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## FERMILPSC - Fermi LAT Second Source Catalog |
HEASARC Archive |

The previous 1-Year Point Source Catalog (1FGL) covered only the first 11 months of the mission. The 2FGL catalog also uses a higher-resolution diffuse model and improved Instrument Response Functions (IRFs), which have more accurate effective area values and better point spread function representation. The catalog also includes extended sources and better spectral representations (not just power law fits). The criteria for including sources is more conservative, and there is no dramatic increase in the number of sources compared to 1FGL.

Sources were detected and characterized in the 100 MeV to 100 GeV range. Source detection was based on a threshold likelihood Test Statistic of 25, corresponding to a significance of just over 4 sigma.

This catalog includes source location regions, defined in terms of elliptical fits to the 95% confidence regions and power-law spectral fits, as well as flux measurements in 5 bands for each source. The Fermi LAT Team has evaluated the populations of gamma-ray sources that are represented in the catalog using a protocol defined before launch. Individual LAT-detected sources have been provided identifications or plausible associations with sources in other astronomical catalogs. Care was taken to characterize the sensitivity of the results to the model of interstellar diffuse gamma-ray emission used to model the bright foreground, and a number of sources at low Galactic latitudes and toward bright local interstellar clouds are flagged as having positions that are strongly dependent on the model or as potentially entirely due to incorrectly modeled structure in the Galactic diffuse emission.

Some work on the analysis procedures for data from Fermi LAT is still in progress. In particular, we emphasize several caveats for use of this catalog:

- This work is a summary of the LAT results covering the time period August 4, 2008, to July 31, 2010.
- The analysis is based on data in the 100 MeV to 100 GeV energy range.
- Source detection is based on the integrated data set, i.e., sources are detected according to their average fluxes.
- The catalog adopted multiple spectral shapes, depending on the observed source properties. Pulsars are modeled by an exponentially cutoff Power Law. Others are modeled as a LogParabola if statistically significant curvature (according to Signif_Curve) is detected, or as a simple Power Law otherwise.
- After computing the maximum likelihood, the threshold for inclusion is likelihood Test Statistic TS > 25, corresponding to somewhat more than 4 sigma significance. The catalog has 1873 sources.
- The corresponding flux limit varies widely according to the hardness of the source spectrum and the brightness of the background Galactic diffuse emission.
- The 2FGL information in five energy bands is not appropriate for detailed spectral modeling. Upper limits are indicated by 0 flux uncertainty; in these cases the corresponding flux entry has a 2-sigma upper limit. Note that some sources have only upper limits in each of the 5 bands; these sources have been detected based on their integrated fluxes.
- A variability index of VI > 41.6 indicates a high probability that the source is variable on a time scale of months. It does not address shorter or longer time variations.
- The diffuse model used in the analysis is still subject to improvements. Matching the model to the large-scale emission is an iterative process. The diffuse model is particularly important for sources near the Galactic Plane.
- Catalog entries include two important quality flags that indicate potential issues: a "c" following the 2FGL name indicates that the source is found in a region with bright and/or possibly incorrectly modeled diffuse emission; any non-zero entry in the "Flags" column indicates that some inconsistency was seen during the analysis (see the table in the preprint for details). Sources with either of these indicators should be used with some care. They may be artifacts of the analysis.
- The catalog analysis assumed point source emission for all but twelve extended sources, which were modeled using spatial templates and are indicated by an "e" following the name. Results for extended sources such as the LMC and Cen A are likely to not be representative of the total emission, and additional catalog point sources may appear near such objects.
- Associations listed in the catalog are not in general to be taken as firm identifications. Associations are positional coincidences that are statistically unlikely to be due to chance between known or candidate gamma-ray producing objects with 2FGL sources. Except for those cases with correlated variability or spatial extent, a physical relationship is not established.

**Name**

The source designation, 2FGL JHHMM.m+DDMM[c], constructed according to the
IAU Specifications for Nomenclature, in which the Right Ascension and
Declination have been truncated to 0.1 decimal minutes and 1', respectively.
The optional 'c' suffix indicates that, based on the region of the sky, the
source is considered to be potentially confused with Galactic diffuse
emission. The '2' refers to the second-year catalog (the first-year catalog
was '1'; another one is planned for year 5) and 'FGL' represents Fermi
Gamma-ray LAT.

