ultraluminous x-ray sources: a mystery for modern x-ray astronomy
DESCRIPTION
Ultraluminous X-ray sources: a mystery for modern X-ray astronomy. Tim Roberts. Tim Roberts. Galaxies in the X-ray regime. M83 - “Southern pinwheel galaxy” ESO VLT. M83 ESO/VLT image. 0.3 – 1 keV ≡ 12 – 40 Å “soft” 1 – 2 keV ≡ 6 – 12 Å 2 – 8 keV ≡ 1.5 – 6 Å “hard”. - PowerPoint PPT PresentationTRANSCRIPT
Ultraluminous X-ray sources: a mystery for modern X-ray astronomy
Tim Roberts Tim Roberts
Thursday 14th June 2007 Tim Roberts - ULXs 2
0.3 – 1 keV ≡ 12 – 40 Å “soft”
1 – 2 keV ≡ 6 – 12 Å
2 – 8 keV ≡ 1.5 – 6 Å “hard”
M83 Chandra ACIS-S – true X-ray colours
Galaxies in the X-ray regime
Point sources – X-ray binaries, supernova remnants
Hot phase of the interstellar medium
Active nucleus
M83 ESO/VLT imageM83 - “Southern pinwheel galaxy”
ESO VLT
Thursday 14th June 2007 Tim Roberts - ULXs 3
X-ray source luminosities
Log
X-r
ay lu
min
osit
y (e
rg s
-1)
2737
47 GRBs
Stars
CVs
SNRs
NSs & BHs
ULXsULXs AGNs
QSOsGalactic populations
Supermassive black holes
Thursday 14th June 2007 Tim Roberts - ULXs 4
The missing link?
Only have evidence for stellar-mass (<20 M) and supermassive (>106 M)
black holes
Is there a missing link between them?
Thursday 14th June 2007 Tim Roberts - ULXs 5
The Eddington limit
Maximum theoretical luminosity for a spherically-accreting object Balance between gravitational pull inwards and
radiative push outwards Directly proportional to mass of accretor
LEdd ~ 1.3 × 1038 (M/M) erg s-1
ULXs too big for stellar-mass black holes; can’t all be displaced AGN A new, intermediate-mass class of black holes?
Thursday 14th June 2007 Tim Roberts - ULXs 6
How do we test this?
Need the best instruments
Chandra (NASA) XMM-Newton (ESA)Unrivalled X-ray imaging
(0.5 arcsecond resolution) High photon collecting area
(0.4 m2 @ 1 keV)
Workhorse instruments for both - CCD imaging spectrometers
Thursday 14th June 2007 Tim Roberts - ULXs 7
Black Hole accretion states
Ph
oton
cm
-2 s
-1 k
eV-1
1 10 100 1 10 100 1 10 100Energy (keV)
High (thermal-dominated)
~ 1 – 2 keV disc + PL tail
Low/hard
Hard PL (Γ ~ 1.5 – 2) dominant, disc absent or truncated, radio jet
emission. Least luminous.
Very high (steep power-law)
Soft PL (Γ > 2.5) plus some hot disc emission.
Most luminous.
Energy spectra from McClintock & Remillard (2006)
Key point - accreting black hole X-ray spectra can be empirically modelled as the combination of an accretion
disc spectrum and a power-law (corona)
Thursday 14th June 2007 Tim Roberts - ULXs 8
XMM-Newton evidence for IMBHs
X-ray spectroscopy – cool accretion discs (Miller et al. 2003)
NGC 1313 X-1
Tin M-0.25
kTin ~ 0.15 keV c.f. kTin ~ 1 keV for stellar BHs→ ~ 1000 M BHs
Thursday 14th June 2007 Tim Roberts - ULXs 9
LX – kTin relationship
IMBH candidates occupy separate part of parameter space to stellar-mass BHs
Strong evidence for IMBHs as new class underlying luminous ULXs
From Miller et al.
(2004)
LX T4
Thursday 14th June 2007 Tim Roberts - ULXs 10
ULXs in starburst galaxies Multiple ULXs (10+) are
found in Starburst galaxies Ongoing star formation
ULXs are short-lived Requires an infeasibly large
underlying population of IMBHs (King 2004)
Alternative: are ULXs in Starbursts “ordinary” high-mass X-ray binaries (HMXBs)?
From Gao et al. (2003)
Thursday 14th June 2007 Tim Roberts - ULXs 11
In support of stellar-mass BHs How to exceed Eddington limit:
Relativistic beaming Radiative anisotropy Truly super-Eddington discs
Super-Eddington mass transfer rates in HMXBs can fuel ULXs
Blue stellar counterparts – high mass companions?
GRS1915+105 – demonstrates super-Eddington does happen in stellar-mass systems
Potential X-ray luminosities for accretion onto a 10 M BH from 2 – 17 M secondaries (Rappaport, Podsiadlowski & Pfahl 2005)
Thursday 14th June 2007 Tim Roberts - ULXs 12
Dichotomy
Highest X-ray luminosities and cool accretion discs point to IMBHs, but…
Other evidence stacking up in favour of smaller black holes
Which one is the correct interpretation?
