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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.

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 source 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.

(info) 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.

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.

More source descriptions to come

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