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## FERMILPSC - Fermi LAT 8-Year Point Source Catalog |
HEASARC Archive |

Relative to the 3FGL (4-year) catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for Galactic diffuse gamma-ray emission. The 4FGL catalog includes 5098 sources above 4 sigma significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall 364 sources are considered as identified based on angular extent or correlated variability observed at other wavelengths. For 1525 sources we have not found plausible counterparts at other wavelengths. More than 2940 of the identified or associated sources are active galaxies of the blazar class, 241 are pulsars.

We emphasize several important points and caveats regarding use of the 4FGL catalog:

- This work is a summary of the LAT results covering the time period August 4, 2008, to August 2, 2016.
- The catalog is based on analysis of Pass 8 data in the 50 MeV to 1 TeV 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 5098 sources.
- The corresponding energy flux limit varies according to the hardness of the source spectrum and the brightness of the background Galactic diffuse emission.
- Catalog entries include quality flags that indicate potential issues: 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 these indicators should be used with great care.**They correspond to significant excesses of photons, but such excesses can result from residual extended emission or confused source pile-up. - The catalog analysis assumed point source emission for all but seventy-five 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.
- The Crab pulsar and PWN are represented by a total of three entries, two of which (designated "i" for inverse compton and "s" for synchrotron) represent spectral components of the PWN. We consider these three entries to represent two sources.
- 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 alignment between known or candidate gamma-ray producing objects
with 4FGL sources. Upper case in the CLASS column means identified.
**Except for those cases with correlated variability or spatial extent, a physical relationship is not established.**The designators for some associated sources may be updated in a future release to use more common or more complete names.

**Name**

The source designation, 4FGL 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 '4' refers
to the eight-year catalog (the first-year catalog was '1'; the second-year
catalog was '2'; the four-year catalog was '3') and 'FGL' represents Fermi
Gamma-ray LAT. The letter at the end can be "c" (coincident with interstellar
clump), "e" (extended source), "i" (for Crab nebula inverse Compton) or "s" (for
Crab nebula synchrotron).

**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_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, over the 100 MeV
to 1 TeV band.

**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 - 1 TeV.

**Flux_1_100_GeV**

The integral photon flux for 1 - 100 GeV, in photons/cm^{2}/s.

**Flux_1_100_GeV_Error**

The 1-sigma uncertainty in the integral photon flux for 1 - 100 GeV, in
photons/cm^{2}/s.

**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.

**Energy_Flux_Error**

The 1-sigma error on energy flux from 100 MeV to 100 GeV.

**Spectrum_Type**

The spectral type in the global model (PowerLaw, LogParabola, PLSuperExpCutoff).

**PL_Flux_Density**

The differential flux at pivot_energy for the PowerLaw fit, in photons/cm^{2}/MeV/s.

**PL_Flux_Density_Error**

The 1-sigma uncertainty in differential flux at pivot_energy in PowerLaw fit,
in photons/cm^{2}/MeV/s.

**PL_Index**

The photon index for the PowerLaw fit.

**PL_Index_Error**

The 1-sigma uncertainty on the photon index for PowerLaw fit.

**LP_Flux_Density**

The differential flux at pivot_energy for the LogParabola fit, in
photons/cm^{2}/MeV/s.

**LP_Flux_Density_Error**

The 1-sigma uncertainty in differential flux at pivot_energy in LogParabola
fit, in photons/cm^{2}/MeV/s.
lp_index
The 1-sigma uncertainty on the photon index for LogParabola fit.

**LP_Index**

Photon Index at Pivot Energy for LogParabola Fit

**LP_Index_Error**

The 1-sigma uncertainty on the photon index for LogParabola fit.

**LP_Beta**

The curvature parameter (beta) for LogParabola spectrum types.

**LP_Beta_Error**

The 1-sigma uncertainty on lp_beta for LogParabola spectrum types.

**LP_Curve_Significance**

The significance in sigma of the fit improvement between PowerLaw and
LogParabola fits.

**PLEC_Flux_Density**

The differential flux at pivot_energy for the PLSuperExpCutoff fit, in
photons/cm^{2}/MeV/s.

**PLEC_Flux_Density_Error**

The 1-sigma uncertainty in differential flux at pivot_energy in
PLSuperExpCutoff fit, in photons/cm^{2}/MeV/s.

**PLEC_Index**

The low energy photon index for PLSuperExpCutoff fit.

**PLEC_Index_Error**

The 1-sigma uncertainty on plec_index for PLSuperExpCutoff fit.

**PLEC_Exp_Factor**

The exponential factor for PLSuperExpCutoff fit.

**PLEC_Exp_Factor_Error**

The 1-sigma uncertainty on exponential factor for PLSuperExpCutoff fit.

**PLEC_Exp_Index**

The exponential index for PLSuperExpCutoff fit.

**PLEC_Exp_Index_Error**

The 1-sigma uncertainty on exponential index for PLSuperExpCutoff fit.

**PLEC_Curve_Significance**

The significance in sigma of the fit improvement between PowerLaw and PLEC fits.

**Npred**

The number of predicted events in the model.

**Extended_Source_Name**

The extended source identification, if any.

**Gamma_Assoc**

The corresponding source in the previous gamma-ray source catalogs, in the
order 3FHL > 3FGL > 2FHL > 1FHL > 2FGL > 1FGL > 0FG.

**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.

**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 active galaxy bcu = active galaxy of uncertain type bin = binary bll = BL Lac type of blazar css = compact steep spectrum quasar fsrq = FSRQ type of blazar gal = normal galaxy (or part) glc = globular cluster hmb = high-mass binary mc = molecular cloud nlsy1 = narrow line Seyfert 1 nov = nova PSR = pulsar, identified by pulsations psr = pulsar, no pulsations seen in LAT yet pwn = pulsar wind nebula rdg = radio galaxy sbg = starburst galaxy sey = Seyfert galaxy sfr = star-forming region snr = supernova remnant spp = special case - potential association with SNR or PWN ssrq = soft spectrum radio quasar unk = unknown

**Assoc_Name**

The designation of the identified or likely associated source.

**Assoc_Name_Alt**

The alternate designation or an indicator as to whether the source is inside
an extended source.

**Assoc_Prob_Bay**

The probability of association according to the Bayesian method. It is set to
NULL for extended sources (which do not enter that calculation) and to 0 for
point sources that did not reach 0.8, the threshold for declaring associations.

**Assoc_Prob_LR**

The probability of association according to the Likelihood Ratio method. It
is set to NULL for extended sources (which do not enter that calculation) and
to 0 for point sources that did not reach 0.8, the threshold for declaring
associations.

**Assoc_RA**

The Right Ascension of assoc_name counterpart, in degrees.

**Assoc_Dec**

The Declination of assoc_name counterpart, in degrees.

**Assoc_Error_Radius**

The position uncertainy at 68% confidence level of the counterpart
localization for assoc_name, in degrees.

**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: Not used. N=3: Flux (> 1 GeV) or energy flux (> 100 MeV) changed by more than 3 sigma when changing the diffuse model or the analysis method. Requires also that the flux change by more than 35% (to not flag strong sources). N=4: Not used. N=5: Not used. N=6: Not used. N=7: Not used. N=8: Not used. N=9: Localization Quality > 8 in pointlike (see Section 3.1 in catalog paper) or long axis of 95% ellipse > 0.25. N=10: Spectral Fit Quality > 30 in pointlike. N=11: Not used. N=12: Highly curved spectrum; LogParabola beta fixed to 1 or PLEC_Index fixed to 0 (see Section 3.3 in catalog paper).