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## FERMILPSC - Fermi LAT 14-Year Point Source Catalog (4FGL-DR4) |
HEASARC Archive |

We emphasize several important points and caveats regarding use of the 4FGL-DR4 catalog:

- This work is a summary of the LAT results covering the time period August 4, 2008, to August 2, 2022.
- The analysis is based on 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 of new sources is likelihood Test Statistic TS > 25, corresponding to somewhat more than 4 sigma significance. The catalog has 7194 sources. All previous 4FGL sources (except for a few discussed in the paper) are retained in the 4FGL-DR4 even if TS < 25.
- 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 the source is affected by systematic errors (see the table in the preprint for details). Sources with 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 82 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 lobes 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 and candidate gamma-ray producing objects with 4FGL-DR4 sources. Upper case in the CLASS1 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.
- The DR4 catalog was developed using a smooth spatial and spectral rescaling of the 4FGL Galactic diffuse model gll_iem_v07, as explained in Sect 2.2 of the document. The adjustment is largely equivalent to modulating the gll_iem_v07 model with a LogParabola spectral shape in each region of interest. It is not recommended as an all-purpose replacement for gll_iem_v07 because its behavior above 10 GeV was not carefully controlled, but it can be used in the GeV domain following the DR4 prescription (modulated by a power law and associated with the fixed isotropic spectrum and the DR4 background of sources). When using it, please quote "the rescaled gll_iem_v07 interstellar emission model associated to the 4FGL-DR4 catalog" and cite the DR4 document (link to document). This rescaled diffuse model has the same limitations as gll_iem_v07, and comparing results obtained with this rescaled model and with gll_iem_v07 cannot quantify systematic errors due to the model of the diffuse emission.

The 4FGL-DR2 update is described in Fermi Large Area Telescope Fourth Source Catalog Data Release 2.

The 4FGL-DR3 update is described in Fermi Large Area Telescope Fourth Source Catalog Data Release 3.

The 4FGL-DR4 update is described in Fermi Large Area Telescope Fourth Source Catalog Data Release 4.

Additional information is available at http://fermi.gsfc.nasa.gov/ssc/data/access/lat/14yr_catalog/.

**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).

**Data_Release**

The data release in which the source was added to the catalog: '1' for the
original 8-year catalog, '2' for the 10-year catalog, '3' for the 12-year
catalog, '4' for the 14-year catalog.

**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, 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.

**LP_Epeak**

Peak energy in nuFnu estimated from LogParabola model.

**LP_Epeak_Error**

The 1-sigma uncertainty on peak energy for LogParabola model.

**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_S**

The best fit local photon index at pivot_energy in the PLSuperExpCutoff model.

**PLEC_Index_S_Error**

The 1-sigma uncertainty on plec_index_s for PLSuperExpCutoff fit.

**PLEC_Exp_Factor_S**

The best fit local spectral curvature at pivot_energy in the PLSuperExpCutoff
model.

**PLEC_Exp_Factor_S_Error**

The 1-sigma uncertainty on plec_exp_factor_s for PLSuperExpCutoff fit.

**PLEC_Exp_Index**

The exponential index for PLSuperExpCutoff fit.

**PLEC_Exp_Index_Error**

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

**PLEC_Curve_Significance**

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

**PLEC_Epeak**

Peak energy in nuFnu estimated from PLSuperExpCutoff model.

**PLEC_Epeak_Error**

The 1-sigma uncertainty on peak energy for PLSuperExpCutoff model.

**Npred**

The number of predicted events in the model.

**Variability_Index**

The sum of the log(likelihood) difference between the flux fitted in each
time interval and the average flux over the full catalog interval; a value
greater than 27.69 over 12 intervals indicates <1% chance of being a steady
source. See the accompanying paper for details.

**Frac_Variability**

The fractional variability computed from the fluxes in each year.

**Frac_Variability_Error**

The 1-sigma uncertainty on exponential index on the fractional variability.

**Significance_Peak**

The source significance in peak interval in sigma units.

**Flux_Peak**

The peak integral photon flux from 100 MeV to 100 GeV, in photon/cm^{2}/s.

**Flux_Peak_Error**

The 1-sigma error on peak integral photon flux, in photon/cm^{2}/s

**Time_Peak**

Time of center of interval in which peak flux was measured, in MJD.

**Time_Peak_Interval**

Length of interval in which peak flux was measured, in seconds.

**Alt_GammaRay_Name_1**

Most recent correspondence to previous FGL source catalogs, if any.

**Alt_GammaRay_Name_2**

Most recent correspondence to previous FHL source catalogs, if any.

**Alt_GammaRay_Name_3**

Name of likely corresponding source from AGL source catalogs, if any.

**Alt_GammaRay_Name_4**

Name of likely corresponding 3EG source, if any.

**Alt_GammaRay_Name_5**

Name of likely corresponding EGR source, if any.

**Alt_GammaRay_Name_6**

Name of corresponding 4FGL-DR2 source, if any.

**Extended_Source_Name**

The extended source identification, if any.

**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

**Source_Type_Alt**

Class designation for an alternative, low-confidence association.

**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 uncertainty 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).