Notes on Sources
Here are some notes about using the gamma-ray sources that are available for use with obsSim.
The XML file for the sources described below, with annotations about the definitions of the parameters, are available here.
FT1 event files generated for these sources used an FT2 file for a one-week scanning/rocking observation. These files are available here. For all of these FT1 files, I used the random seed 34621 in obsSim.par. There's nothing special about this number, but don't be surprised when you get different gamma rays if you use a different seed. All FT1 files are for the FRONT/BACK (i.e., DC1) response functions.
I can't personally vouch for the correctness of the output for any of these sources. I haven't seen anything that looks obviously wrong yet, but I haven't tried quantitatively studying the numbers of gamma rays that the sources generate and I haven't seriously tried any timing studies.
TransientTemplate (Chiang)
Here is a very simple example of a transient source. Note that observationSim/v5r1p3/xml/obsSim_source_library.xml has more examples of TransientTemplate sources.
The parameter values in params="..." below are as follows:
1. source flux (photons m -2 s -1 above 100 MeV)
2. photon spectral index for a power-law spectrum
3. start time (in s)
4. stop time (in s) for the flare (both measured with respect to whatever the 0 of time is
5. name of the file that has the flare profile
NOTE: Two other arguments can follow the file name: Emin and Emax (MeV). The gamma rays will be generated within the energy range [Emin, Emax]. The values for the limits default to 30 MeV and 1e5 MeV if not specified.
The file (item 5 above) contains two columns, time and flux. They are BOTH
RESCALED; the time column is scaled to match the start and stop times specified in the XML file and the flux entries are scaled so that the MEAN flux is the same as specified in the XML file. transient_profile.dat is available here. The profile rises linearly from 0 for the first 1/3rd of the time, plateaus for the next 1/3rd, and then falls linearly to 0 again in the final 1/3rd of the time interval.
<source name="flaring_source">
<spectrum escale="MeV">
<SpectrumClass name="TransientTemplate"
params="0.2,2,1e5,1.2e5,transient_profile.dat"/>
<celestial_dir ra="80" dec="20"/>
</spectrum>
</source>
Here is the distribution of arrival times for the gamma rays from this simulation. Note that during the 20 ks that the source is on, the source is seen on 3 different orbits (with 3 different aspect angles). Also shown are the RA and Dec of the LAT z-axis on the same time scale. The count rates are lower for the first and third orbits because the LAT is pointed further away from the source (which has RA, Dec = 80 deg, 20 deg) and because the source has lower flux at those times.
PulsarSpectrum (Omodei & Razzano)
The source below is lifted from celestialSources/Pulsar/v1r0p0/xml/Pulsar_user_library.xml. It more or less describes the Crab for some epoch.
This source does not presently apply arrival time corrections to take into account the motion of the LAT or general relativistic effects.
The parameters in the params="..." attribute below are as follows
1. I don't know
2. Pulsar fluence (erg cm -2)
3. Period (s)
4. Period derivative (s per s)
5. Number of peaks (in light curve, I guess)
6-9. Parameters of the functional form describing the spectrum (description to be linked in)
NOTE: The period derivative is ignored in Science Tools v5r1; you will want at least v5r1p1 to get the version of the source that actually uses the period derivative.
<source name="PSRCrabPolarPhen" >
<spectrum escale="MeV"> <SpectrumClass name="PulsarSpectrum" params=" 1,2
.3e-6,0.033,422e-15,2,1e6,30e6,-1.9,0.29"/>
<galactic_dir l="184.56" b="-5.78"/>
</spectrum>
</source>
Here is the distribution of the events on the sky (with some outliers cropped off) for this simulation. Also shown is the overall counts spectrum for the one-week observation.
Note: This source apparently also leaves behind a file called pulsar.root in the directory from which you run obsSim.
GalPulsars (Hierath)
There's nothing really to specify in the XML file. All of the information is in the file named in the params attribute, which is human readable. The file flux/v8r10p2/sources/pulsars.txt has many more pulsars.
This source does keep track of where the LAT is and where the earth is in its orbit in order to make arrival time corrections. I have not personally tested this yet. Because this source needs to use an actual ephemeris for the solar system, the absolute times will matter.
<source name="vela-galactic">
<spectrum escale="GeV">
<SpectrumClass name="GalPulsars" params="vela.txt"/> <use_spectrum frame="galaxy"/>
</spectrum>
</source>
Here is the distribution of the events on the sky (with some outliers cropped off) for this simulation. Also shown is the overall counts spectrum for the one-week observation.
