Constraining the accretion flow evolution around the best intermediate mass black hole candidate HLX-1 in the ESO galaxy Olivier Godet Collaborators D. Barret N. Webb T. Alexander V. Braito S. Corbel D. Cseh S.W. Davis G. Dubus S. Farrell R. Fender C. Maraston R. Narayan S. R. Oates J. Pforr B. Plazolles M. Servillat K. Wiersema Y. Zhu N. Gehrels A.J. Gosling I. Heywood T. Kawaguchi C. Knigge J.-P. Lasota E. Lenc D. Lin T. Maccarone C. Maraston

Accretion Flow Instabilities 2012 Science rationale Two varieties of BHs known:  stellar-mass BH (3- ~20 M - e.g. Crowther et al. 2010, Belczynski et al. 2010, maybe up to ~80-90 M - e.g. Fryer 1999, Belczynski et al. 2004, 2010)  supermassive BHs ( M, possibly down to a few x 10 5 M - e.g. Peterson et al. 2005, Greene & Ho 2004, 2007) The cosmological growth of supermassive BHs is a key to understanding the formation and evolution of galaxies.  SMBH seeds proposed to be of intermediate masses ( M - Madau et al. 2001, Bellovary et al. 2011)  Growth by IMBH mergers, gas accretion episodes / accreting stars (e.g. Bromley et al. 2012)

Accretion Flow Instabilities 2012 Science rationale Two varieties of BHs known: stellar-mass and supermassive BHs The cosmological growth of supermassive BHs is a key to understanding the formation and evolution of galaxies. Do IMBHs exist? If so, where to search for them? IMBHs are also of great interest for: detecting strong gravitational wave signals (e.g. Miller et Colbert 2004) & dark matter (Fornasa & Bertone 2008) investigating the unification of some properties of accreting BHs of all masses One possibility is in Ultra Luminous X-ray sources.

Accretion Flow Instabilities 2012 Ultra Luminous X-ray sources ULXs are X-ray sources exibiting L X > 3x10 39 erg s -1. ULXs are located outside the nucleus of their host galaxy (spiral & elliptical galaxies). Most ULXs are thought to be accreting BHs. Assuming spherical, steady accretion, L Edd (M BH ~ 20 M ) ~ 3x10 39 erg s -1 To avoid L X > L Edd, M should be in the IMBH range. Soft components in some ULX spectra with kT ~ keV may also provide evidence for IMBHs (e.g. Roberts 2007)

Accretion Flow Instabilities 2012 Ultra Luminous X-ray sources Not all ULXs contain IMBHs ( Gao 2003, King 2004, Grimm et al. 2003, Walton et al. 2011, Roberts 2007) Alternatives:  Collimated emission (geometrical collimation – King et 2001, King 2008; relativistic boosting – Freeland et al. 2003)  super- or critical Eddington accretion onto a stellar mass BH (e.g. Mizuno et al. 2001, Vierdayanti et al. 2006, Feng & Kaaret 2007, Kajava & Poutanen 2008, King 2009, Zampieri & Roberts 2009, Gladstone et al. 2009) Kajava & Poutanen (2008)

Accretion Flow Instabilities 2012 Discovery of HLX-1 (Farrell et al. 2009) Discovery made using the 2XMM catalogue (Watson et al. 2009) during a work to search for new compact objects Name = 2XMM J (observation in 2004 Nov. 23) Associated with the near-edge-on S0-type galaxy ESO – 8” from nucleus z galaxy = (i.e. 95 Mpc) 9% chance for a random association HST image HLX-1 at most Brightest detected ULX! Source variable Assuming L 500 M

Accretion Flow Instabilities 2012 Redshift measurement (Wiersema et al ) Chandra position with error circle of 0.3” (95% c.l.) – Webb et al Optical counterpart in R (23.8 mag) & V (24.5 mag) bands ( Soria et al. 2010)  F X /F opt ratio ~ 1000 unlikely to be a background AGN (< 10) VLT DDT in Nov.-Dec to measure the redshift of HLX-1 Galaxy redshift = At 6643Å, rest-frame 6497Å blend line well known to be associated to late G & early K stars – consistent with ~5-6 Gyr old stellar population from ESO (Soria et al. 2010) Galaxy HLX-1 Hα in absorption Na ID in absorption Blend Fe I & Ca I line

