FERMILPSC - Fermi LAT Second Source Catalog

Overview

The Fermi LAT 2-Year Source Catalog (2FGL) is a catalog of high-energy gamma-ray sources detected by the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (Fermi) mission during the first 24 months of the science phase of the mission, which began on 2008 August 4.

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.

Bulletin

The FERMILPSC database table was last updated on 8 August 2012.

Caveats

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.

References

A full description of the LAT Second Source Catalog is given in the "LAT 2-year Catalog Paper" preprint by the Fermi LAT team, which is available at http://fermi.gsfc.nasa.gov/ssc/data/access/lat/2yr_catalog/.

Provenance

This database table was updated by the HEASARC in July 2011 using electronic data obtained from the Fermi Science Support Center (FSSC) available at http://fermi.gsfc.nasa.gov/ssc/data/access/lat/2yr_catalog/.

Data Products

OGIP-compliant FITS format lightcurve files and quicklook plots are available for each source. These were created by the HEASARC from the flux histories tabulated in the FITS version of the catalog. The tabulated flux histories are the integrated fluxes (in photons/cm²/s) along with the associated errors in the 100 MeV to 100 GeV energy band.

Parameters

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/cm²/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/cm²/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/cm²/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/cm²/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/cm²/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/cm²/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/cm²/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/cm²/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/cm²/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/cm²/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/cm²/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 nebula

Table 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_68²) 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.

Contact Person

Questions regarding the FERMILPSC database table can be addressed to the HEASARC User Hotline.

A service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysics Observatory (SAO)

HEASARC Director: Dr. Alan P. Smale
HEASARC Associate Director: Dr. Roger Brissenden
NASA Official: Phil Newman
Web Curator: Meredith Gibb
Last Modified: 19-Jul-2011

Page Author: Browse Software Development Team
Last Modified: 19-Jul-2011