Ultraluminous X-ray sources: a mystery for modern X-ray astronomy

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


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


The missing link?

Only have evidence for stellar-mass (<20 M) and supermassive (>106 M)

black holes

Is there a missing link between them?


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?


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


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)


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


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


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)


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)


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?


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)


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


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)


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


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


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)


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!


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


New HST imaging

AC

S W

FC

F60

6WF

330W

F43

5W F

606W

mV ~ 24 mV > 27 mV ~ 25


New HST imaging (2)

mV ~ 22

mV ~ 25

mV ~ 26

Very blue colours - optical emission from accretion discs?


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?


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

Documents

Ultraluminous X-ray sources: a mystery for modern X-ray astronomy