly 0.1° above 10 GeV•

No obvious optimal method to handle that in (α,δ,E) space•

Combine several energy bands, merge seeds from several detection
methods. Poster by E. Massaro
Raw counts map, 75 x 45°

Wavelet filtered
Front > 5 GeV
Back > 10 GeV
Very few events,
very well localized

Front > 200 MeV
Back > 400 MeV
Many events, not sowell localizedSource significance•3D maximum likelihood analysis (position and energy) was used todetermine source significance assuming power-law spectra on top of
standard diffuse model•

Define TS = 2 Δlog(likelihood) comparing models with and without thesource. Cut at TS = 25, corresponding to about 4 σ or 2.5E-5 probability(4 degrees of freedom including source position)Works well at high latitudes.
Peak in TS distribution atthreshold.
TS distribution flatter close to
Galactic plane. Faint sources
are not detected.

> 1000 LAT sources•Front > 200 MeV, Back > 400 MeV, log color scale•

Galactic coordinates, Aitoff projection

Sensitivity map•••

Structure is mostly that of the interstellar medium
Below 10-8 ph/cm2/s outside the Galaxy (|b| > 30°)
Strong dependence on spectral index (hear B. Lott)
10-9 ph/cm2/s

Flux > 100 MeV required to reach TS=25 for average E-2.2 spectrum

Galactic coordinates, Aitoff projection

Source localization•Error ellipses adjusted on TS maps,
on average close to circles•

Conservative 0.012° absolute limit
based on bright pulsars

Average r95 is 10’Average axis ratio is 0.83•A difficult example is below.
Cross is 1σ 1D error fromanother localization method forcomparison

Source association•

Likelihood ratio between true association (gaussian distribution withwidth defined from r95) and random association (flat at counterpartdensity)•

Typically one half of the sources are associated to a plausible
counterpart (radio blazar, pulsar, PWN, SNR, XRB), down from 2/3 at0FGL (brighter)

Point sources only

No doubt that most of theseassociations are true.
r95 was multiplied by 1.2 tocover the tail.
The distance distribution maybe more complex than asimple gaussianThe variable Fermi sky•

1-day snapshots, > 100 MeV, viewed from the poles (orthographic proj).
Red is significant.

•The Sun is moving down right of North pole and up right of South poleSource variability• Build light curves on one-month time•scale, assuming constant spectral index• Pulsars are stable within 3%• Bright blazars are very clearly variable• Upper limit in intervals in which sources •are not significant

Variability index: χ2 against
constant hypothesis. 250
variable sources at 99%
confidence level
Relative variations: ΔF/F where
ΔF2 = measured variance minus
Poisson varianceFaint source, TS = 35
Var. index = 13.5,not significant

Average relative variations
are 67% in variable sources
Variability cannot bemeasured in faint sources

Source spectra•

Extract flux in 5 bands from 100
to 300 MeV, 300 MeV to 1 GeV, 1to 3 GeV, 3 to 10 GeV, 10 to 100
GeV
Upper limit in bands in whichsources are not significant
Same faint source,
TS = 35, Г = 2.4

Single band to two bands flux ratio••

Sources not significant in all
bands, total flux not wellmeasured•

Average spectrum is broken,power-law estimate is too highTotal flux taken from sum of
bands rather than power-law fit

Index > 1 GeV – index < 1 GeV

Source confusion
Average distance between sources
outside the plane is about 3°•

15°region of the Galactic ridge
above 1 GeV•More than r68 at typical detectionenergy (0.8° at 1 GeV)

•Crosses are sources, pixel is 0.2°

•Sources not clearly separated

Important for soft sources, andintroduces strong additional bias
against very soft sources (Г ≥ 3)•

Could be unmodeled diffuseemission•

PRELIMINARY•

Diffuse emission uncertainties•

Use two different diffuse models toassess sensitivity of sources•

Inside Galactic plane, flux dispersion
due to diffuse model is 1.8 sigma•

Outside plane, dispersion is 0.7 sigma•

Source to background ratio
within r68 is not very large inthe Galactic plane even above
1 GeV.

•

Has to go above 3 GeV to beabove 50%

Galactic ridge and dense clouds••

The Galactic ridge (|lat| < 1°, |lon| < 60°) has serious difficulties: sources areclose to each other, are not high above the background below 3 GeV, and theGalactic diffuse model is very uncertain there (hear Troy Porter and SethDigel). This even affects sources statistically very significant (TS > 100).
We now plan to set Galactic ridge sources apart entirely (some 120 sources),and warn against using them without detailed analysis. Of course there are still
many true sources in there, including pulsars and SNRs.
ρ Ophiuchi
yellow flagged
green not flagged
Sources outsidethe Gal. ridge canbe flaggedindividuallyOrionFirst LAT source catalog• Extends 0FGL to much fainter sources• Typical 95% error radius is 10’. Absolute accuracy is better
than 1’• About 250 sources show evidence of variability• Half the sources are associated positionally, mostly withblazars and pulsars• Other classes of sources exist in small numbers (XRB, PWN,SNR, starbursts, globular clusters, radio galaxies, narrow-lineSeyferts)• Uncertainties due to the diffuse model, particularly in theGalactic ridge• Detailed results on many of these sources at this meeting• Catalog will be available before the end of November

The sky viewed from above
Fermi-LAT
11 months
E > 1 GeV

Orthographicprojection