Generating Spectra from Data Objects¶

If you have a yt data object (such as a sphere, box, disk) and a source model of any sort, then you can also generate spectra from the entire object. This can be done in two modes–either in the rest frame of the source, in which case the spectrum will be a “count rate” spectrum in units of counts s^-1 keV^-1, or in an observer frame at some distance in which case the spectrum will be in units of counts s^-1 cm^-2 keV^-1.

Assuming one has a dataset and (say) a sphere object, you can generate spectra like this:

# Here's a power-law source model, but any source model will do

emin = 0.5
emax = 40.0
e0 = 1.0
alpha
norm_field = ("gas", "power_law_emission")

plaw_model = pyxsim.PowerLawSourceModel(e0, emin, emax, norm_field, alpha)

# Make a count rate spectrum in the source frame

nbins = 1000

spec_src = plaw_model.make_spectrum(sp, emin, emax, nbins)

The resulting spectrum spec_src is a CountRateSpectrum object, which has a number of methods in SOXS that can be used to analyze and visualize it.

If we instead want to find a spectrum of a source measured by an observer at a specific distance in their own reference frame, then we can specify either a redshift and a cosmology (which uniquely specifies a distance) or we can set a distance explicitly for a local source.

Here’s an example where only a redshift is specified. In this case, a cosmology is assumed by default from yt, usually the one associated with the dataset:

# Make a flux spectrum in the observer frame at some redshift

emin_obs = 2.0
emax_obs = 20.0
redshift = 0.1

spec_obs = plaw_model.make_spectrum(sp, emin, emax, nbins, redshift=redshift)

The resulting spectrum spec_obs is a Spectrum object, which has a number of methods in SOXS that can be used to analyze and visualize it.

If you want to choose a different cosmology, specify a yt Cosmology object:

# Make a flux spectrum in the observer frame at some redshift
# at a specified cosmology

from yt.utilities.cosmology import Cosmology

cosmo = Cosmology(hubble_constant=0.704, omega_matter=1.0-0.728)

emin_obs = 2.0
emax_obs = 20.0
redshift = 0.1

spec_obs = plaw_model.make_spectrum(sp, emin, emax, nbins, redshift=redshift,
                                    cosmology=cosmo)

You can also simply specify a distance in the dist keyword argument, if the source is local (but note that in this case you cannot specify a redshift at the same time):

# Make a flux spectrum in the observer frame at some local distance

emin_obs = 2.0
emax_obs = 20.0

spec_obs = plaw_model.make_spectrum(sp, emin, emax, nbins, dist=(8.0, "kpc"))

Finally, it is also possible to simulate the Doppler shifting of the spectrum from the velocity field of the source. This is done by specifying the normal keyword argument, which can be either be a string corresponding to one of the coordinate axes (e.g. “x”, “y”, or “z”), or a 3-vector of the form [a, b, c] that specifies the normal direction to the source. This will pick out the component of the velocity along the sight line from each cell or particle and Doppler-shift the spectrum from that volume element before adding it to the total summed spectrum.

emin_obs = 2.0
emax_obs = 20.0

# Make a flux spectrum in the observer frame at some local distance with
# Doppler shifting along the z-axis

spec_obs1 = plaw_model.make_spectrum(sp, emin, emax, nbins, dist=(8.0, "kpc"),
                                     normal="z")

# Make a flux spectrum in the observer frame at some redshift with
# Doppler shifting along an arbitrary direction

spec_obs2 = plaw_model.make_spectrum(sp, emin, emax, nbins, redshift=0.05,
                                     normal=[3.0, -0.2, 1.0])

An example of how the Doppler shifting works is in the cookbook example Thermal_Emission.

Note

Doppler shifting via the normal keyword is only possible if a distance or redshift is specified, since otherwise the spectrum is by definition computed in the rest frame of the source.