**RA**

The Right Ascension of the gamma-ray source in the selected equinox.
This was given in J2000 decimal degree coordinates in the original table.

**Dec**

The Declination of the gamma-ray source in the selected equinox.
This was given in J2000 decimal degree coordinates in the original table.

**LII**

The Galactic Longitude of the gamma-ray source, as derived from the
J2000 position in the original table.

**BII**

The Galactic Latitude of the gamma-ray source, as derived from the
J2000 position in the original table.

**Semi_Major_Axis_68**

The semi-major axis of the error ellipse at 68% confidence, in degrees.

**Semi_Minor_Axis_68**

The semi-minor axis of the error ellipse at 68% confidence, in degrees.

**Position_Angle_68**

The position angle of the 68%-confidence semi-major axis, from celestial
North, positive toward increasing R.A. (eastward), in degrees.

**Semi_Major_Axis_95**

The semi-major axis of the error ellipse at 95% confidence, in degrees.

**Semi_Minor_Axis_95**

The semi-minor axis of the error ellipse at 95% confidence, in degrees.

**Position_Angle_95**

The position angle of the 95%-confidence semi-major axis, from celestial
North, positive toward increasing R.A. (eastward), in degrees.

**Detection_Significance**

The source detection significance, in Gaussian sigma units, as derived from
the likelihood analysis Test Statistic.

**Pivot_Energy**

The energy, in MeV, at which the error in the differential photon flux is
minimal (i.e., the decorrelation energy for the power-law fit). This is
derived from the likelihood analysis for 100 MeV - 100 GeV.

**Flux_Density**

The differential photon flux at the pivot energy, in photons/cm^{2}/MeV/s. This
is derived from the likelihood analysis for 100 MeV - 100 GeV.

**Flux_Density_Error**

The 1-sigma uncertainty in the differential flux at the pivot energy.

**Spectral_Index**

The best fit for the photon number power-law index, as derived from the
likelihood analysis for 100 MeV - 100 GeV. For LogParabola spectra (see
Spectrum_Type), it is the index at Pivot_Energy.
For PLExpCutoff spectra, it is the low-energy index.

**Spectral_Index_Error**

The 1-sigma uncertainty in the photon spectral index.

**Flux_1_100_GeV**

The photon flux for the 1 GeV - 100 GeV range, in photons/cm^{2}/s, as obtained
by summing the photon flux values from the likelihood analysis in the three
bands from 1 GeV to 100 GeV. If delta(flux)/flux exceeds 0.5, then flux +
2*delta(flux) is given instead as an approximate 2-sigma upper limit.

**Flux_1_100_GeV_Error**

The 1-sigma uncertainty for the integral flux in the 1 to 100 GeV range, as
obtained by summing in quadrature the errors from the three energy bands from
1 GeV to 100 GeV. If delta(flux)/flux exceeds 0.5, then this is set to 0.

**Energy_Flux**

The energy flux, in erg/cm^{2}/s, in the 100 MeV to 100 GeV range obtained
by spectral fitting from 100 MeV to 100 GeV. If delta(flux)/flux exceeds
0.5, then flux + 2*delta(flux) is given instead as an approximate 2-sigma
upper limit.

**Energy_Flux_Error**

1-sigma error on energy flux from 100 MeV to 100 GeV.
If delta(flux)/flux exceeds 0.5, then this is set to 0.

**Curve_Significance**

Significance (in sigma units) of the fit improvement between power-law and
either LogParabola (for ordinary sources) or PLExpCutoff (for pulsars) spectrum
type. A value greater than 16 triggers switching to the curved spectrum.

**Spectrum_Type**

The type of spectrum, either PowerLaw, LogParabola, or PLExpCutoff.

**Beta**

The curvature parameter (beta) for LogParabola spectrum types. It is blank
for other spectrum types.

**Beta_Error**

The 1-sigma error on beta for LogParabola spectrum types. It is blank
for other spectrum types.

**Cutoff**

Cutoff energy as exp(-E/Cutoff) for PLExpCutoff spectrum types. It is blank
for other spectrum types.

**Cutoff_Error**

1-sigma error on cutoff energy for PLExpCutoff spectrum types. It is blank
for other spectrum types.

**Powerlaw_Index**

The best-fit power-law index. This is equal to the Spectral_Index if
Spectrum_Type is PowerLaw.

