Accretion-ejection morphology of the microquasar SS433 resolved at sub-au scale
GRAVITY Collaboration, P.-O. Petrucci, I. Waisberg, J.-B. Le Bouquin, J. Dexter, G. Dubus, K. Perraut, P. Kervella, R. Abuter, A. Amorim, N. Anugu, J.P. Berger, N. Blind, H. Bonnet, W. Brandner, A. Buron, ?. Choquet, Y. Clénet, W. de Wit, C. Deen, A. Eckart, F. Eisenhauer, G. Finger, P. Garcia, R. Garcia Lopez, E. Gendron, R. Genzel, S. Gillessen, F. Gonte, X. Haubois, M. Haug, F. Haussmann, Th. Henning, S. Hippler, M. Horrobin, Z. Hubert, L. Jochum, L. Jocou, Y. Kok, J. Kolb, M. Kulas, S. Lacour, B. Lazareff, P. Lèna, M. Lippa, A. Mérand, E. Müller, T. Ott, J. Panduro, T. Paumard, G. Perrin, O. Pfuhl, J. Ramos, C. Rau, R.-R. Rohloff, G. Rousset, J. Sanchez-Bermudez, S. Scheithauer, M. Schöller, C. Straubmeier, E. Sturm, F. Vincent, I. Wank, E. Wieprecht, M. Wiest, E. Wiezorrek, M. Wittkowski, J. Woillez, S. Yazici, G. Zins
AAstronomy & Astrophysics manuscript no. ss433petrucciV10 c (cid:13)
ESO 2018March 14, 2018 L etter to the E ditor Accretion-ejection morphology of the microquasar SS433 resolvedat sub-au scale (cid:63)
GRAVITY Collaboration (cid:63)(cid:63) : P.-O. Petrucci , I. Waisberg , J.-B. Le Bouquin , J. Dexter , G. Dubus , K. Perraut ,P. Kervella , , R. Abuter , A. Amorim , N. Anugu , J.P. Berger , , N. Blind , H. Bonnet , W. Brandner , A. Buron ,´E. Choquet , , Y. Cl´enet , W. de Wit , C. Deen , A. Eckart , , F. Eisenhauer , G. Finger , P. Garcia , R. GarciaLopez , E. Gendron , R. Genzel , , S. Gillessen , F. Gonte , X. Haubois , M. Haug , , F. Haussmann , Th. Henning ,S. Hippler , M. Horrobin , Z. Hubert , , L. Jochum , L. Jocou , Y. Kok , J. Kolb , M. Kulas , S. Lacour ,B. Lazare ff , P. L`ena , M. Lippa , A. M´erand , E. M¨uller , , T. Ott , J. Panduro , T. Paumard , G. Perrin , O. Pfuhl ,J. Ramos , C. Rau , R.-R. Rohlo ff , G. Rousset , J. Sanchez-Bermudez , S. Scheithauer , M. Sch¨oller ,C. Straubmeier , E. Sturm , F. Vincent , I. Wank , E. Wieprecht , M. Wiest , E. Wiezorrek , M. Wittkowski ,J. Woillez , S. Yazici , , and G. Zins (A ffi liations can be found after the references) Received ... / Accepted ...
ABSTRACT
We present the first optical observation at sub-milliarcsecond (mas) scale of the microquasar SS 433 obtained with the GRAVITY instrumenton the VLT interferometer. The 3.5 hour exposure reveals a rich K-band spectrum dominated by hydrogen Br γ and He i lines, as well as (red-shifted) emission lines coming from the jets. The K-band continuum emitting region is dominated by a marginally resolved point source ( < ff use background accounting for 10% of the total flux. The jet line positions agree well with the ones expected fromthe jet kinematic model, an interpretation also supported by the consistent sign (i.e. negative / positive for the receding / approaching jet component)of the phase shifts observed in the lines. The significant visibility drop across the jet lines, together with the small and nearly identical phases forall baselines, point toward a jet that is o ff set by less than 0.5 mas from the continuum source and resolved in the direction of propagation, with atypical size of 2 mas. The jet position angle of ∼ ◦ is consistent with the expected one at the observation date. Jet emission so close to the centralbinary system would suggest that line locking, if relevant to explain the amplitude and stability of the 0.26c jet velocity, operates on elementsheavier than hydrogen. The Br γ profile is broad and double peaked. It is better resolved than the continuum and the change of the phase signalsign across the line on all baselines suggests an East-West oriented geometry alike the jet direction and supporting a (polar) disk wind origin. Key words.
