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

Thursday 14th June 2007 Tim Roberts - ULXs2 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 image M83 - “Southern pinwheel galaxy” ESO VLT

Thursday 14th June 2007 Tim Roberts - ULXs3 X-ray source luminosities Log X-ray luminosity (erg s -1 ) GRBs Stars CVs SNRs NSs & BHs ULXs AGNs QSOs Galactic populations Supermassive black holes

Thursday 14th June 2007 Tim Roberts - ULXs4 The missing link? Only have evidence for stellar-mass ( 10 6 M  ) black holes Is there a missing link between them?

Thursday 14th June 2007 Tim Roberts - ULXs5 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 L Edd ~ 1.3 × (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 - ULXs6 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 m 1 keV) Workhorse instruments for both - CCD imaging spectrometers

Thursday 14th June 2007 Tim Roberts - ULXs7 Black Hole accretion states Photon cm -2 s -1 keV Energy (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 - ULXs8 XMM-Newton evidence for IMBHs  X-ray spectroscopy – cool accretion discs (Miller et al. 2003) NGC 1313 X-1 T in  M kT in ~ 0.15 keV c.f. kT in ~ 1 keV for stellar BHs → ~ 1000 M  BHs

Thursday 14th June 2007 Tim Roberts - ULXs9 L X – kT in 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) L X  T 4

Thursday 14th June 2007 Tim Roberts - ULXs10 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 - ULXs11 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?  GRS – 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 - ULXs12 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 - ULXs13 NGC 1313 X-1 From Miller et al. (2003) kT in ~ 0.15 keV “diskbb” – optically-thick accretion disc power-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! kT in ~ 1.16 keV Γ ~ 2.5 M33 X-8 From Foschini et al. (2004)

Thursday 14th June 2007 Tim Roberts - ULXs14 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 - ULXs15 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 J : 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 - ULXs16  Archetypal luminous ULX (L X > 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 EPIC spectrum of Ho II X-1 Data: pn/MOS1/MOS2 Model: diskpn + comptt

Thursday 14th June 2007 Tim Roberts - ULXs17 X-ray timing measurements – PSDs Courtesy P. Uttley Adapted 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 - ULXs18 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 in VHS?  Band-limited PSD - but don’t see variability, so must be at high-f  M BH < 100 M  EPIC-pn light-curve of Ho II X-1 (0.3 – 6 keV, 100 s binning)

Thursday 14th June 2007 Tim Roberts - ULXs19 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) = M X 3 sin 3 i/(M * +M X ) 2 = PK * 3 sin 3 i/2πG  But ULX counterparts have m V ~ 22 – 26 (where identified) in crowded fields – not trivial!

Thursday 14th June 2007 Tim Roberts - ULXs20 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 - ULXs21 New HST imaging ACS WFC F606W F330W F435W F606W m V ~ 24m V > 27m V ~ 25

Thursday 14th June 2007 Tim Roberts - ULXs22 New HST imaging (2) m V ~ 22 m V ~ 25 m V ~ 26 Very blue colours - optical emission from accretion discs?

Thursday 14th June 2007 Tim Roberts - ULXs23 Radial velocity studies Gemini-S GMOS spectrum of NGC 1313 X-2 Possible to derive P, K BH 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 - ULXs24 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 L edd  Require dynamical mass measurements to prove this; a work in progress