What are ultraluminous X-ray sources? Tim Roberts

Wednesday 5th September 2007 Tim Roberts - What are ULXs?2 A definition, and an opportunity  ULX: an X-ray source in an extra-nuclear region of a galaxy with an observed luminosity in excess of erg s -1  Now accessible Chandra has resolved populations in starburst galaxies XMM-Newton: detailed timing and spectroscopy The Antennae - Chandra ACIS

Wednesday 5th September 2007 Tim Roberts - What are ULXs?3 A new class of black hole?  But Eddington limit for spherical accretion: L Edd ~ 1.3 × (M/M  ) erg s -1 hence ULXs contain  10 M  compact objects – larger still if accretion sub-Eddington – massive black holes.  Not super-massive BHs (M BH  10 6 M  ); fall to Galactic centre in a Hubble time due to effects of dynamical friction.  Too massive for stellar remnants (3M   M BH  18M  ). NB. Evolution of a single star very unlikely to form black hole more massive than ~ 20 M  (Fryer & Kalogera 2001).  Are we observing a new, 10 2 – 10 5 M  “intermediate mass” class of accreting black hole (IMBHs; e.g. Colbert & Mushotzky 1999)?

Wednesday 5th September 2007 Tim Roberts - What are ULXs?4 X-ray evidence for IMBHs  X-ray spectroscopic evidence – cool accretion discs (Miller et al. 2003). NGC 1313 X-1 T  M kT in ~ 0.15 keV c.f. kT in ~ 1 keV for stellar BHs → ~ 1000 M  BHs

Wednesday 5th September 2007 Tim Roberts - What are ULXs?5 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

Wednesday 5th September 2007 Tim Roberts - What are ULXs?6 Vanishing IMBHs problem  But some problems with IMBHs, most notably…  X-ray luminosity function (XLF), normalised to star formation rate, unbroken over 5 decades,  XLF break at ~ 0.1 L Edd for 1000-M  IMBHs.  No other source population switches off at 0.1 L Edd like this. From Grimm, Gilfanov & Sunyaev (2003) Break at ~ 2 × erg s -1

Wednesday 5th September 2007 Tim Roberts - What are ULXs?7 ULXs in starburst galaxies  Multiple ULXs (10+) are found in starburst galaxies.  Ongoing star formation  ULXs are intrinsically short- lived.  Requires an infeasibly large underlying population of IMBHs (King 2004).  Alternative: are ULXs in starbursts high-mass X- ray binaries (HMXBs)? From Gao et al. (2003)

Wednesday 5th September 2007 Tim Roberts - What are ULXs?8 In support of stellar-mass BHs  How to exceed Eddington limit: Relativistic beaming (e.g. Körding et al. 2002) Radiative anisotropy (e.g. King et al. 2001) Truly super-Eddington discs (e.g. Begelman 2002; Heinzeller & Duschl 2007)  Super-Eddington mass transfer rates in HMXBs can fuel ULXs.  Blue stellar counterparts – high mass companions? Potential X-ray luminosities for accretion onto a 10 M  BH from 2 – 17 M  secondaries (Rappaport, Podsiadlowski & Pfahl 2005)

Wednesday 5th September 2007 Tim Roberts - What are ULXs?9 Evidence from our own Galaxy  GRS has intermittently exceeded the Eddington limit over its ~15 year outburst (Done et al. 2004)  SS433 is super-critically accreting (perhaps exceeding mdot Edd by >10 3 ) - if seen face- on it would be an ULX (Begelman et al. 2006, Poutanen et al. 2007) Precessing jet/outflow - link to ULX nebulae? SS433: cartoon showing jet precession & inclination

Wednesday 5th September 2007 Tim Roberts - What are ULXs?10 The situation in early 2005 …  Dichotomy X-ray evidence such as extreme luminosities, cool accretion discs and M82 X-1 QPOs point to IMBHs, but… Other evidence stacking up in favour of smaller black holes. Which one is the correct interpretation?

Wednesday 5th September 2007 Tim Roberts - What are ULXs?11  Key evidence for IMBHs from “soft excess” in XMM-Newton ULX spectra. 10+ examples.  But mass estimate only valid from disc-dominated HS spectrum; ULXs patently aren’t in HS!  Furthermore, 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! 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 kT in ~ 1.16 keV Γ ~ 2.5 M33 X-8 From Foschini et al. (2004)

Wednesday 5th September 2007 Tim Roberts - What are ULXs?12 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

Wednesday 5th September 2007 Tim Roberts - What are ULXs?13 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)

Wednesday 5th September 2007 Tim Roberts - What are ULXs?14 Other explanations for spectral break  “Slim” accretion discs (e.g. Watarai et al. 2000) Accretion disc structure changes at highest accretion rates (close to the Eddington limit). Model disc profile T(r)  r -p ; standard disc has p = 0.75, slim disc p = 0.5. Recent work finds p ~ 0.6 for ULXs (e.g. Tsuneda et al. 2006, Vierdayanti et al. 2006, Mizuno et al. 2007). Physical similarities to optically-thick corona?  Fully comptonised VHS with spectrum modified by ionised fast outflow (Goncalves & Soria 2006).  Common thread: high accretion rate, small black holes.

