Lecture 11: X-ray Spectra of Hot Ionized Medium
(9/28/11)
Two kinds of X-ray emission are seen in interstellar medium:
(1)
thermal emission – Bremsstrahlung and lines
this applies to thermal plasma
(2)
nonthermal emission – power law
this is synchrotron radiation and requires very energetic
electrons (TeV); in pulsar wind nebulae and superbubbles
X-ray emission is absorbed by the intervening medium.
The absorbed X-ray emission passes through the X-ray
telescope/detector.
Both the interstellar absorption and telescope/detector response
are energy (wavelength) dependent. It is impossible to
disentangle their effects. To decipher the observed spectrum, we
have to assume a plasma emission model, pass it through an
absorption column density, modify it with the instrument response
function, and used 2 statistics to determine the best-fit model.
Thermal plasma spectra show lines superposed on
Bremsstrahlung continuum emission.
More lines at high energies show up as temperature increases, and
distinct Fe emission is seen at 107 K.
Thermal plasma emission:
Raymond & Smith 1977, ApJS, 35, 419
Mewe, Gronenschild, & Oord 1986, A&AS, 62, 197
Raymond’s commentary on historical development of X-ray
emission models of hot plasma (see handout).
Interstellar absorption:
Ride & Walker 1977, A&A, 61, 339
Balucinska-Church & McCammon 1992, ApJ, 400, 699
Watch out for the E3 in the y-axis. The cross-section itself is highest
at the lower energy range.
Note also that C, N, O are the main elements providing the opacity
at low energy range.
X-ray absorption column density (NH) includes H+, HI, and H2
components of the ISM; therefore, it is different from the column
density of HI (NHI).
The gas to dust ratio of the Galactic ISM has been reported by
Bohlin, Savage, & Drake 1978, ApJ, 224, 132
NHI = 5.8x1021 E(B-V) H-atoms cm-2
When the absorption column density is unknown, the HI column
density implied by extinction is frequently used to approximate
the absorption column density.
Guerrero, Chu, Gruendl 2000, ApJS, 129, 295
These plots show that thermal emission from plasma of
temperatures below 2x106 K is heavily absorbed, but the ISM is
quite transparent to plasma emission of higher temperatures. This
is mainly because hotter plasma emits at higher energies.
The Art of X-ray Spectral Fitting
Metz, et al. 2004, ApJ, 605, 725
Hot Interstellar Gas and Stellar Energy Feedback in the Antenna
Galaxies
There are not enough photons or spectral resolution to distinguish among
these different models. We adopted one thermal component models to
determine plasma temperatures and densities.
Baldi, Raymond, Fabbiano. Et al. 2006, ApJS, 162, 113
CHEMICAL ENRICHMENT OF THE COMPLEX HOT ISM OF THE ANTENNAE GALAXIES. I. SPATIAL
AND SPECTRAL ANALYSIS OF THE DIFFUSE X-RAY EMISSION
Some regions need power-law components to explain the hard X-ray
emission.
They find a variety of temperatures (from 0.2 to 0.7 keV) and NH (from
Galactic to 2x1021 cm-2). Metal abundances for Ne, Mg, Si, and Fe vary
dramatically throughout the ISM from subsolar values (~0.2) up to several
times solar.
Townsley et al. 2011, ApJS, 194, 1, 15, & 16
Three papers based on 1 Ms Chandra observations of the Carina Nebula
(a)
(b)
(c)
(d)
8 um in red, Ha in green
8 um in red, Ha in green, X-ray in blue
color composite of X-ray images
same as (c) but with different stretch to show faint emission
Components 1-3 : hot plasma model (red, green, blue)
Component 4: young stars, coronal emission (cyan)
Component 5: hard thermal background, AGNs, galaxies (magnetic)
Component 6, cosmic background, Galactic Ridge emission 10 (purple)
Watch out for point sources. The detected faint point sources have
the following spectrum. There are many more fainter sources, such
as T Tauri stars. They make contribution to the “diffuse” emission,
too.
Extra line components are needed to fit the spectrum.
Line-like correlated residuals in the X-ray spectral fits suggest that
substantial X-ray emission is generated by charge exchange at the
interfaces between Carina's hot, rarefied plasma and its many cold neutral
pillars, ridges, and clumps.