Background Models in SOXS¶

SOXS simulates background for every observation. The background in SOXS is comprised of three components: a uniform galactic foreground, a point-source background, and an instrumental/particle background. The former two components are not tied to any particular instrument specification, whereas the latter depends on the instrument being simulated. We will describe each of these background components in turn.

Galactic Foreground Model¶

Two models for the galactic foreground are available in SOXS, "default" and "halosat". The

The `"default"` Foreground Model¶

The "default" galactic foreground component is modeled as a sum of two thermal models with thermal broadening of emission lines, one absorbed, bapec+tbabs*bapec, with parameters:

tbabs*bapec Model 1, Hot Halo

nH: \(0.018 \times 10^{22}~\rm{cm}^{-2}\)
kT: \(\rm{0.225~keV}\)
abund: \(\rm{1.0~Z_\odot}\)
redshift: \(0.0\)
norm: \(\rm{7.3 \times 10^{-7}~10^{-14}\frac{\int{n_en_HdV}}{4{\pi}D_A^2(1+z)^2}}\)

bapec Model 2, Local Hot Bubble

kT: \(\rm{0.099~keV}\)
abund: \(\rm{1.0~Z_\odot}\)
redshift: \(0.0\)
norm: \(\rm{1.7 \times 10^{-6}~10^{-14}\frac{\int{n_en_HdV}}{4{\pi}D_A^2(1+z)^2}}\)

This model is from McCammon et al. (2002).

The `"halosat"` Foreground Model¶

The "halosat" foreground model is the same as the "default", except that it includes an additional absorbed thermal model (with tbabs) for the Hot Halo, based on HaloSat observations:

tbabs*bapec Model 3, Hot Halo

nH: \(0.018 \times 10^{22}~\rm{cm}^{-2}\)
kT: \(\rm{0.7~keV}\)
abund: \(\rm{1.0~Z_\odot}\)
redshift: \(0.0\)
norm: \(\rm{8.76 \times 10^{-8}~10^{-14}\frac{\int{n_en_HdV}}{4{\pi}D_A^2(1+z)^2}}\)

In either case, the background is diffuse and uniformly fills the entire field of view of the instrument you choose to simulate. The absorption model, neutral hydrogen column density, the abundance for the hot halo components, or the APEC version for the foreground can be changed. One can also add velocity broadening to the emission lines for the hot halo components. For more information, see SOXS Configuration File.

Note

The power-law component from unresolved point sources is not included in this model since it is modeled separately in SOXS, as we detail next.

Point Source Background Model¶

Another astrophysical component of the background in SOXS comes from resolved point sources. The emission of these sources is assumed to originate from cosmologically distant AGN and galaxies. The fluxes for these sources are drawn from \(\rm{log}~N-\rm{log}~S\) distributions taken from Lehmer et al. (2012)’s study of the Chandra Deep Field South. The point sources have fluxes in the 0.5-2 keV band in the \(7.63 \times 10^{-22} - 1.0 \times 10^{-12}~\rm{erg}~\rm{s}^{-1}~\rm{cm}^{-2}\) range.

Each point source is given a power-law spectrum. Galaxies are assumed to have a spectral index of \(\alpha = 2.0\). The spectral indices of AGN sources are drawn from a fit to the spectral index distribution of sources given in Figure 13a of Hickox & Markevitch 2006. Sources are absorbed by foreground Galactic neutral hydrogen assuming a neutral hydrogen column of \(n_H = 0.018 \times 10^{22}~\rm{cm}^{-2}\) and the wabs model by default. The absorption model and the value of the hydrogen column can be changed using the SOXS Configuration File. The position of each point source is uniformly randomly distributed within the field of view. A uniform background across the field of view, associated with many completely unresolved point sources, is also added, with a spectral index of \(\alpha = 2.0\) and a flux of \(1.352 \times 10^{-12}~\rm{erg}~\rm{s}^{-1}~\rm{cm}^{-2}~\rm{deg}^{-2}\) in the 0.5-2 keV band.

Though a point-source population is automatically created as a background component when an observation is simulated, one can also create a SIMPUT catalog of point sources using the same machinery, with the ability more finely control the input parameters of the model. For more information, see Point Source Catalog.

