The FSSC is now serving Pass 7 LAT data for analysis. The new version of LAT data provides a number of improvements over Pass 6, and is considered the best dataset for all types of LAT analysis. Pass 6 data will no longer be served. However, pre-existing Pass 6 data are archived. Here we discuss the changes and how they affect your analysis.
As a result of the updated event classification, many (~30%) of the events in the Pass 7 data are new events that were not present in Pass 6. Some of the Pass 6 events have been reclassified as background, and are no longer in the event data. Pass 7 should be treated as a new LAT data set. (Related Cicerone discussion)
In Pass 6, both photon and extended data files contained all events. However, with the increase in effective area at low energies, the Pass 7 Transient class is now significantly larger than the Pass 6 Transient class. As a result, Pass 7 Transient class events are only present in the Extended data files. Pass 7 Source class events and better (those used for most non-transient (>200 s) analyses) continue to be present in both file types.
In order to allow events to belong to multiple classes, the EVENT_CLASS data field is now a bitfield. The four main classes are: Transient (bit 0), Source (bit 2), Clean (bit 3), and Ultraclean (bit 4). Bit 1 is unused. As before, the analysis classes (Transient through Ultraclean) are hierarchical. (Related Cicerone discussion)
The precalculated difuse response columns have been populated with values for the Pass 7 Source and Pass 7 Clean event classes. These precalculated diffuse responses were generated using the new Pass 7 Galactic diffuse model and class-specific isotropic models (described below). If you choose to use any other diffuse models, you must use gtdiffrsp to calculate the diffuse reponse of those model prior to performing an unbinned likelihood analysis. This includes when you are using one of the 12 individual extended sources included in the 2FGL catalog.
Weekly files for the Pass 7 data contain the Source, Clean, and dataclean event classes. Transient events are only available from the LAT Data Server. Pass 7 weekly data and spacecraft files are available at the ftp site, and can be retrieved with wget:
wget -m -P . -nH --cut-dirs=4 -np -e robots=off http://fermi.gsfc.nasa.gov/FTP/fermi/data/lat/weekly/photon/
wget -m -P . -nH --cut-dirs=4 -np -e robots=off http://fermi.gsfc.nasa.gov/FTP/fermi/data/lat/weekly/spacecraft/
Pass 6 weekly files created prior to August 1, 2011 have been archived and are available via ftp:
Pass 6 Version 11: http://heasarc.gsfc.nasa.gov/FTP/fermi/data/lat/weekly/p6v11/
Pass 6 Version 3: http://heasarc.gsfc.nasa.gov/FTP/fermi/data/lat/weekly/p6v3/
No new data (acquired since August 1, 2011) will be processed using the Pass 6 analysis.
Recovering effective area at low energies was a priority in the Pass 7 event classification. Biases, for example in the exposure calculation, caused by the energy dispersion in Pass 6 became very apparent in energy ranges where the effective area is changing rapidly (below a few hundred MeV). For Pass 7, the increase in effective area helped, but did not eliminate the behavior. Energy dispersion effects still should be considered at low energies. (Related Cicerone discussion)
Dependence of the effective area on the photon azimuthal angle (phi) was first introduced in the P6_V11 IRFs. When the livetime cube is generated using a specified number of phi bins, the exposure calculation (e.g., in gtexpmap or gtexcube2) will reflect this. Otherwise, the average aziumuthal response will be used. (Related Cicerone discussion)
The instrument PSF was characterized post-launch by using a population of AGN and is included in the Pass 7 IRFs. To accommodate low statistics in the measurement of the PSF, there is no longer dependence of the PSF on inclination angle (theta) and instead a description representing the two-year average is given. This does not apply to Transient class, which still uses the pre-launch PSF formulation. The use of the theta-averaged PSF is only a concern in analyses for which the source is not observed over a large range of inclination angles. The worst cases are generally GRBs. (Related Cicerone discussion)
The pixel size is now 0.125° and has been adjusted to place a row centered at |b|=0°.
A new HI map that does not include the Magellanic stream was used to generate the model. In addition, several non-pointlike structures seen in the all-sky residuals were removed through the used of dedicated templates. These structures include: Loop 1 north polar spur, Loop 1 excess, high-energy galactic lobes, and three prominent Galactic plane excesses. As a result, the described features may disappear from the dataset when this model is used. (Full description of Galactic diffuse model)
The isotropic diffuse model is always paired with a specific Galactic diffuse model and a specific data set. Once the Galactic model changed, it was necessary to redefine the isotropic model for the Pass 7 class selections using that new Galactic model. You should not use this model with Pass 6 data, nor with a different Galactic diffuse model. And you should not use Pass 6 models with Pass 7 data.
A new version of the Fermi Science Tools has been released and is available for download. You will need these tools to be able to properly analyze Pass 7 data.
gtselect now includes a new parameter, evclass, which is used to specify the bit number corresponding to the event class for the data you wish to analyze. gtselect is also backwards compatible and will accept evclsmin and evclsmax for use when analyzing previously downloaded Pass 6 data.
A new tool, gtexpcube2 has been introduced to generate binned exposure maps for use in binned likelihood analysis. This tool replaces the on-the-fly generation of an exposure map by gtsrcmaps. In addition, gtsrcmaps now requires that you account for exposure out to 10° beyond your source region (due to the large PSF at low energies). Practically speaking, this means generating an exposure map with gtexpcube2 that exceeds the boundaries of the counts cube used in your analysis by 10° on all sides. (see the Binned Likelihood analysis thread)
If you need access to the previous set of documentation and analysis threads appropriate for Pass 6 data analysis, click here.