X-ray Study of the Local Hot Gas

Taotao Fang

UCB

WithClaude 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

Local Group

R=1 Mpc

MS 0737+7441 PG 1211+143NGC 3227 Mkn 509NGC 4258 Ton S180MCG 6-30-15 NGC 7469NGC 4593 Mkn 766H 1426+428 Ton 1388PKS 0558-504 Mkn 501NGC 3783 1H 1219+301H 1821+643 3C 273NGC 5548 NGC 4051PKS 2155-304 Mkn 421

Target:

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 × R2 × nhalo × L

ROSAT ALL SKY MAP

• 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. X-ray data

showed (Garmire et al. 1992) the emission measure from this component is:

• Combining with X-ray absorption measure, we found:

• CAUTION: the temperature of this hot halo component is extremely uncertain, varying from 106 to 106.5 K.

Soft X-ray Background

€

ne2L = 2.5 ×10−3ZO

−1 cm−6 pc

€

ne ≈ 5 ×10−4 cm−3; L ≈ 20ZO−1 kpc

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

€

ne ≈ 5 ×10−4 cm−3; L ≈ 20ZO−1 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:

• Since the total mass within these X-ray “cloud” must be smaller than the total baryon mass of the halo, we have:

€

Ncl =4

3

R

r

⎛

⎝ ⎜

⎞

⎠ ⎟2

C

€

R ≤ 0.5 MpcZO1

⎛

⎝ ⎜

⎞

⎠ ⎟

1

2 C

0.5

⎛

⎝ ⎜

⎞

⎠ ⎟−

1

2 Ωb

0.044

⎛

⎝ ⎜

⎞

⎠ ⎟

1

2 Mhalo

1012 M

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

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:

• At the temperature where O VII ionization peaks, the collisional excitation rate

We then have:€

IOVII ≤ (5 −10) photons cm-2s-1sr -1

€

S ≥10−12 cm3s−1

€

ne ≤ 9 ×10−3 cm−3; r ≥ 0.2ZO−1 kpc

X-ray Study of the Local Hot Gas