**Flux_30_100_MeV**

The integral photon flux in the 30 to 100 MeV range, in photon/cm^{2}/s. This
column is intentionally set to null in the initial versions of this catalog.

**Flux_30_100_MeV_Error**

The 1-sigma uncertainty in the integral photon flux in the 30 to 100 MeV
range, in photon/cm^{2}/s. This column is intentionally set to null in the
initial versions of this catalog.

**Sqrt_TS_30_100_MeV**

The square root of the likelihood Test Statistic for the 30 to 100 MeV range.
This column is intentionally set to null in the initial versions of this
catalog.

**Flux_100_300_MeV**

The integral photon flux for the 100 to 300 MeV range, in photon/cm^{2}/s, as
derived from the likelihood analysis in that band with a fixed photon
power-law index from the 100 MeV-100 GeV fit. This value is a 2-sigma upper
limit if the source is not significant in this band.

**Flux_100_300_MeV_Error**

The 1-sigma uncertainty in the integral photon flux for the 100 to 300 MeV
range. This value is set to 0 if the source is not significant in this band.

**Sqrt_TS_100_300_MeV**

The square root of the likelihood Test Statistic for the 100 to 300 MeV range.

**Flux_0p3_1_GeV**

The integral photon flux for the 0.3 to 1 GeV range, in photon/cm^{2}/s, as
derived from the likelihood analysis in that band with a fixed photon
power-law index from the 100 MeV-100 GeV fit. This value is a 2-sigma upper
limit if the source is not significant in this band.

**Flux_0p3_1_GeV_Error**

The 1-sigma uncertainty in the integral photon flux for the 0.3 to 1 GeV
range. This value is set to 0 if the source is not significant in this band.

**Sqrt_TS_0p3_1_GeV**

The square root of the likelihood Test Statistic for the 0.3 to 1 GeV range.

**Flux_1_3_GeV**

The integral photon flux for the 1 to 3 GeV range, in photon/cm^{2}/s, as
derived from the likelihood analysis in that band with a fixed photon
power-law index from the 100 MeV-100 GeV fit. This value is a 2-sigma upper
limit if the source is not significant in this band.

**Flux_1_3_GeV_Error**

The 1-sigma uncertainty in the integral photon flux for the 1 to 3 GeV
range. This value is set to 0 if the source is not significant in this band.

**Sqrt_TS_1_3_GeV**

The square root of the likelihood Test Statistic for the 1 to 3 GeV range.

**Flux_3_10_GeV**

The integral photon flux for the 3 to 10 GeV range, in photon/cm^{2}/s, as
derived from the likelihood analysis in that band with a fixed photon
power-law index from the 100 MeV-100 GeV fit. This value is a 2-sigma upper
limit if the source is not significant in this band.

**Flux_3_10_GeV_Error**

The 1-sigma uncertainty in the integral photon flux for the 3 to 10 GeV
range. This value is set to 0 if the source is not significant in this band.

**Sqrt_TS_3_10_GeV**

The square root of the likelihood Test Statistic for the 3 to 10 GeV range.

**Flux_10_100_GeV**

The integral photon flux for the 10 to 100 GeV range, in photon/cm^{2}/s, as
derived from the likelihood analysis in that band with a fixed photon
power-law index from the 100 MeV-100 GeV fit. This value is a 2-sigma upper
limit if the source is not significant in this band.

**Flux_10_100_GeV_Error**

The 1-sigma uncertainty in the integral photon flux for the 10 to 100 GeV
range. This value is set to 0 if the source is not significant in this band.

**Sqrt_TS_10_100_GeV**

The square root of the likelihood Test Statistic for the 10 to 100 GeV range.

**Variability_Index**

The sum of 2*Log(Likelihood) comparison between the flux fitted in 24 time
segments and a flat lightcurve over the full 2-year catalog interval.
A value greater than 41.64 indicates there is a less than 1% chance of being
a steady source. See the accompanying paper for details.

**Significance_Peak**

The source significance for the time interval with the maximum monthly value
of the photon flux, as derived from the likelihood analysis for 100 MeV - 100
GeV. If the probability of steady emission is > 1%, this is null.

**Flux_Peak**

The maximum monthly value of the integral photon flux, in photon/cm^{2}/s, as
derived from the likelihood analysis for 100 MeV - 100 GeV. If the
probability of steady emission is > 1%, this is null.

**Flux_Peak_Error**

The 1-sigma uncertainty in the peak flux.

