Fermi GBM Spectral Fits Data (gdt.missions.fermi.gbm.scat)

GBM provides standard spectral fits for each GRB triggered on-board. These spectral fits are hosted in FITS format at HEASARC as “SCat” (Spectral Catalog) files. The SCat files contain the spectral fit data, including fit parameters, uncertainties, fluxes/fluences, fit statistic, covariance. The files also contain metadata about the detectors and data that were used in the fit. All of this information can be easily accessed using the Scat class.

We can read a SCat FITS file:

>>> from gdt.core import data_path
>>> from gdt.missions.fermi.gbm.scat import Scat
>>> filepath = data_path.joinpath('fermi-gbm/glg_scat_all_bn170817529_flnc_comp_v00.fit')
>>>
>>> scat
<Scat: glg_scat_all_bn170817529_flnc_comp_v00.fit;
 4 detectors; 1 fits>

We can acces the list of model fits by:

>>> scat.model_fits
[<GbmModelFit: Comptonized, Epeak>]

The GbmModelFit class contains the all of the relevant info for a fit. Let’s look at a few properties contained in this class:

>>> one_fit = scat.model_fits[0]
>>> one_fit.time_range
(-0.192, 0.064)
>>> one_fit.stat_name
'Castor C-STAT'
>>> one_fit.stat_value
516.5009155273438
>>> one_fit.dof
479
>>> one_fit.name
'Comptonized, Epeak'
>>> one_fit.parameter_list()
['Amplitude', 'Epeak', 'Index', 'Pivot E =fix']

For the actual fit data, they are stored as Parameter objects, which group together the fit value, uncertainty, and metadata such as the parameter name and units. If we want to quickly see the fit information, we can simply print the GbmModelFit object:

>>> print(one_fit)
Comptonized, Epeak
   Amplitude: 0.03 +/- 0.02
   Epeak: 215.09 +/- 54.22
   Index: 0.14 +/- 0.59
   Pivot E =fix: 1.00e+02 +/- 0.00e+00

Otherwise, we can directly access the parameter fit info by attributes and methods of the Parameter class:

>>> epeak = one_fit.parameters[1]
>>> epeak.name
'Epeak'
>>> epeak.value
215.09434509277344
>>> epeak.uncertainty
(54.219116, 54.219116)
>>> epeak.one_sigma_range()
(160.87522888183594, 269.31346130371094)

For each fit, the photon and energy fluxes and fluences are also recorded, and those are stored as special Parameter objects as well:

>>> print(one_fit.photon_flux)
Photon Flux: 2.81 +/- 0.44 ph/cm^2/s

>>> print(one_fit.energy_fluence)
Energy Fluence: 1.40e-07 +/- 3.03e-08 erg/cm^2

As for the detector metadata used in the fits, they are stored in GbmDetectorData objects:

>>> scat.detectors
[<GbmDetectorData: BGO_00; TTE;
  time range: (-0.192, 0.064) s;
  energy range: (284.65, 40108.0) keV>,
 <GbmDetectorData: NAI_01; TTE;
  time range: (-0.192, 0.064) s;
  energy range: (8.501, 903.45) keV>,
 <GbmDetectorData: NAI_02; TTE;
  time range: (-0.192, 0.064) s;
  energy range: (9.119, 902.57) keV>,
 <GbmDetectorData: NAI_05; TTE;
  time range: (-0.192, 0.064) s;
  energy range: (8.698, 909.58) keV>]

We can access various properties stored in these objects. For example:

>>> [det.detector for det in scat.detectors]
['BGO_00', 'NAI_01', 'NAI_02', 'NAI_05']
>>> [det.channel_range for det in scat.detectors]
[(3, 124), (5, 124), (5, 124), (5, 124)]
>>> # was the detector used in the fit?
>>> [det.active for det in scat.detectors]
[True, True, True, True]

In addition to these properties, the deconvolved photon counts, errors, and detector-convolved photon model are stored as well. These can be used for making the model count rate plots. For example:

>>> # the detector-convolved photon model values for the second detector
>>> scat.detectors[1].photon_model
array([1.96157284e-02, 1.99605655e-02, 2.02203058e-02, 2.04198491e-02,
       2.05748081e-02, 2.06954200e-02, 2.07886994e-02, 2.08651833e-02,
       ...
       5.15691772e-06, 3.84230407e-06, 2.66911161e-06, 3.89260578e-07],
     dtype=float32)

Reference/API

gdt.missions.fermi.gbm.scat Module

Classes

Scat()	A container class for the spectral fit data in an SCAT (Spectroscopy CATalog) file.
GbmDetectorData()	A container for the detector info, with values used in the GBM SCAT files.
GbmModelFit()	A container for the info from a model fit, with values used in the GBM SCAT files.

Class Inheritance Diagram