X-ray Study of the Local Hot Gas
Taotao Fang
UCB
With
Claude Canizares, Chris Mckee and Mark Wolfire
Z = 0 X-ray Absorber
Where are the local X-ray absorbers?
•
Typically these lines are
unresolved, which implies
an upper limit of line width
of ~ 0.025 Å, or ~ 350 km
s-1 at 21.6 Å. This means a
upper limit of distance of ~
5 Mpc if the Hubble
constant is 70 km s-1 Mpc-1.
• The typical column
density of O VII absorbers
is ~ 1016 cm-2.
300 kpc
R=1 Mpc
Local Group
Target:
MS 0737+7441
NGC 3227
NGC 4258
MCG 6-30-15
NGC 4593
H 1426+428
PKS 0558-504
NGC 3783
H 1821+643
NGC 5548
PKS 2155-304
PG 1211+143
Mkn 509
Ton S180
NGC 7469
Mkn 766
Ton 1388
Mkn 501
1H 1219+301
3C 273
NGC 4051
Mkn 421
ALL SKY MAP, O VI AND O VII
O VI data from Sembach et al. (2003)
X-ray Absorption in the Intervening Systems:
(z > 0)
• PKS 2155-304 (Fang et al. 2002)
– 4 x 1015 cm-2
• H 1821+643 (Mathur et al. 2003)
– 2-3 
• Mkn 421 (Nicastro et al. 2004)
– (0.7 - 1) x 1015 cm-2
• 3C 120 (Mckernan et al. 2004)
– Based on very low counts (<10 counts per bin)
• Why we see so many local (z = 0) absorbers
with high column densities, but so little
intervening absorbers with small column
densities?
• One solution: these X-ray absorbers are
associated with our Milky Way, in stead of
the Local Group.
– Expected number of absorber along LOS;
– Soft X-ray background emission measurement;
– Some diagnostic observations;
Expected Number of Absorbers:
• Basic assumption: X-ray absorbers are associated with halos, either MW type,
or LG type.
• Model A: halo distribution (PS) + gas distribution (NFW) + metal distribution
– TOO MANY UNCERTAINTIES, CAN FIT ANY DATA!
• Model B: start from observations
– Covering factor: C
– Uniformly distributed within the halo
R
R
N  C  R  n halo  L
2
ROSAT ALL SKY MAP
Soft X-ray Background
• Three components:
– Extragalactic X-ray background from point sources (and WHIM?),
power law spectrum.
– Local hot bubble, producing thermal emission around 106K, within
a bubble with radius of ~ a few hundred pc around the Sun.
– Halo component, producing thermal emission around 106.3 K. Xray data showed (Garmire et al. 1992) the emission measure from
this component is:
3
1
O
6
n L  2.5 10 Z cm pc
2
e
• Combining with X-ray absorption measure, we found:
4

3
1
O
n e  5 10 cm ; L  20Z kpc
• CAUTION: the temperature of this hot halo component is
extremely uncertain, varying from 106 to 106.5 K.
Diagnostic Observations
• 4U 1820 (LMXB)
• D < 7.6 kpc
• Futamoto et al
(2004)
• GX 339 (LMXB)
• D < 4.0 kpc
• Miller et al (2004)
• Caution: high
column density of
O VIII!
Total Baryon Mass and Fraction
Summary
• Observation:
– In a total of 22 los, Chandra & XMM detected 9 los show z = 0
X-ray absorption lines with high column density;
– All los with more than 60 counts per bin showed z = 0 lines;
– Very few intervening absorption systems were reported, with
very low ion column densities.
• We argue that these local X-ray absorbers are possibly associated
with MW halo, instead of intragroup medium in LG
– Expected number of the absorbers
– Soft X-ray background emission measurement;
– Diagnostic observations of nearby targets
• From SXB and X-ray absorption measurement, we constrain the
the properties of the X-ray absorbers as
4
3
1
O
n e  5 10 cm ; L  20Z kpc
Total Baryon Mass and Fraction
• Given a covering factor C, total number of X-ray
“cloud” with a radius of r and within a halo of radius
R, are:
4 R 
N cl    C
3 r 
2
• Since the total mass within these X-ray “cloud” must
be smaller than the total baryon mass of the halo, we
have:

ZO   C   b   M halo
R  0.5 Mpc     
 
12

 1  0.5  0.044  10 M
1
2
1

2
1
2
1
2




Ionization fraction for O VI, O VII, and O VIII
McCammon et al. (2002)
Chandra Moon Observation
Chandra Moon Spectrum
Wergerlin et al.(2004)
X-ray Emission Line Measurement
• In most case, the line intensity of O VII
triplet is:
IOVII  (5 10) photons cm s sr
-2 -1
-1
• At the temperature where O VII ionization
peaks, the collisional excitation rate
12

S  10
3 1
cm s
We then have:
3
3
1
O
n e  9 10 cm ; r  0.2 Z kpc

X-ray Study of the Local Hot Gas