Thursday 14th June 2007 Tim Roberts - ULXs 13
NGC 1313 X-1From Miller et al. (2003)
kTin ~ 0.15 keV
“diskbb” – optically-thick accretion discpower-law – hot, optically-thin corona
ULX X-ray spectra revisited
Key evidence for IMBHs from “cool disc” in XMM-Newton ULX spectra. 10+ examples
But not all ULXs show this spectral form: several have an “inverted” spectrum e.g. NGC 55 ULX (Stobbart et al.
2004), NGC 5204 X-1 (Roberts et al. 2005)
Difficult to explain dominant soft power-law physically!
kTin ~ 1.16 keV
Γ ~ 2.5
M33 X-8From Foschini et al. (2004)
Thursday 14th June 2007 Tim Roberts - ULXs 14
A sample of bright ULXs
Look at best archival XMM-Newton data
Demonstrate that 2-10 keV spectrum fit by a broken power-law in all of the highest quality data Invalidates IMBH
model - hard component is not a simple power-law
Stobbart, Roberts & Wilms 2006
Disc
Power-law
Thursday 14th June 2007 Tim Roberts - ULXs 15
ULX accretion physics Physical accretion disc plus
corona model: cool discs, optically-thick coronae ULXs operate differently to common
Galactic black hole states, except… “Strong” VHS in XTE J1550-564:
energetically-coupled corona/ disc (Done & Kubota 2006). Key features are a disc that appears
cool as its inner regions are obscured by an energetic, optically-thick corona.
From Done & Kubota (2006)
Thursday 14th June 2007 Tim Roberts - ULXs 16
Archetypal luminous ULX (LX > 1040 erg s-1) – top IMBH candidate
Deep XMM-Newton obs (110 ks, though > 60% spoiled by bad space weather)
Best fit spectrum: cool disc (~ 0.2 keV) plus optically-thick ( ~ 6) corona
Holmberg II X-1 Goad, Roberts et al. 2006
EPIC spectrum of Ho II X-1
Data: pn/MOS1/MOS2
Model: diskpn + comptt
Thursday 14th June 2007 Tim Roberts - ULXs 17
X-ray timing measurements – PSDs
Courtesy P. UttleyAdapted from
Vaughan et al. (2005)
Power Spectral Densities for two AGN and Galactic BH Cygnus X-1
Approximate scaling of break frequencies with mass
Frequency regime
probed by XMM for
bright ULXs
Thursday 14th June 2007 Tim Roberts - ULXs 18
Ho II X-1: timing Lack of strong variability PSD analysis – compare to
classic black hole states Insufficient power for HS May be in L/H or VHS -
energy spectrum says latter Similar to “χ”-class of GRS
1915+105 in VHS? Band-limited PSD - but don’t
see variability, so must be at high-f MBH < 100 M
EPIC-pn light-curve of Ho II X-1
(0.3 – 6 keV, 100 s binning)
Thursday 14th June 2007 Tim Roberts - ULXs 19
How to progress? X-ray measurements suggest smaller black holes;
but no direct mass determination As with Galactic BHs, ultimate test of the compact
object mass in ULXs is dynamical studies Mass function f(M) requires measurement of orbital
period (P) and velocity amplitude of donor star (K*), from line features in optical spectrum:
f(M) = MX3sin3i/(M*+MX)2 = PK*
3sin3i/2πG But ULX counterparts have mV ~ 22 – 26 (where
identified) in crowded fields – not trivial!
Thursday 14th June 2007 Tim Roberts - ULXs 20
3 steps to the black hole mass
(1) Identify counterpart from HST imaging Time awarded in cycle 14 to complete local ULXs
(2) Obtain spectra – suitable features? Gemini spectra awarded for three ULX counterparts
(3) Monitoring campaign
Thursday 14th June 2007 Tim Roberts - ULXs 21
New HST imaging
AC
S W
FC
F60
6WF
330W
F43
5W F
606W
mV ~ 24 mV > 27 mV ~ 25
Thursday 14th June 2007 Tim Roberts - ULXs 22
New HST imaging (2)
mV ~ 22
mV ~ 25
mV ~ 26
Very blue colours - optical emission from accretion discs?
Thursday 14th June 2007 Tim Roberts - ULXs 23
Radial velocity studies
Gemini-S GMOS spectrum of NGC 1313 X-2
Possible to derive P, KBH from broad He II 4686Å accretion disc line?
Done for Galactic BHs (e.g. Soria et al. 1998). Dynamic mass within
reach?
Thursday 14th June 2007 Tim Roberts - ULXs 24
State of play Break present in 2 – 10 keV spectrum of many ULXs
– not expected for sub-Eddington IMBHs Cool disc, optically thick corona - similarities to very high
accretion rate Galactic BHs Ho II timing result – mass limit < 100 M
New theory - possible to produce BHs of up to 100 M in young (low metallicity) stellar populations
Strongly suggests ULXs are larger stellar-mass BHs accreting at around Ledd
Require dynamical mass measurements to prove this; a work in progress