Accretion Flow Instabilities 2012 Redshift measurement (Wiersema et al ) Confirmation of HLX-1 distance (~95 Mpc using WMAP5 parameters) Possible velocity offset ~ +170 km s -1 Galaxy HLX-1 Galaxy HLX σ detection of a Hα line in emission at the position of HLX-1 only

Spectral states Hardness-Intensity track reminiscent of those in Galactic BH binaries, but at much brighter luminosity (L keV ~ 2x erg s -1 ) BH nature in HLX-1 strengthened Godet et al. (2012) Meyer-Hofmeister et al. (2009) GX339-4 Accretion Flow Instabilities 2012

Spectral states State transitions: soft-to-hard and hard-to-soft, XMM1 = steep- powerlaw state (Godet et al. 2009; Servillat et al. 2011)

Accretion Flow Instabilities 2012 Detection of radio flares (Webb et al. 2012, Science) Detection of radio flares (Webb et al. 2012, Science) Stellar-mass & supermassive BHs known to launch jets Stellar-mass BH binaries known to show radio flares following the hard-to-soft transition (e.g. Körding et al. 2005) 7 x 12 hrs ATCA observations at 5 & 9 GHz spread over 2 outbursts (Fender et al. 2004)

Accretion Flow Instabilities 2012 Detection of radio flares (Webb et al. 2012, Science) Detection of radio flares (Webb et al. 2012, Science) Stellar-mass & supermassive BHs known to launch jets Stellar-mass BH binaries known to show radio flares following the hard-to-soft transition (e.g. Körding et al. 2005) 7 x 12 hrs ATCA observations at 5 & 9 GHz spread over 2 outbursts Hard-to-soft transition

Accretion Flow Instabilities 2012 Detection of radio flares (Webb et al. 2012, Science) Detection of radio flares (Webb et al. 2012, Science) Co-added 5+9 GHz detection: 8.2σ, F = 45 μJy All non detections (5+9 GHz): 3σ upper limit = 21 μJy Detection of a variable radio emission following the hard-to-soft transition Radio nebula as seen in some ULXs excluded Data consistent with a transient jet ejection event First ever detection of a jet event in a ULX!

Accretion Flow Instabilities 2012 Constraining the BH mass From peak luminosity and assuming hyper-accretion with L X 500 M (Farrell et al. 2009) Eddington scaling from Galactic BH sources: Radio flares appear when L X ~10%-100% L Edd (e.g. Fender et al. 2004) 9200 < M < M (Webb et al. 2012) Low hard state: L X ~ 1% L Edd M ~ M (Servillat et al. 2011) Steep powerlaw state: L X ~ 100% L Edd M ~ 2x10 4 M (Servillat et al. 2011) Soft-to-hard luminosity transition: L X ~ 1-4% L Edd (e.g. Maccarone 2003) < M < M Dynamical measurements of the BH mass very challenging given the source distance (~95 Mpc) Need to rely on some indirect ways to weigh the BH

Accretion Flow Instabilities 2012 Constraining the BH mass To detect a 6.4 keV Fe line (Gaussian) with EW = 30 eV, exposure time > 1 Ms with XMM – even worse for relativistic lines Spectral fitting of the soft component by physically motivated accretion flow models: BHSPEC (Davis et al. 2005) & the SLIMDISK model (Kawaguchi 2003) Motivation: To put constraints on the BH mass & accretion flow Both models applied to either Galactic BH sources or/and ULXs (e.g. for SLIMDISK, Foschini et al. 2006; Okajima et al – Hui & Krolik for BHSPEC). SLIMDISK = sub- and super-Eddington accretion regime - radial advection Methodology: multi-epoch and multi-instrument (XMM, Swift-XRT & Chandra) spectral fitting within Xspec

Accretion Flow Instabilities 2012 Results from Davis et al. (2011) BHSPEC = relativistic sub-Eddington thin disk including Comptonization and electron opacity effects around a rotating BH (Davis et al. 2005) Key parameters: Data selection: Swift1 Swift2

Accretion Flow Instabilities 2012 Results from Davis et al. (2011) Large degeneracy due to lack of constraints on i & a* uncertainty in the BH mass by a factor of 100 XMM: 3000 M (i=0 o, a*=-1, l=0.7) < M < 3x10 5 M (i=90 o & a*=0.99) at 90% c.l.  Assuming a binary system, i < 75 o due to lack of eclipses & M < 10 5 M Swift & Chandra: a*< 0 & a* < -0.5 inconsistent with Swift & Chandra data, respectively. l ≥ 1 for sufficiently low a*. All data: 6000 M (i=0 o & l=1) < M All BH mass estimates favour the IMBH solution. 68% 90% 99%