Instrumental Background¶

Each instrument specification in the SOXS instrument registry has a default instrumental/particle background given by its "bkgnd" entry, which specifies a PHA file for the background count rate, and the area in square arcminutes that the background in the file was created with:

from soxs import get_instrument_from_registry
hdxi = get_instrument_from_registry("lynx_hdxi")
print(hdxi)

{
    "name": "lynx_hdxi",
    "arf": "xrs_hdxi_3x10.arf",
    "rmf": "xrs_hdxi.rmf",
    "bkgnd": ["lynx_hdxi_particle_bkgnd.pha", 1.0],
    "fov": 22.0,
    "num_pixels": 4096,
    "aimpt_coords": [0.0, 0.0],
    "chips": [["Box", 0, 0, 4096, 4096]],
    "focal_length": 10.0,
    "dither": True,
    "psf": ["image", "chandra_psf.fits", 6],
    "imaging": True,
    "grating": False
}

The background model FITS table file must contain (at minimum) an extension named "SPECTRUM" which has a table of two columns: (1) instrument channels (must be the same as those in the RMF) and (2) either counts or count rate. The HDU containing the spectrum must also have the exposure time of the simulated spectrum in seconds stored in the "EXPOSURE" item in the header.

The "bkgnd" entry can also be set to None, which corresponds to no particle background. To change the particle background, one would need to define a new instrument specification with a different background.

Adjusting Background Components¶

All components of the background are turned on in the instrument simulator by default. The various components of the background can be turned on or off entirely in the call to instrument_simulator() by setting the parameters ptsrc_bkgnd, foreground, and/or instr_bkgnd to True or False:

# turn off the astrophysical foreground
soxs.instrument_simulator(simput_file, out_file, exp_time, instrument,
                          sky_center, overwrite=True, foreground=False)

# turn off the instrumental background
soxs.instrument_simulator(simput_file, out_file, exp_time, instrument,
                          sky_center, overwrite=True, instr_bkgnd=False)

# turn off all backgrounds
soxs.instrument_simulator(simput_file, out_file, exp_time, instrument,
                          sky_center, overwrite=True, ptsrc_bkgnd=False,
                          instr_bkgnd=False, foreground=False)

If you want to change the neutral hydrogen column used for the background point sources, set the bkg_nH (default value is 0.05) in the call to instrument_simulator():

# change the value of the neutral hydrogen column
soxs.instrument_simulator(simput_file, out_file, exp_time, instrument,
                          sky_center, overwrite=True, bkg_nH=0.02)

One can also take finer control of the point-source contribution to the background by supplying an ASCII table of point-source properties generated by make_point_sources_file() or make_point_source_list() using the input_pt_sources keyword argument:

# supply a list of point sources
fov = 20.0 # arcmin
soxs.make_point_source_list('my_ptsrc.dat', fov, sky_center)
soxs.instrument_simulator(simput_file, out_file, exp_time, instrument,
                          sky_center, overwrite=True, input_pt_sources="my_ptsrc.dat")

See Saving the Point Source Properties to Disk for Later Use for more information on this feature.

Using a Background From an Event File¶

Creating a new background every time SOXS is run may be time-consuming for long exposures. SOXS provides a way to generate background events for a particular instrument, save them to a standard event file, and then use this file as input to instrument_simulator(). The make_background_file() allows for this:

out_file = 'bkgnd_evt.fits'
exp_time = (1.0, "Ms")
instrument = "hdxi"
sky_center = [24., 12.] # degrees
soxs.make_background_file(out_file, exp_time, instrument, sky_center,
                          overwrite=True, foreground=True, instr_bkgnd=False,
                          ptsrc_bkgnd=True)

As can be noted from this example, make_background_file() allows one to turn any of the three background components on or off using the boolean arguments foreground, instr_bkgnd, or ptsrc_bkgnd.

fov = 20.0 # arcmin
out_file = 'bkgnd_evt.fits'
exp_time = (1.0, "Ms")
instrument = "hdxi"
sky_center = [24., 12.] # degrees
soxs.make_point_source_list('my_ptsrc.dat', fov, sky_center)
soxs.make_background_file(out_file, exp_time, instrument, sky_center,
                          overwrite=True, input_pt_sources='my_ptsrc.dat')

See Saving the Point Source Properties to Disk for Later Use for more information on this feature.

instrument_simulator() can use this background file when it is supplied with the bkgnd_file argument, provided that the same instrument is used and the exposure time of the source observation is not longer than the exposure time that the background was run with:

exp_time = (500.0, "ks") # smaller than the original value
instrument = "hdxi"
simput_file = "beta_model_simput.fits"
out_file = "evt.fits"
sky_center = [30., 45.]
soxs.instrument_simulator(simput_file, out_file, exp_time, instrument,
                          sky_center, overwrite=True, bkgnd_file="bkgnd_evt.fits")

Note that the pointing of the background event file does not to be the same as the source pointing–the background events will be re-projected to match the pointing of the source.

Background Models in SOXS¶

Galactic Foreground Model¶

The "default" Foreground Model¶

The "halosat" Foreground Model¶

Point Source Background Model¶

Instrumental Background¶

Adjusting Background Components¶

Using a Background From an Event File¶

The `"default"` Foreground Model¶

The `"halosat"` Foreground Model¶