Stars: individual: SS433 – ISM: jets and outflows – Techniques: interferometric – Infrared: stars
1. Introduction
SS 433 (K = ∼ M (cid:12) (e.g. Cherepashchuk et al. 2013 and refer-ences therein) accreting matter from a companion massive star(e.g. Gies et al. 2002; Hillwig & Gies 2008). It is located at 5.5 ± ∼ ◦ around a precession axis of position angle PA ∼ ◦ .The jet of SS 433 has been intensively studied from arcmin-utes down to milliarcsecond (mas) scale in the radio. VLBI ob-servations show that jet signatures are already present at about (cid:63) Based on observations made with VLTI / Gravity instrument. (cid:63)(cid:63)
GRAVITY is developed in a collaboration by the Max PlanckInstitute for extraterrestrial Physics, LESIA of Paris Observatory / CNRS / UPMC / Univ. Paris Diderot and IPAG of Universit´e GrenobleAlpes / CNRS, the Max Planck Institute for Astronomy, the Universityof Cologne, the Centro Multidisciplinar de Astrof´ısica Lisbon andPorto, and the European Southern Observatory. ∼ . c (Eikenberry et al. 2001). The powerful and contin-uous accretion flow is provided by the donor star at a rate of ∼ − M (cid:12) / yr , forming a complex structure around the system.Orbital phase-resolved optical spectra show evidence of an ac-cretion disc in the inner region, which is also thought to powerthe ejection of matter (e.g. Perez & Blundell 2009). The accre-tion disc is not directly observable due to the presence of a densewind outflowing from the disc itself but was revealed by the de-tection of a pair of widely-separated, hence rapidly rotating ( ∼
500 km s − ), narrow components in the profile of the ”station-ary” (i.e. not associated with the jet) Br γ and H α lines (Perez& Blundell 2009, 2010). The disc is expected to be perpendicu-lar to the jet axis and precessing with it at the same period (e.g.Blundell et al. 2011). The IR spectrum of SS 433 is compati-ble with bremsstrahlung emission produced by the accretion discwind, the estimated size at 2 microns of the accretion disc + windsystem then being smaller than 1 mas (Fuchs et al. 2006).We present in this letter the observation of SS 433 in theK band performed in July 2016 with the VLTI / GRAVITY in- a r X i v : . [ a s t r o - ph . H E ] M a y lease give a shorter version with: \authorrunning and / or \titilerunning prior to \maketitle Fig. 1. a) Normalized K-band spectrum (with a binning over 2spectral channels) obtained with GRAVITY. The Br γ and He i lines as well as the expected position of the corresponding jetlines ( jet / jet / receding jet) are indicatedby vertical lines. We have also reported the expected position ofthe Br δ jet line from the receding jet. The solid blue / red line cor-responds to the emission of the approaching / receding jet com-ponents. b) Visibility amplitudes and c) phases on the UT1-UT3baseline (the visibility amplitudes and phases of all the baselinesas well as the uv -plane at the time of the observation are shownin the Appendix, in Fig. B.1). For the phase, we follow the signconvention of Pauls et al. (2005), i.e. negative phases point to thebaseline direction. The solid black line corresponds to the bestfit model of the jet lines (see Sect. 3.2.2).strument. The spectro-interferometric capabilities of GRAVITYallow us to resolve, for the first time in the Optical, this micro-quasar at sub-mas spatial resolution. This gives us the opportu-nity to study on such scales and simultaneously the propertiesof the di ff erent accretion-ejection components (jets, wind, disk),providing a new look at this famous source.
2. Observations and data reduction
We used GRAVITY (GRAVITY collaboration, 2017, submitted;Eisenhauer et al. 2011) to perform spectral-di ff erential interfer-ometry on SS 433 with the Unit Telescopes (UTs) of the VLTI.We recorded 18 data sets at high spectral resolution ( R = . We used the GRAVITY stan-dard pipeline (Lapeyrere et al. 2014) to reduce and calibrate theobservations. We have checked that the di ff erent files are simi-lar within errors. Then, to increase the signal-to-noise ratio, we The ”uniform disk” diameters in the K band are 0.214 ± ± merged all the files and binned over 2 spectral channels the in-terferometric observables. We used the interferometric data pro-vided by the fringe tracker (working at a higher frequency, i.e.around 900 Hz) to calibrate the continuum. The uv -plane at thedate of observation is indicated in the Appendix (Fig. B.1).