**Time_Peak**

The time at the center of the interval with the maximum photon flux. If the
probability of steady emission is > 1%, this is null.

**Time_Peak_Interval**

The length of the interval, in seconds, in which the peak integral photon
flux was measured.

**Alt_GammaRay_Name_1**

The designation of a corresponding (0FGL) source in the Fermi Bright Source
List, if any.

**Alt_GammaRay_Name_2**

The designation of the likely corresponding source from the 1AGL catalog, if
any.

**Alt_GammaRay_Name_3**

The designation of the likely corresponding source from the 3EG catalog, if
any.

**Alt_GammaRay_Name_4**

The designation of the likely corresponding source from the EGR catalog, if
any.

**Alt_GammaRay_Name_5**

The designation of the likely corresponding source from the 1FGL catalog, if
any.

**Source_Type**

The primary class designation of the identification or likely associated
source. Refer to the accompanying paper for details of class designations and
the association method. Note that designations shown in capital letters are firm
identifications; lower case letters indicate associations. The following
abbreviations are used:

agn = other non-blazar agn agu = active galaxy of uncertain type bzb = BL Lac type of blazar bzq = FSRQ (flat spectrum radio quasar) type of blazar glc = globular cluster gal = normal galaxy (or part) hmb = high-mass binary nov = nova rdg = radio galaxy sbg = starburst galaxy sey = Seyfert galaxy snr = supernova remnant spp = special case - potential association with SNR or PWN PSR = pulsar, identified by pulsations psr = pulsar, no pulsations seen in LAT yet pwn = pulsar wind nebulaTable 10 in the LAT 2-year catalog paper lists possible associations for sources near supernova remnants, designated spp.

**Alt_Source_Type**

Secondary class designation of the identification or likely associated source.

**Assoc_Name_1**

The designation of the identified or likely associated source.

**Assoc_Name_2**

An alternate designation of the identified or likely associated source.

**Assoc_Name_3**

An alternate designation to an extended source name, if any.

**Analysis_Flags**

These analysis flags indicate possible issues noted in the detection or
characterization of the source. A value of 0 indicates no known problem. Each
condition present raises a unique bit flag. The flag values are bit-encoded
into a single integer, with flag N contributing 2^{N-1} to the value, where
N is given as:

N=1: Source with TS > 35 which went to TS < 25 when changing the diffuse model. Note that sources with TS < 35 are not flagged with this bit because normal statistical fluctuations can push them to TS < 25. N=2: Moved beyond its 95% error ellipse when changing the diffuse model. N=3: Flux (> 1 GeV) or energy flux (> 100 MeV) changed by more than 3 sigma when changing the diffuse model. Requires also that the flux change by more than 35% (to not flag strong sources). N=4: Source-to-background ratio less than 20% in highest band in which TS > 25. Background is integrated over the 68%-confidence area (pi*r_68^{2}) or 1 square degree, whichever is smaller. N=5: Closer than theta_ref from a brighter neighbor, where theta_ref is defined in the highest band in which source TS > 25, or the band with highest TS if all are < 25. theta_ref is set to 2.17 degrees (FWHM) below 300 MeV, 1.38 degrees between 300 MeV and 1 GeV, 0.87 degrees between 1 GeV and 3 GeV, 0.67 degrees between 3 and 10 GeV and 0.45 degrees about 10 GeV (2*r_68). N=6: On top of an interstellar gas clump or small-scale defect in the model of diffuse emission. N=7: Deprecated. N=8: Unstable position determination; best position from optimization outside the 1-sigma (39% in 2D) contour from the TS map (see Section 3.1.4 in catalog paper). N=9: Elliptical quality > 4 in pointlike (i.e., TS contour does not look elliptical). N=10: Spectral Fit Quality > 16.3 (see Equation 3 in catalog paper). N=11: Possibly due to the Sun (see Section 3.6 in catalog paper). N=12: Highly curved spectrum; LogParabola beta fixed to 1 or PLExpCutoff Spectral Index fixed to 0 (see Section 3.3 in catalog paper).

**TeVCat_Flag**

This flag parameter indicates a possible association with the TeVCat. The
flag values have the following meanings:

N = No TeV association P = Small TeV source E = Extended TeV source (diameter > 40 arcminutes)

**TeVCat_Assoc**

The designation of the likely corresponding source from the TeVCat catalog, if
any.