X-ray Absorption Line
Spectroscopy of the Hot ISM
Q. Daniel Wang (UMass),
Yangsen Yao (MIT), et al.
Why X-ray absorption lines?
• Tracing all K transitions
of metals  all three
phases of the ISM.
• Not affected by photoelectric absorption
unbiased measurements
of the global ISM.
LETG+HETG spectrum of LMXB X1820-303
Yao & Wang 2006, Yao et al. 2006, Futamoto et al 2004
Fe XVII K
Spectroscopic diagnostics
OVII
OVII
Ne IXI
Ne VIII
OVI
Ne X
Assuming solar abundances and CIE
One line (e.g., OVII K) 
velocity centroid and EW
 constraints on the
column density, depending
on assumed b and T
Two lines of different
ionization states (OVII
and OVIII K)  T
Two lines of the same state
(K and K)  b
Lines from different species
 abundance fa
Absline:
an XSPEC-based joint-fit routine
• Allow fits to individual spectral lines as well as
broad-band spectra
• Accounting for line saturation:
I()=Ic() exp[-()]
()NHfafi(T)flu(,0,b)
b=(2kT/mi+2)1/2
• Allow for measurements of thermal, chemical,
and kinetic properties of the hot ISM.
• Rigorous error estimates of all parameters.
Yao & Wang 2005
Add velocity and/or depth
X-ray binary
AGN
X-ray binary
ROSAT all-sky survey
in the ¾-keV band
Comparison of the emission and
absorption data
• From emission data (McCammon et al. 2002)
– ~50% of the background intensity is thermal
and local (z < 0.01)
– Measurement of emission measure
• X-ray absorption line spectroscopy
– Measure the column density, thus the mass
The combination of the emission/absorption
data  physical scale of the hot gas
Mkn 421
LETG/HRC
LETG/ACIS
PKS 2155
3C 273
Z~0
absorption:
Evidence for the warm-hot IGM?
• Suggested initially based on
1-T modeling
• But demands a too large
total baryon mass (Fang et
al. 2006)
• No enhancement observed
through the inner region of
the local group (Bregman et
al, 2007)
• No detection through other
groups of galaxies
Mrk 421: Joint absorption/emission
line analysis
•1-T model is inconsistent with
the OVIII and OVII K
emission line ratio (McCammon
et al. 2002).
•The absorption and emission
lines are consistent with the
model:
n=n0e-z/hn; T=T0e-z/hT
 n=n0(T/T0), =hT/hn,
L=hn/sin b
(d)-(f) include the far-UV OVI absorption line
(Yao & Wang 2006)
Properties of the hot gas towards
Mkn 421
• Not isothermal
• Consistent with a exponential hot disk model
with T0 = 2.8 x 106 K and n0=2.4 x 10-3 cm-3
and hn~ 2 kpc, which is comparable to the
scale height of OVI-absorbing gas (2.3 kpc;
Savage et al. 2003)
• This thick hot disk most likely represents
the Galactic corona, heated and enriched by
supernovae in the Galactic disk.
Differential
absorption analysis
3C 273
Mkn 421
Assuming a 1-T thermal
plasma of solar
abundances:
NH ~ 2.2 x 1019 cm-2
T ~ 2.0 x 106 K
vb ~ 2x102 km/s
Yao & Wang 2007
Joint-fit with the emission
enhancement
3C 273
Mkn 421
ROSAT PSCP ¾-keV band
• Scale = 2.4 (1-10) kpc
(90% interval)
• Indicating primarily
Galactic bulge/center
origin
• Total thermal energy ~
1056 ergs, consistent
with estimate from
dynamic modeling
(Sofue 2000; BlankHawthorn & Cohen 2003)
LMC X-3 as a distance marker
• BH X-ray binary,
typically in a
high/soft state
• Roche lobe
accretion
• 50 kpc away
• Vs = +310 km/s
• Away from the
LMC main body
H image
Wang et al. 2005
LMC X-3: absorption lines
•The line centroid of the OVI
and OVII lines are consistent
with the Galactic origin.
•No~1.9 x 1016 atoms/cm2,
similar to those seen in AGN
spectra!
•T ~ 1.3 x 106 K
•vb ~ 79 km/s
Joint-fit to the Suzaku XIS
diffuse emission spectrum
• 100 ks
• 0.5 deg off LMC X-3
• Assuming the thick
hot disk model
LMXB X1820-303
• In GC NGC 6624
– l, b = 2o.8, -8o
– Distance = 7.6 kpc 
tracing the global ISM
– 1 kpc away from the
Galactic plane  NHI
• Two radio pulsars in the
GC: DM  Ne
• Chandra observations:
– 15 ks LETG (Futamoto
et al. 2004)
– 21 ks HETG
LETG+HETG spectrum
Yao & Wang 2006, Yao et al. 2006
Fe XVII K
X1820-303: Results
• Hot gas accounts for ~ 6% of the total O
column density
• Mean temperature T = 106.34 K
• O abundance:
– 0.3 (0.2-0.6) solar in neutral atomic gas
– 2.0 (0.8-3.6) solar in ionized gas
• Hot Ne/O =1.4(0.9-2.1) solar (90% confidence)
• Hot Fe/Ne = 0.9(0.4-2.0) solar
• Velocity dispersion 255 (165–369) km/s
Exponential distribution models
Disk model
•nH = 5.0(-1.8,+2.6)x10-3 cm-3
exp[-|z|/1.1(-0.5,+0.7) kpc]
•Total NH~1.6 x1019 cm-2
Sphere model
•nH = 6.1(-3.0,+3.6)x10-2 cm-3
exp[-R/2.7(-0.4,+0.8) kpc]
~3 x 10-3 cm-3 at the Sun
•Total NH~6.1 x1019 cm-2
X-ray absorption is primarily around
•MH~7.5(2.5-16)x108 Msun
the Galactic disk within a few kpc!
Yao & Wang (2005)
Global hot gas properties
• Spatial extent:
– Locally, the data are consistent with a thick Galactic disk
with a density scale height ~2 kpc, ~ the values of OVI
absorbers and free electrons
– Much more extended around the Galactic bulge
• Thermal structure: mean T ~ 106.3 K toward the inner
region, ~ 106.1 K at solar neighborhood, but nonisothermal even locally.
• Velocity dispersion: ~200 km/s in the inner region,
80 km/s in the solar neighborhood
• O and Ne abundances: consistent with solar
abundance ratios
NGC 3556 (Sc)
•Active star forming
•Hot gas scale height
~ 2 kpc
•Lx ~ 1% of SN
energy
Red – optical
Green – 0.3-1.5 keV band
Blue – 1.5-7 keV band
Wang et al. 2004
NGC 4565 (Sb)
Wang (2006)
Red – optical
Green – 0.3-1.5 keV band
Blue – 1.5-7 keV band
Very low specific SFR
William McLaughlin (ARGO Cooperative Observatory)
No sign for any outflows from the
disk in radio and optical
Conclusions
• X-ray absorption line spectroscopy has led to
the firm detection of the global hot ISM.
• The spectroscopy allows for the study of the
thermal, chemical and kinetic properties of the
hot ISM.
• The detected hot gas is strongly concentrated
toward Galactic disk and bulge.
• Heating is most due to SNe. But the bulk of
their energy is not detected and is probably
propagated into the halo, balancing the cooling.
• But so far there is little X-ray evidence for a
hot halo on scale ~ 100 kpc.
NGC 2841 (Sb)
• D=15 Mpc
• Lx ~ 7 x 1039 ergs/s
Red: optical
Blue: 0.3-1.5 keV diffuse emission
Wang 2006