Fermi Gamma-ray Space Telescope

Caveats About the Fermitools

As the Fermitools mature, issues and errors will be identified and corrected. This page will document such errors, and provide information on which version of the tools contain the correction. For the current release, the known bugs are documented here.

The Science Tools releases that are addressed on this page are:

Science Tools - Release v9r27p1

gtmktime:

When analyzing short time periods (e.g. during GRBs), keep in mind that you might lose some good events for time intervals shorter than a few seconds.

gtltcube:

Using values of dcostheta such as 0.01 can result in a different binning being used on different platforms.

gtlike:

Binned Likelihood:

  • gtfindsrc cannot be used with binned likelihood
  • When using a FITS image as an input model to gtlike, care needs to be exercised to make sure that the non-zero part of the image approximately fills the input map. If this is not true, then when integrating the instrumental parameters for the source the program may not sample the source sufficiently which can lead to biased results.

pulsar tools:

When the pulsar tools read an orbital ephemeris from pulsar ephemeris database (text or FITS), it converts the unit of OMDOT parameter (the first time derivative of periastron longitude) from "degrees per year" (as stored in pulsar ephemeris database) to "radians per second" (for use in computations of binary demodulation). A bug was found in the conversion. Explicitly speaking, the current pulsar tools use 365 days for a year for the conversion, when it should be 365.25 days. As a result, the pulsar tools uses a larger value for OMDOT than the actual value.

The bug affects all users who used orbital ephemeri(de)s with non-zero OMDOT value. Due to the nature of the bug, it is difficult to give an estimate on the impact on analysis results in general.

Certain pulsar tools require that the GTIs be listed so that the earliest time is first and the latest time is last. The Fermi Science Tools correctly maintain ordering of GTIs within the file. However, use of outside packages to manipulate or modify event files could cause incorrect GTI sequencing, producing scientifically inaccurate results.

gtorbsim:

The sign convention used for the rocking profile in gtorbsim is the opposite of that used in the spacecraft file. For the input rocking profile, a positive rocking angle is used to indicate a rock toward the north orbit pole. However, a positive rocking angle in the spacecraft file indicates a rock toward the south orbit pole.

Science Tools - Release v9r15p2

gtmktime:

When analyzing short time periods (e.g. during GRBs), keep in mind that you might lose some good events for time intervals shorter than a few seconds.

gtltcube:

Using values of dcostheta such as 0.01 can result in a different binning being used on different platforms.

gtlike:

Binned Likelihood:

  • gtfindsrc cannot be used with binned likelihood
  • When using a FITS image as an input model to gtlike, care needs to be exercised to make sure that the non-zero part of the image approximately fills the input map. If this is not true, then when integrating the instrumental parameters for the source the program may not sample the source sufficiently which can lead to biased results.

Unbinned Likelihood:

  • The fit to the diffuse components consistently applies more flux to the isotropic component than is appropriate. A quantification of this effect is not yet available, but is within the systematics indicated by the LAT instrument team.
    This issue was corrected in version v9r27p1 of the Science Tools, released April 18, 2012

pulsar tools:

When the pulsar tools read an orbital ephemeris from pulsar ephemeris database (text or FITS), it converts the unit of OMDOT parameter (the first time derivative of periastron longitude) from "degrees per year" (as stored in pulsar ephemeris database) to "radians per second" (for use in computations of binary demodulation). A bug was found in the conversion. Explicitly speaking, the current pulsar tools use 365 days for a year for the conversion, when it should be 365.25 days. As a result, the pulsar tools uses a larger value for OMDOT than the actual value.

The bug affects all users who used orbital ephemeri(de)s with non-zero OMDOT value. Due to the nature of the bug, it is difficult to give an estimate on the impact on analysis results in general.

Certain pulsar tools require that the GTIs be listed so that the earliest time is first and the latest time is last. The Fermi Science Tools correctly maintain ordering of GTIs within the file. However, use of outside packages to manipulate or modify event files could cause incorrect GTI sequencing, producing scientifically inaccurate results.

gtorbsim:

The sign convention used for the rocking profile in gtorbsim is the opposite of that used in the spacecraft file. For the input rocking profile, a positive rocking angle is used to indicate a rock toward the north orbit pole. However, a positive rocking angle in the spacecraft file indicates a rock toward the south orbit pole.

The FSSC is working with the instrument teams to develop solutions to these known issues.

Please contact the FSSC helpdesk for assistance.

Last updated by: Elizabeth Ferrara - 09/28/2012