Observational Evidence for Quasi-soft X-Ray

Sources in Nearby Galaxies and the link to Intermediate-mass Black Holes

Albert Kong and Rosanne Di Stefano

Harvard-Smithsonian Center for Astrophysics

X-ray Point Sources in Nearby Galaxies

• Chandra and XMM-Newton found many point sources in nearby galaxies.

• They are mainly X-ray binaries, but some of them are SNRs or foreground/background objects.

• We found some extreme X-ray objects in nearby galaxies.

• Some off-nucleus point sources have luminosity > 1039 erg/s, often referred as ultra-luminous X-ray sources (ULXs).

• We also found some very soft X-ray sources typically with temperature of ~100eV; some of them are also ULXs (e.g., Mukai et al. 2003; Kong & Di Stefano 2003; Fabbiano et al. 2003).

Very Soft X-ray SourcesSupersoft Sources and Quasi-soft Sources

• SSSs are sources with kT ~ tens of eV, and with L typically between 1036-1038 erg/s.

• Some may be nuclear-burning WDs; some of these may be progenitors of Type Ia supernovae.

• Because absorption is usually low comparing to our Galaxy, external galaxies are good to search for very soft sources (VSSs).

• We have developed a method to search for VSSs in external galaxies. (Di Stefano & Kong 2003 a,b,c)

Very Soft X-ray SourcesSupersoft Sources and Quasi-soft Sources

• QSSs typically have 100eV < kT < 250 eV.

• Or else they have a softer dominant spectrum, but may include a small hard component.

• We have tested our algorithm on simulated data.

• We have tested our algorithm on real data Chandra and some XMM-Newton from ~20 galaxies.

• We find many SSSs and QSSs; their luminosity is between 1036-1039 erg/s.

• QSSs and SSSs occupy between 11% and 45% of all X-ray sources.

Recurrent SSS in NGC 300kT=57 eVLbol=1.5x1039 erg/s

kTbb=91 eVkTRS=74 eVLbol=7x1039 erg/s

kT=71 eVLbol=2.3x1038 erg/s kT=76 eV

Lbol=4x1039 erg/s

Supersoft Sources

NGC1399-162

kTMCD=0.35 keVL0.3-7=1.7x1039 erg/s

QSS-σM83-35

QSS-MNOH

kT=0.12 keVL0.3-7=1038 erg/s

M51-10QSS-σ

kT=0.24 keVL0.3-7=3.6x1038 erg/s

kT=0.13 keVL0.3-7=4.3x1037 erg/s

M101-114QSS-MNOH

Quasi-soft Sources

A Luminous SSSs in M31

kT=70eV, α=1.8L0.3-7=4x1038 erg/s

M83 M101

M51 NGC4697

Red: SSSs Blue: QSSs

What are the Quasi-soft Sources?

• They are almost certainly not WDs.• They may be SNRs, but variability can distinguish them

from accreting objects.• The most natural explanation may be that they are

accreting IMBHs.• If the accretion is mediated by a disk which is

geometrically thin, but optically thick, the spectrum must be soft. (M/103MSun)= h (42eV/kTlso)2 [(ξ/0.1) Lobs/3x1037 erg/s]1/2

where h is a factor of order unity.• They may represent a simple extension of observed

properties of Galactic BHs.

Cyg X-1kT=0.3 keV, α=2.4L0.5-200=7x1037 erg/sMBH=6.9-13.2 MSun XTE J1550-564

kT=0.8 keV, α=2.3L0.5-10=1038 erg/sMBH=8.4-10.8MSun

Chandra+RXTE (Miller et al. 2003)

IC 342 X-1kTMCD=1.98 keVL0.5-10=1x1039 erg/s

Kong 2003

kTMCD=0.2 keVα=1.8L0.5-10=5x1039 erg/s

Miller et al. 2003

A Recurrent SSS in NGC 300

Recurrent SSS in NGC 300kT=57 eVLbol=1.5x1039 erg/s

• If the 5.4 hr variability is orbital origin, then the system is consistent with a ~1000 MSun IMBH.

• If the donor fills its Roche lobe, then Porb (8.9 hr)(Md/M ), where Md is the mass of the donor star. If Porb = 5.4 hr, then Md = 0.61 MSun.

• See Rosanne’s poster (#13) for details.

Alternative Models

• Neutron star: the very soft X-ray emission would presumably emanate from a photosphere is much larger than the neutron star itself.

• Stellar-mass BH: Compton thick outflow from a stellar-mass BH, accreting near the Eddington limit (King & Pounds 2003).

Summary• Galaxies are rich in SSSs and in QSSs.• We are faced with the challenge of understanding the

nature(s) of the members of each of these classes of X-ray sources.

• Some SSSs and some QSSs may be IMBHs.

Whatever the nature(s) of SSSs and QSSs(See Rosanne’s poster #13)

• IMBHs need not be ultraluminous.• IMBHs can be found in anywhere in ellipticals and spirals.• Binary evolution can provide important and testable

predictions about the properties of accreting IMBHs.