Wednesday 5th September 2007 Tim Roberts - What are ULXs?15 X-ray timing – PSDs & QPOs  Break frequencies in PSDs related to black hole mass and accretion rate by McHardy et al. (2006)  QPOs are detected; but only low-frequency limited diagnostic value? Suggest M  IMBHs in M82 X-1, NGC 5408 X-1 (Strohmayer et al. 2003, 2007) Adapted from Vaughan et al. (2005) Frequency regime probed by XMM for bright ULXs Scaling of break frequencies with mass, assuming accretion at mdot Edd

Wednesday 5th September 2007 Tim Roberts - What are ULXs?16 Ho II X-1: timing  PSD analysis – compare to classic BH states. Insufficient power for high or classic very high states Energy spectrum not low/hard state  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 .  Lack of variability predicted for hyper-accretion (Ohsuga 2007) EPIC-pn light-curve of Ho II X-1 (0.3 – 6 keV, 100 s binning) Goad et al. 2006

Wednesday 5th September 2007 Tim Roberts - What are ULXs?17 Monitoring NGC 5204 X-1  First XMM-Newton or Chandra programme looking at ULX variability on time scales days - weeks.  Similar amplitude of variability to GRS (tho’ no limit-cycle variations). Roberts et al. 2006

Wednesday 5th September 2007 Tim Roberts - What are ULXs?18 Spectral variability in NGC 5204 X-1  As flux increases, gain hard counts.  Spectral modelling - heating/cooling of optically thick corona.  Behaviour the same as XTE J – strong VHS!  Could also describe disc heating + 50-ks + 5-ks

Wednesday 5th September 2007 Tim Roberts - What are ULXs?19 NGC 4490 revisited  Population of ULXs in NGC 4485/90 pair - spectral variability follows harder-as- brighter trend  Seen in many ULXs now  Recent examples - flaring event in NGC 1365 X1 (Soria et al. 2007), cool disc component of NGC 1313 X-2 not following L X  T 4 (Feng & Kaaret 2007) Gladstone & Roberts in prep.

Wednesday 5th September 2007 Tim Roberts - What are ULXs?20 A sky full of ULXs  ULXs are now showing up as a minority population in deep surveys (Hornschemeier et al. 2004, Watson et al. 2005)  Cross-correlation of 2XMMp with RC ULXs ~15% level) This is with many ULXs in starbursts excluded! ~ 1 in 5 with L X > erg s -1 3 candidate hyper-luminous X-ray sources (HLXs) Some indication of spectral changes with luminosity/ energy - but data quality mixed

Wednesday 5th September 2007 Tim Roberts - What are ULXs?21 A multi-wavelength perspective  Optical - counterparts and bubbles  Bubbles also seen in radio (e.g. Lang et al. 2007)  Spitzer observations of NGC AGN-like emission lines from ULXs (Vazquez et al. 2007)  Optical/IR modelling of counterparts - small black holes (~100 M  or less; Copperwheat et al. 2007) HST/ACS images of NGC 2403 X-1 - Roberts et al. (in prep) F330W F435W F606W

Wednesday 5th September 2007 Tim Roberts - What are ULXs?22 So, what are ULXs?  Bulk of evidence - few keV X-ray spectral breaks, spectral variability, star formation link etc - argues most ULXs are extreme accretion rate, smallish (<100 M  ) black holes  ULX is an accretion state, not a source class  Formation of black holes up to 100 M  possible in massive binary stars (Belczynski et al. 2006) or very massive, low metallicity stars (Fryer & Kalogera 2001)  Cannot rule out individual cases of larger IMBHs - HLXs are the best candidates?

Wednesday 5th September 2007 Tim Roberts - What are ULXs?23 (Re-)fueling problem  Best way of providing fuel supply: companion star.  Alternative: molecular cloud disruption (Krolik 2004).  New modelling – very difficult to form stable IMBH-ULXs, underpredict ULXs by (Madhusudhan et al 2006).  Though plausible in dense stellar clusters (Baumgardt et al. 2006).

Wednesday 5th September 2007 Tim Roberts - What are ULXs?24 A note on BH 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 (2003)

Wednesday 5th September 2007 Tim Roberts - What are ULXs?25 M82 X-1: Best IMBH candidate?  Extreme ULX in M82 (L X,peak ~ erg s -1 ).  Central density of MGG 11 sufficient for creation of IMBH via stellar mergers/collapse.  Detection of X-ray QPOs – flux is relatively isotropic.  BUT – could be nucleus of accreted galaxy etc. From Portegies-Zwart et al. (2004) See also Kaaret et al. (2001), Strohmayer & Mushotzky (2003), King & Dehnen (2005), Mucciarelli et al. (2006)