Generating Photon Lists

A photon list in pyXSIM is a three-dimensional HDF5 dataset that contains a distribution of cell or particle positions, velocities, and photon energies within each cell or particle. Once created, it be used to generated simulated X-ray events from a particular line of sight.

Generating a New Photon List from a Data Source

A photon list is generated from a data source, by which we mean any spatially extended object in three dimensions represented in yt. For example, we may have a dataset of a galaxy cluster simulation, and wish to generate photons from the thermal emission of the hot cluster plasma. We can load up the dataset in yt and create a spherical data source centered on the cluster potential minimum:

import yt
from yt.utilities.cosmology import Cosmology
import pyxsim

ds = yt.load("GasSloshing/sloshing_nomag2_hdf5_plt_cnt_0150")
sp = ds.sphere("c", (1.0, "Mpc"))

This sphere object sp can then be used as input to the make_photons() function to create a new photon list. The arguments taken by make_photons() are as follows:

  • photon_prefix: The prefix of the filename(s) to be written. If run in serial, the filename will be "{photon_prefix}.h5", if run in parallel, the filenames will be "{photon_prefix}.{mpi_rank}.h5".

  • data_source: A YTSelectionContainer. The 3D data source from which the photons will be generated.

  • redshift: The cosmological redshift for the photons. Determines the angular diameter and luminosity distances to the source given a cosmology, which determines the number of photons.

  • area: The collecting area to determine the number of photons. If units are not specified, it is assumed to be in \(cm^2\).

  • exp_time: The exposure time to determine the number of photons. If units are not specified, it is assumed to be in seconds.

  • source_model : A SourceModel used to generate the photons. see Source Models for X-ray Emission for options.

  • parameters (optional): A dictionary of parameters to be passed to the source model if necessary.

  • point_sources (optional): If True, the photons will be assumed to be generated from the exact positions of the cells or particles and not smeared around within a volume. Default: False

  • center (optional): string or array-like object. The origin of the photon spatial coordinates. Accepts “c”, “max”, or a coordinate. If not specified, pyXSIM attempts to use the “center” field parameter of the data_source.

  • dist (optional): The angular diameter distance, only should be used for nearby sources. This may be supplied instead of it being determined from the redshift and cosmology. If units are not specified, it is assumed to be in Mpc. To use this, the redshift must be set to zero.

  • cosmology (optional): A Cosmology object which supplies cosmological informaton. If the data_source has cosmological parameters, they will be used, otherwise a \(\Lambda{\rm CDM}\) cosmology with the following parameters are assumed: \(H_0\) = 71 km/s/Mpc, \(\Omega_m\) = 0.27, \(\Omega_\Lambda\) = 0.73.

  • velocity_fields (optional): A list of fields to use for the velocity to Doppler-shift the photons. If not specified, the following will be assumed: ['velocity_x', 'velocity_y', 'velocity_z'] for grid datasets, and ['particle_velocity_x', 'particle_velocity_y', 'particle_velocity_z'] for particle datasets.

  • bulk_velocity (optional): A 3-element array or list specifying the local velocity frame of reference. If not a YTArray, it is assumed to have units of km/s. Default: [0.0, 0.0, 0.0] km/s.

  • observer (optional): Whether the observer is “external”, and the flat-sky approximation can be assumed for the purposes of projecting photons onto the sky, or the observer is “internal” and a spherical projection is assumed. Default is “external”. For more details on the “internal” observing mode see “Internal” Observers and “All-Sky” Observations.

  • fields_to_keep (optional): A list of fields to add to the HDF5 photon list in addition to the cell or particle positions, velocities, and sizes. Default: None

As an example, we’ll assume we have created a source_model representing the thermal emission from the plasma (see Source Models for X-ray Emission for more details on how to create one):

redshift = 0.05 # The redshift to the object.
area = (3000., "cm**2") # A constant effective area to generate the photons with.
exp_time = (100., "ks") # The exposure time to generate the photons with.
center = # A center in 3D for the photon positions. If not specified,
                   # the center of the `data_source` will be chosen.

# Optionally, construct a cosmology object.
cosmo = Cosmology(hubble_constant=0.68, omega_matter=0.31, omega_lambda=0.69)

n_photons, n_cells = pyxsim.make_photons("my_photons", sp, redshift, area,
                                         exp_time, source_model,
                                         center=center, cosmology=cosmo)

If you run on one core, this will write a file called "my_photons.h5" containing the photon list. If run on (say) 6 cores, it will write 6 files, called "my_photons.[0-5].h5". The total number of photons is returned in n_photons, and the total number of cells with photons is returned in n_cells.