3. Results
The K-band GRAVITY spectrum is plotted on the top panel ofFig. 1. It is very rich with hydrogen Br γ and He i lines, as wellas broad features around 2.08, 2.18 and 2.3 µ m. The stationaryBr γ line is clearly double peaked (see also Fig. 2) while the sta-tionary He I line shows a P Cygni profile arising in the wind. Wehave overplotted in Fig. 1 the positions of the He i , Br γ and Br δ approaching (hereafter jet1, pointing to East) and receding (here-after jet2, pointing to West) jet lines as predicted by the standardkinematic model formula (see Fabian et al. 2004, hereafter F04,and reference therein):1 + z ± = γ (1 ± β sin θ sin i cos φ ± β cos θ cos i ) (1)where the - and + signs correspond to the jet1 and jet2 compo-nent respectively, β the jet velocity in unit of light speed, θ theprecession angle between the jets and the precessional axis, i theangle between the precessional axis and the line of sight and ψ the precessional phase at the observation date. We use the val-ues determined by Eikenberry et al. (2001) for β , θ and i i.e. β = θ = ◦ and i = ◦ . At the date of the GRAVITYobservation, the estimated precessional phase is ψ ∼ .
71. Thejet line positions computed with these parameter values (verti-cal dashed line in Fig. 1) agree quite well with the position ofthe observed emission features, clearly supporting a jet origin.The fact that the signs of the phase shifts (Fig. 1c, see below)are the same for all the ”jet1” lines and all the ”jet2” lines, i.e.positive (negative) for jet1 (jet2), also supports that the spatiallyresolved lines originate in the jets. These features will be called”jet lines” in the following. The parameters of our jet lines fit(redshift, FWHM, equivalent width) are reported in Appendix(Table A.1).Note that at the precession phase of ∼ ff ect, thereceding and approaching shifts z ± are both redshifts. Moreover,the approaching He i and receding Br γ lines are very close andnot distinguishable, producing a blended emission line profilearound 2.18 µ m. This precessional phase also corresponds toa near edge-on disk orientation, a precessional phase whereP Cygni profiles are commonly observed in H and He I lines(F04). The absolute visibility amplitudes of the continuum are allhigher than 0 . ∼ ff usebackground accounting for 10% of the total flux. \authorrunning and / or \titilerunning prior to \maketitle In the following, we normalize the visibility amplitudes andphases by their values in the continuum in order to work with dif-ferential quantities. Consequently the measured visibility ampli-tudes in the continuum (taken from the fringe tracker), hereaftercalled V c , are included into the modeling. As an example, thenormalized visibility amplitudes and phases for the UT1-UT3baseline are shown in the middle and at the bottom of Fig. 1respectively (the normalized visibility amplitudes and phases ofthe six baselines are plotted in the Appendix in Fig. B.1). We decompose the jet lines contribution into two parts, F jet1 ( λ ) / F jet2 ( λ ) from the jet1 (approaching) / jet2 (receding) componentsseparately as detailed in Appendix A. Looking at the visibilityand phases, we infer that jet1 and jet2 are roughly symmetricaround the continuum and that the Br γ and He I lines are emittedfrom the same regions. Consequently the interferometric di ff er-ential observables can be modeled with: V Norm ( u , v , λ ) = F jet1 ( λ ) V ( u , v ) + F jet2 ( λ ) V ( − u , − v ) + V c ( u , v ) V c ( u , v )[ F jet2 ( λ ) + F jet1 ( λ ) +
1] (2)where V ( u , v ) is the Fourier Transform of the geometrical modelfor Jet1.The significant visibility drop across the lines (with deeperdrops on the longest baselines), together with the significant ( > ◦ for each spectral line) and nearly identical phases for allbaselines, point toward an overall jet geometry which is signif-icantly resolved, with a typical size of 2 mas and only slightlyo ff set from the continuum, typically by less than 1 mas.This is confirmed by tentative adjustments with componentslocated further away (one blob, several blobs, varying PA, fluxratio, elongation, intensity profile...). Secondary minima exist inthe χ r cube for solutions located further away (typically 10 maswith PA ≈
70 deg and 15 mas with PA ≈
90 deg). However the fitqualities are low ( χ r >
3) and they predict a wrong sign ofthe visibility phase for at least one baseline. In fact, fitting alljet lines together under the assumption that they have similarorigins and structures, makes the fit rather well constrained be-cause of the additional spatial frequency coverage. We concludethat elongated models ( > ff setGaussian provides a statistically better fit ( χ r = . χ = V ( u , v , s , a ) = exp(2 i π f a )1 + i π f s (3)with f = v cos( PA ) − u sin( PA ). This is the Fourier Transformof an exponentially decreasing profile exp( − ( r − a ) / s ) H ( r − a ),where H ( r ) is the Heaviside function. It is limited to the positiveordinates, decays on a spatial scale s and is translated by a fromthe continuum position. Positive r are defined toward PA (Northto East), and the model is infinitely thin in the direction perpen-dicular to PA. The fit is significantly better ( χ r = σ errors ) are s = . ± . To compute the errors, we assume 60 observables (6 baselines × γ , Br δ and He I) × δ ) × a , s and PA ), giving 57 degrees of freedom Fig. 2.
