A confirmed stellar-mass ULX with recurrent X-ray occultations from its precessing disk

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1 A confirmed stellar-mass ULX with recurrent X-ray occultations from its precessing disk C. Motch, R. Soria and M. Pakull Thanks to F. Grisé and J. Greiner

2 P13 in NGC 7793 : a key ULX D = 3.7 Mpc In Sculptor N E P13 B,V,I and Ha image (ESO) 2

3 The optical counterpart of P13 B Band 90% confidence Chandra error circle P13 First identified by M. Pakull Clear identification with a blue V ~ 20.5 object in NGC 7793 The optically brightest of all known ULX counterparts. Hα emitter. At the rim of prominent HII region and star cluster H α 3

4 L X ~ 3-10 x erg/s A supergiant B9Ia mass-donor star: M v ~ (m v ~ 20.5) T eff ~ K M ~ M R ~ 80 R L x /L bol ~ 8 High-order Balmer absorption lines revealing the companion star Stellar absorption lines Disk emission lines He II λ Bowen CIII-NIII complex in emission Motch, Pakull, Soria, Grisé et al., Nature,

5 Porb 63.5d from an optical photometric campaign in , and a Swift XRT/UV OT monitoring Amplitude 0.5 mag in V and 1.0 mag in Swift UVOT u band (3470 Ang). Photometric modulation largely due to X- ray irradiation of the heated hemisphere of the donor star Light curve modelling (ELC, Orosz+ 2000) yields e = and in order to fit the B9Ia star in its Roche lobe requires: u V Light curve fitting M BH < 15 M L X > L Edd Potential BH+NS merger (Beldycki & Belczynski 2016) Orbital Phase 5 5 C. Motch ULX and their environments Strasbourg June

6 diskbb + comptt Chandra 2003 XMM 2013 kt e ~ 2 kev ktin ~ 0.2 kev The X-ray energy (Super)-Eddington Canonical ULX spectrum with two-temperature accretion can generate feature Ultraluminous (Comptonized like) X-ray spectra distribution of P13 shows a soft excess and a high energy break typical of the Ultraluminous state 6 6 C. Motch ULX and their environments Strasbourg June

7 X-ray heating effects No heating X-ray heating! No heating Orbital Phase 7

8 A recent outburst reaching Lx ~ erg/s Orbital Phase 0.5 (apoastron) May

9 X-ray heating effects Optically bright V ~ 20.0 mag X-ray source close to companion star Optically faint V ~ 20.4 mag X-ray source too far to significantly heat the companion Lower Balmer absorption (reemission..), stronger Bowen (heated atmosphere). HeII emission remains steady as expected if emitted in the disc wind 9

10 Radial velocity curves Radial velocity curves changing with season / X-ray state He II emission velocity varies with orbital phase and seems to somehow trace BH velocity Complex Balmer absorption velocity patterns, possibly contaminated by disc wind as in SS433 (Barnes+2006; Hillwig & Gies 2007) (X-ray bright) 2011 (X-ray faint) Amplitudes consistent with M BH < 15 M Orbital Phase C. Motch ULX and their environments Strasbourg June

11 A precessing disc in P13? 2011 X-ray low state X-ray faint state (Lx ~ few erg/s) observed on occasion w/o any change in optical light curve amplitude and He II emission strength. => Companion star remains illuminated while X-ray source is shielded from our view. HeII & Balmer Radial velocity curves seem to depend on X-ray state (see previous slide). There is a long term oscillation of the phase of maximum light

12 Porb = 63.5d 2013 Grond Photometric maximum has a modulation of ~ +/- 0.1 in phase 2010 & 2011 ESO-VLT La Campanas 12 12

13 Porb=1.7d Phasing 11 optical and u band photometric maxima ~ 7 years Her X-1 optical light curves Gerend & Boyton

14 kev Swift/XRT Count Rate (ct s 1 ) L x = 1e40 erg/s 2012 disk occultation Possible long periods precession mechanisms: Tidal forced induced precession with periodic mass transfer (e ~ 0.3) Time (MJD) Her X-1 Leahy 2002 Irradiation driven warped discs 14 14

15 The low state X-ray spectrum Lx ~ 3e39 erg/s Chandra Lx ~ 2e39 erg/s XMM-Newton Nov 2013 Sep 2003 Lx ~ 5e37 erg/s Aug +Dec 2011 (142ksec, PI Soria) May 2012 (20ksec) Faint state spectrum cannot be fitted with a power-law, a single-temperature thermal-plasma, or a Comptonization spectrum. Unexplained line features at ~ 1 kev and at ~ 4.5 kev 1 kev line reminiscent of the 1 kev excess seen in ULX EPIC fits residuals (Middleton+2015) explained as line emission + blue shifted absorptions (Pinto+2016) Is the 4.5 kev line receding Fe line at v x cos(θ) 0.4 c? (the corresponding blueshifted peak would be at E ~10 kev, outside the Chandra and XMM-Newton band) 15 15

16 Conclusions The low state spectrum of P13 is consistent with the idea that the disk blocks the view to the central source during low states and allows us to directly observe outflows produced by the super-critical accretion disc. Future planned faint state X-ray observations may reveal/confirm the presence of high velocity winds and perhaps jets. Precession may offer the possibility to observe the same accretion disc and outflows at varying angle. Therefore P13 may be an interesting object for testing models of supercritical accretion disks (e.g. Ohsuga+2011, Jiang+2014, Sadowski+2015, King+2015). Observing state transitions will be very useful to distinguish between hard <-> ultrasoft or hard <-> absorbed mechanism. Thank you 16 16