If you want to simulate photons from a a nearby object, set the redshift to zero and specify a distance using the dist keyword argument:

n_photons, n_cells = pyxsim.make_photons("my_photons", sp, 0.0, area,
                                         exp_time, source_model,
                                         center=center, dist=(4., "kpc"))

By default, the photons generated from the cells or particles in the simulation will be smeared throughout the volume of those elements. To treat all of the cells or particles in the dataset as if they are point sources, set point_sources=True:

n_photons, n_cells = pyxsim.make_photons("my_photons", sp, redshift, area,
                                         exp_time, source_model,
                                         center=center, point_sources=True)

By default, for computing the Doppler shifts of the photons, pyXSIM uses the default velocity fields of the dataset, which are "velocity_x", "velocity_y", and "velocity_z" for grid/cell-based datasets and "particle_velocity_x", "particle_velocity_y", and "particle_velocity_z" for particle-based datasets. If you need to use other fields, you can specify them using the velocity_fields keyword argument:

vfields = [("flash", "velx"), ("flash", "vely"), ("flash", "velz")]
n_photons, n_cells = pyxsim.make_photons("my_photons", sp, redshift, area,
                                         exp_time, source_model,
                                         center=center, dist=(4., "kpc"),

We can also add other fields to the file using the fields_to_keep option, which will not be used for photon projection in later steps but can be used for diagnostic and/or analysis purposes. These represent the fields at the positions where photons have been generated from. For example, to add density and temperature fields to the file:

fields_to_keep = [("gas", "density"), ("gas", "temperature")]
n_photons, n_cells = pyxsim.make_photons("my_photons", sp, redshift, area,
                                         exp_time, source_model,
                                         center=center, dist=(4., "kpc"),

Merging Photon Lists

Photon lists which have been written to files can be merged together, using the merge_files() function. This may be useful if you generate photons from different sources or source types that are co-spatial.

merge_files() takes a list of input filenames, and an output filename. The optional keyword arguments are overwrite, which decides whether or not an existing file will be overwritten, and add_exposure_times decides whether or not the final file will have an exposure time of the sum of the times in the separate files or that of the longest exposure time between the files.

from pyxsim import merge_files
merge_files(["photons_0.h5","photons_1.h5","photons_3.h5"], "photons.h5",
            overwrite=True, add_exposure_times=True)

The PhotonList Class

Generated photons stored to an HDF5 file can be examined with the PhotonList class. Currently, this class can be used to examine parameters of a photon list in the parameters dictionary:

photons = pyxsim.PhotonList("therm_photons.h5")
{'bulk_velocity': array([0., 0., 0.]),
 'center': array([0., 0., 0.]),
 'data_type': 'cells',
 'fid_area': 500.0,
 'fid_d_a': 122.21820987067642,
 'fid_exp_time': 100000.0,
 'fid_redshift': 0.03,
 'hubble': 0.71,
 'observer': 'external',
 'omega_lambda': 0.73,
 'omega_matter': 0.27,
 'velocity_fields': array([[b'gas', b'velocity_x'],
        [b'gas', b'velocity_y'],
        [b'gas', b'velocity_z']], dtype='|S10')}

as well as other pertinent information in the info dictionary:

{'data_source': 'YTSphere (UniformGridData): , center=[0. 0. 0.] cm, radius=1.5428387904811624e+24 cm',
 'dataset': 'UniformGridData',
 'pyxsim_version': '4.1b1.dev29+g1c09873.d20221228',
 'source_model': "CIESourceModel(
                      emin=1 keV
                      emax=80.0 keV
                      temperature_field=('gas', 'temperature')
                      emission_measure_field=('gas', 'emission_measure')
 'soxs_version': '4.2.1',
 'yt_version': '4.2.dev0'}

If this photon list file has originated from merged photon lists, then there will be multiple instances of each piece of information, numbered by the file, e.g. "soxs_version_0", "soxs_version_1", and so on. The original files used to make the merge will be stored in the key "original_files".


To produce a spectrum binned on energy, call write_spectrum().

specfile = "myspec.fits" # filename to write to
emin = 0.1 # minimum energy of spectrum
emax = 10.0 # maximum energy of spectrum
nchan = 2000 # number of bins in spectrum
photons.write_spectrum(specfile, emin, emax, nchan, overwrite=False)

This bins the photon energies using the emin, emax, and nchan arguments into a histogram which will be written to the file as a spectrum. As usual, the overwrite argument determines whether or not a file can be overwritten.