Note: I do not know right now why this source generated so many fewer gamma rays than the one above (470 vs. 1842). Nominally Vela is brighter. I used the source specifications exactly as they were given in the XML files in the repository.
GRBmanager (Omodei & Cohen-Tanugi?)
The source below is mostly lifted from celestialSources/GRB/v4r0p7/xml/GRB_user_library
The parameters in the params="..." attribute are as follows
1. Time of initial GRB (s)
2. Interval between GRBs (s)
I don't know what the time reference is, i.e., what absolute time 0 corresponds to.
Nothing else other than the coordinates of the GRB can be specified. You must be careful to have the GRBs go off when they are in the FOV of the LAT. One unphysical way to do this (pointed out to me by Johann) is to replace the galactic_dir line with, e.g., <direction frame="glast" theta="0" phi="0"/>, which fixes the GRB to be right on the LAT z-axis no matter where it is pointed.
Note: this source will also make grb_#####.root files in your current directory. These files are alleged to contain 2-dim histograms of the GRB and are generated whether or not the GRB was within the FOV. -->
<source name="GRB_50_60">
<spectrum escale="MeV"> <SpectrumClass name="GRBmanager" params="10000
,20000"/>
<galactic_dir l="50" b="60"/>
</spectrum>
</source>
Here are the times and energies of the 104 gamma rays that this source generated for the one week observation period. The second plot is the time history of the gamma rays in the intense burst near the end of the week.
Diffuse interstellar emission with MapSource (Chiang?)
The source below is lifted from observationsSim/v5r1p3/xml/obsSim_source_library.xml
The 'MapSpectrum' source can also be used for this purpose. I don't know whether one is superior to the other. The definitions of the params="..." attributes are given below:
- Total photon flux from the map (#/m^2/s) = 17.
- Photon spectral index = 2.1
- Fits file (plate-carree only) = $(FLUXROOT)/sources/gas_gal.fits
- Minimum photon energy (MeV) = 30.
- Maximum photon energy (MeV) = 2e5
gas_gal.fits is the EGRET team's interstellar emission model (basically Hunter et al. 1997). The file is a 2-dimensional array of the integrated intensity (>100 MeV) on a 30' grid in Galactic coordinates. For MapSource there's nothing special about the grid size, or using Galactic vs. celestial coordinates. The important point is that the image be in a flat (plate-caree) projection, as mentioned above.
This source generated 502320 gamma rays for the run that I made. In obsSim.par, I had set a limit of 200k events per output FITS file, so the output spans 3 FITS files.
Note that gas_gal.fits does not contain extragalactic diffuse emission. This was determined with EGRET to have an isotropic distribution with integrated intensity 1.5 x 10 -5 cm -2 s -1 sr -1 (>100 MeV, Sreekumar et al. 1998). The expectation is that the LAT will resolve a significant fraction of this into point sources (blazars). Still, the isotropic intensity or its equivalent in point sources should be included in the model for realistic representation of the celestial gamma-ray background. You can do this either by adding a constant intensity to gas_gal.fits and writing it out in a new FITS file, or you can define an additional, isotropic source for obsSim.
<source name="Galactic_diffuse">
<spectrum escale="MeV">
<SpectrumClass name="MapSource"
params="17.,2.1,$(FLUXROOT)/sources/gas_gal.fits,30.,2e5"/>
<use_spectrum frame="galaxy"/>
</spectrum>
</source>
Isotropic emission as a source in instrument coordinates (who wrote it?)
This source is actually defined in terms of cos(theta), i.e., with respect to the instrument axis rather than in some celestial system. You can get away with this for an isotropic celestial source. I assume that occultation by the earth is properly taken into account, although it hardly matters for the step-rocking observation strategy. Occultation of this source would be worth checking, though, for a pointed observation.
The source below implements the extragalactic diffuse inferred by EGRET (Sreekumar et al. 1998). The flux has units of photons m -2 s -1 integrated between Emin and Emax. The photon spectral index is gamma, i.e., dN/dE = Const*E -gamma .
<source name="extragalactic" flux="0.545">
<spectrum escale="MeV">
<particle name="gamma">
<power_law emin="30." emax="100000." gamma="2.1"/>
</particle>
<solid_angle mincos="-0.4" maxcos="1."/>
</spectrum>
</source>
This source generated 163,716 gamma rays in the one-week observation.