This tutorial provides a step-by-step guide to using the gtburst GUI for GRB and solar flare analysis of GBM and LAT data. You can also watch a video tutorial
Gtburst can be used to do the following:
Before starting this analysis thread, make sure you have the latest version of gtburst. Gtburst adopts a "release early, release often" model, thus it is a good habit to check often for updates. Gtburst can be easily updated via the Update functionality. Menu: Update -> Update to the latest version.
This syncs with the github repository. You may need to sync twice initially to get to the latest version. After each update, you'll need to restart gtburst, which is automatically prompted.
The interface of gtburst consists of:
For this thread, we will analyze GRB080916C, one of the brightest LAT GRBs on record. All outputs from GUI window are in gtburst.log.
Gtburst can download GBM, LLE, and LAT data from the FSSC, can be run on datasets previously downloaded, or can be loaded as a custom dataset.
The trigger details can either be entered manually, or retrieved from the trigger catalog which includes both Swift-BAT and Fermi-GBM triggers.
Click "Browse triggers"
Triggers can be sorted by any of the columns by clicking on the header, or filtered by trigger type.
Note: GBM triggers may not appear immediately in this database, as they depend on the GBM team populating the table, which is done within 48 hours of the trigger.
Select the trigger and click Done (bn080916009 for GRB 080916C in this example).
A box will pop up to confirm data selection. Time Tagged Events (TTE) versus binned CTIME data. TTE data files have finer time resolution and are therefore larger, and can be binned as needed.
Light curves of the selected detectors with be created.
Click Run to select interactively.
Click Tools->Make navigation plots
The navigation plots will pop up in a separate window. The upper plot is the angle between the GRB RA/Dec and the Fermi zenith angle. This indicates when the GRB is and is not occulted by the Earth. A zenith angle cut of 100 is fairly standard for a GRB, but can be adjusted slightly higher if the source is very bright at that time, or lower if the source is fainter and the user is concerned about Earth limb contamination.
The lower panel of the navigation plot is the angle between the GRB RA/Dec and the boresight of the LAT. This indicates when the source is within the LAT FoV. The size of the LAT FoV is dependent on energy and event class.
The navigation plots are in reference to the GRB localization in the GBM GRB catalog, which may be the best available GBM position (~few deg), an announced LAT position (~0.1-1 deg), or a much more accurate position from follow-up (~arcsec). The user can manually adjust the position in the GUI window at this time, or later based upon the counts map. If the user changes the R.A. and Dec. in the initial window, making the navigation plots again will update the plots using the new position.
Click Tasks->Make likelihood analysis
The first step is filtering for the counts map, which can be repeated and optimized. The parameters:
rad - radius of interest (degrees). Customary values corresponds to the 95% containment of the PSF at the emin energy. If you use emin=100 MeV, rad should be 12 deg for any Transient class and 10 deg for Source or cleaner classes.
irf - the event class - in GRB analysis transient class is usually sufficient for short (<100 s) timescales and spectral analysis, and source class is better for longer intervals and localizations. Event Class recommendations for different analyses are discussed at: http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Cicerone/Cicerone_Data/LAT_DP.html
zmax - Zenith angle cut. If the parameter strategy=time (default), any time interval where any part of the ROI is at a Zenith angle larger than zmax is excluded. This is the normal choice for GRB and SF analysis. If strategy=events, all events with a Zenith angle larger than zmax will be excluded. The user is strongly advised against using strategy=events, unless he/she understands exactly its implication, since such choice can introduce systematic uncertainties in the analysis difficult to estimate.
Tstart - time to start analysis relative to GRB trigger. This can be specified either as a time from the trigger time, or as a Mission Elapsed Time (MET). The interface will automatically understand, since MET numbers are very big.
Tstop - time to stop analysis relative to GRB trigger. This can be specified either as a time from the trigger time, or as a Mission Elapsed Time (MET). The interface will automatically understand, since MET numbers are very big.
Emin - minimum energy for analysis in MeV. Normal value is 100 MeV, as going below that requires special attention.
Emax - maximum energy for analysis in MeV
Skybinsize - binning for map
Thetamax - This is an additional cut which will exclude from the analysis time intervals in which the position of the source is at more than Thetamax degrees from the center of the LAT field of view. Since the PSF of the LAT becomes worse and more uncertain at high off-axis angles, this can be used when analyzing bright bursts to reduce the errors on the localization. It is usually not necessary to change this value.
Strategy - method of zenith angle cut
Using strategy=time (the standard value) will exclude from the analysis all time intervals in which any part of the ROI is at Zenith angles larger than the zmax value. Do not change this value unless you know what you are doing! Using strategy=events will exclude from the analysis all events with a Zenith angle larger than zmax, which can introduce subtle systematic errors in the analysis difficult to judge.
You will then see the resulting counts map and photon energy as a function of time. You can click on photons on the right plot, which will be highlighted on the left plot with a small white circle. This is helpful for determining if a particular high energy photon is clustered near others. A small text box will also appear with the ID of the run in which the event was detected, the event ID, the Zenith and the off-axis (theta) angle of that event. You can also zoom in the left plot, and only the photons within your zoomed area will remain in the right plot. This is useful for example to figure out which photons are close to the source position. Note that f you zoom in the right plot, the left plot will NOT change since this would require a new run of the command.
Next we choose the components of our likelihood model. This command will produce a XML file containing your likelihood model, as described here (add a link to the likelihood tutorial). For source class, a particle model of isotr template is appropriate. The other defaults are sufficient. Click Run, once that finishes, click next.
Gtburst will automatically add nearby bright catalog sources to your XML file. Once the dialog box finishes, click Run.
A window summarizing the fit parameters of the model will pop up. You can modify the parameters (e.g. fixing index to some value), or simply click Done to leave everything as it is. If you make changes, be sure to click save, and click Done when finished. Then, once the window with the list of parameters is closed, click Next.
Now you will be given options on the outputs of the likelihood analysis. Optimizeposition=yes will call gtfindsrc at the end of the likelihood analysis and attempt to improve the GRB localization. Showmodelimage=yes will create a model map and display it. This does not have any impact on the actual analysis, but allow you to see a representation of your final model. Spectralfiles=yes will create the pha and rsp files necessary to do spectral analysis in XSPEC or rmfit. Each of these steps will make the analysis take longer. Click run.
Likelihood fit result parameters of GRB, relevant nearby sources, and background models. You must close this window to proceed.
Resulting count map and likelihood model image. GRB 080916C is well detected!
If a substantially improve position is available, enter this position in the start window, which can be reached by clicking Finish, then repeat likelihood analysis from that position.
Tasks->Find source in TS Map
Follow similar steps as the likelihood analysis, ending up with a map and localization. For GRB 080916C, the localization is not improved by the TS map because the statistical error is smaller than the TS map binsize.
Tasks -> Interactively recenter ROI
Make photon selections, click Run. Once it finishes, click Next, and Run.
Click on a new center position on the left counts map. Click Run, and finish. Then repeat the likelihood analysis step at this new location.
Last updated: 02/14/2018