Accretion Flow Instabilities 2012 Results from Godet et al. (2012) SLIMDISK = face-on (i=0 o ) sub- and super-Eddington accretion disks around a non-rotating (a*=0) BH (Kawaguchi 2003) SLIMDISK includes relativistic and electron opacity effects, Comptonization & effects of radial advection (computed at every radii) key parameters: Data selection: XMM, Chandra & Swift-XRT (over different intensity ranges) Consistent mass estimate (90% level) between 3 instruments: M < M < M SLIMDISK model also favours the IMBH solution.

Accretion Flow Instabilities 2012 Results from Godet et al. (2012) Accretion rate (L Edd /c 2 ) keV unabs. luminosity (erg s -1 ) Accretion flow radiating with an efficiency of η ~ 0.11 At peak, Otherwise, Sub-Eddington accretion regime Assuming an average BH mass of M, l ~ 1.1 at the peak  Inner part of the disk is radiation-pressure dominated Kawaguchi (2003)

Accretion Flow Instabilities 2012 Results from Godet et al. (2012) Evolution of the accretion rate At peak, “Plateau” around the peak lasting for 2-3 weeks with

Accretion Flow Instabilities 2012 Results from Godet et al. (2012) Rapid decrease of the accretion rate towards the end of the outbursts  Soft-to-hard state transition In low/hard,

Accretion Flow Instabilities 2012 Results from Godet et al. (2012) Rapid decrease of the accretion rate towards the end of the outbursts  Soft-to-hard state transition In low/hard, Inner parts of the disk switch to an ADAF geometry (e.g. Esin et al. 1997) ? Innermost radius supposed to recede at larger radii in this case

Accretion Flow Instabilities 2012 Results from Godet et al. (2012) Evolution of the X-ray luminosity vs disk temperature Measured: (1σ error) Computed: using η=0.11, R ISCO =3R S & M = M

How is the BH fed? (Lasota et al. 2011) Accretion Flow Instabilities 2012 Swift-XRT monitoring since 2009 (> 600 ks) FRED-like outbursts  presence of reflares  plateau phase at the peak (~2-3 weeks) Recurrence timescale of nearly a year (~367 days) Very steep rise (over a week) Are HLX-1’s outbursts produced by a thermal-viscous instability as seen in X-ray transients & dwarf novae? Answer: observed outburst timescales inconsistent with thermal- viscous timescales ~1 yr

How is the BH fed? (Lasota et al. 2011) Accretion Flow Instabilities 2012 From the SLIMDISK & BHSPEC results, the inner part of the disk appears to be radiation-pressure dominated (L X > 0.06 L Edd ). Radiation-pressure dominated disk shown to be viscously and thermally unstable (e.g. Shakura & Sunyaev 1976, Piran 1978) Local thermal instability should result in the “limit-cycle” behaviour (e.g. Honma et al. 1992) However, not seen in high-state BH binaries with L X up to 0.5 L Edd (e.g. Gierliński & Done 2004) except maybe in GRS (Belloni et al. 1997, Xue et al. 2011) MHD simulations (e.g. Hirose et al. 2009) showed that there are no such thermal instabilities “Limit-cycle” behaviour not seen in HLX-1 lightcurve

How is the BH fed? (Lasota et al. 2011) Accretion Flow Instabilities 2012 Recurrence timescale = orbital modulation Modulated mass-transfer from a donor star in an eccentric orbit AGB star e ~ 0.7 P ~ 1 yr At periastron, the star fills its Roche lobe and matter falls onto the disk. pre-existing disk C/O AGB stars known to have significant mass loss up to M Θ yr -1 (e.g. Bowen & Wilson 1991) Matter will diffuse inwards on a viscous timescale

How is the BH fed? (Lasota et al. 2011) Accretion Flow Instabilities 2012 Recurrence timescale = orbital modulation Modulated mass-transfer from a donor star in an eccentric orbit “Plateaus” seen in the lightcurve around the peak consistent with an enhanced mass-transfer mechanism Caveat: viscous timescale possibly too long with respect to the recurrence timescale

Accretion Flow Instabilities 2012 The nature of the host? (Farrell et al. 2012) Faint optical counterpart detected in 2010 (Soria et al. 2010) Swift/UVOT and GALEX (FUV & NUV) images show evidence for extended emission towards the HLX-1 position (Webb et al. 2010) Host: A starburst region (IMBH = run-away collisions & mergers of massive stars – Freitag et al. 2006) ? An interaction between an IMBH dwarf galaxy and ESO producing a trail of stars (e.g. Sun et al. 2010)? DDT with HST to constrain the nature of the host in 2010 (FUV, NUV, C, V, I, H) – simultaneous observations in X-rays with Swift GALEX - NUV GALEX - FUV Magellan contours HST - FUV HLX-1 background galaxy (z~0.03) dust lanes HLX-1 HST - NUV

Accretion Flow Instabilities 2012 The nature of the host? (Farrell et al. 2012) HLX-1 counterpart detected from FUV to H band. Optical source is not resolved in any filters. Host size < 40 pc in diameter  Globular clusters have half-mass radius ~ 10 pc (e.g. Harris 1996)  Young stellar cluster have half-mass radius < 50 pc (Portegies Zwart et al. 2010) HLX-1

Accretion Flow Instabilities 2012 The nature of the host? (Farrell et al. 2012) Spectral fitting of the SED (X-rays & optical/NIR) 2 possible solutions Age: Mass: Disc irradiation: 2 (d.o.f.): < 1.3 x 10 7 yrs 4 x 10 6 M 8 x (27) 1.3 x yrs 6 x 10 6 M (27) Young * pop. Old * pop. Stellar pop. model (Maraston et al. 2005) Irradiation model diskir (Gierliński et al. 2008, 2009) Γ = 2.1 (fixed) kT cor. = 100keV (fixed) R out ~ R in kT disk ~ 0.2 keV Between 200 Myrs & 10 Gyrs, Irradiation fraction >> 10%

Accretion Flow Instabilities 2012 The nature of the host? (Farrell et al. 2012) GCs dominated by old stars in our Galaxy (Forbes 2003) However, GCs with young stellar pop. observed around disrupted galaxies (e.g. Antennae – Bastian et al. 2006) & Magellanic Clouds (Elson & Fall 1985) A GC with ~4 x 10 6 M corresponds to the upper end of standard GC mass range (Maraston et al. 2004). Alternative: Accreted dwarf galaxy scenario (e.g. Knierman et al. 2010, Mapelli et al. 2012)  a large fraction of gas & stars tidally stripped from the dwarf galaxy during interaction leaving only stars most closely bound to the BH (GC-like)  star formation triggered by tidal interactions  compatible with eccentric binary scenario  presence of dust lanes in ESO could provide evidence for a recent or on-going gas-rich interaction (Shabala et al. 2011)  Number density of X-ray sources similar to HLX-1 ~ Mpc -3 following this scenario (Mapelli et al. 2012) – HLX-1 distance ~ 100 Mpc

Accretion Flow Instabilities 2012Conclusions HLX-1 located at a distance of ~100 Mpc is the brightest ULX known. Probably emitting close to the Eddington limit around the peak. Properties (spectral state transitions, radio flares) similar to what is seen in stellar-mass BH binaries. Spectral fitting and observations in X-rays/radio provide a strong support for the presence of an IMBH - Mass range = M Sub-Eddington radiation-pressure dominated accretion disk with a high (~4x10 -4 M yr -1 ) at the outburst peak. ~1yr recurrence timescale in the outbursts seen in the X-rays could be interpreted as the result of modulated mass-transfer from an AGB star with an eccentric orbit. HLX-1 host is likely to be a young stellar cluster that is the remnant of a recent or on-going interaction of an IMBH dwarf galaxy with ESO

Accretion Flow Instabilities 2012 What will come next? 4x12h ATCA observation in the low/hard state to “test” the BH fundamental plane EVLA/ATCA observations triggered on the 2012 outburst  to further investigate the properties of the radio flares HST/XMM observations at different outburst epochs  to constrain the nature of the stellar population high-resolution ATCA observations in HI  to search for evidence of an interaction between HLX-1 host and ESO (tidal tails) VLT/Swift monitoring campaign to track the rise of the 2012 outburst  to constrain the outburst mechanism (inside-out or outside-in) o to test Lasota et al. (2011) binary scenario o to estimate the accretion rate at the outburst peak Swift-XRT monitoring still on-going