Normalized spectrum (top), visibility phases (left) andamplitudes (right) around the Br γ line with a spectral binning of2. The symbol colors correspond to the baseline colors indicatedin Fig. B.1. a = − . ± .
34 mas and PA = ± ◦ . The resulting best fit isreported in Fig. 1 in black solid line. We tested the thicknessof the jet in the transverse direction by convolving the intensityprofile with a gaussian. The jet profile is unresolved in the trans-verse direction, with an upper limit of its transverse size of ∼ σ . This upper limit is quite large however due to thecoincidental alignment of the baselines with the jet PA (see the uv -plane in the Appendix in Fig. B.1). γ The Br γ profile is broad and double peaked. The visibilitiesclearly drop across the line for all the baselines, with a decreasethat can reach 20% with respect to the continuum (Fig. 2). Thedrop is also deeper for longer baselines. The emitting regionsize is found to be ∼
4. Discussion
Our GRAVITY observations of SS 433 allow us to resolve itsaccretion-ejection structure at sub-mas scale. A sketch summa-rizing our results is shown in Fig. 3. Most (90%) of the infraredcontinuum comes from a partially resolved central source of typ-ical size ∼ \authorrunning and / or \titilerunning prior to \maketitle > 15 mas / e1 . m a s j e t i n t en s i t y p r o fi l e - NE - . Fig. 3.
Sketch of the inner region of SS 433. Most (90%) ofthe infrared continuum comes from a partially resolved cen-tral source of typical size ∼ > γ line emitting region size is found to be ∼ ∼ ◦ . Thejet profile is unresolved in the transverse direction ( < > ∼ ◦ ) derived from the kinematic model at a precessionphase ψ = .
71. The jet profile is unresolved in the transversedirection ( < σ ), as expected since both optical andX-ray emission line widths indicate a jet opening angle θ j ≈ ◦ (Borisov & Fabrika 1987; Marshall et al. 2002). The inten-sity profile along the jet axis is best characterized in our data byan emission peaking at the continuum position and decreasingexponentially on a scale of 1.7 mas = × cm.This profile for the jet intensity is certainly not unique.However, the quite strong decrease of the visibility amplitudeacross the jet lines for all the baselines as well as the similarphase behavior of the jet lines produced by the same jet compo-nent strongly support a sharply decreasing two-sided intensityprofile for the jet.During the ∼
4h observation, the jet material traveling at0.26c moves by ∼ α profile.They found an o ff set of the jet emission by 4 × cm (4.8 mas) from the core and with a larger spatial extent of 6.7 × cm(8.1 mas) (see also F04). A direct comparison is di ffi cult how-ever given the non-simultaneity of these observations and theknown variability of the jet structure. Moreover, the NIR andoptical lines may have di ff erent emissivity profiles (e.g. becauseof increased extinction in the optical compared to the NIR).Our observation unambiguously fixes the size of the jet in-frared line emission region ( ∼ . × cm). Given the averagejet line luminosities, this size is far too large for the (conical) jetto be entirely filled of emitting gas and a quite small clump fill-ing factor ( ∼ − ) is required (e.g. Davidson & McCray 1980;Begelman et al. 1980; Panferov & Fabrika 1997). Clumping islikely to be due to thermal instabilities in a hot outflow, withthe outflow initially driven by radiation pressure in the funnel ofan accretion disk (Davidson & McCray 1980). X-ray line vari-ability limits the hot outflow region to < × cm (Marshallet al. 2013). Our observation shows that the IR emission is dis-placed by less than 0.2 mas = × cm from the central bi-nary system, confirming that clumping originates early on in thehot outflow. It also suggests that line locking (Milgrom 1979),if relevant to explain the amplitude and stability of the 0.26c jetvelocity, operates on elements heavier than hydrogen to be e ffi -cient on this small scale (Shapiro et al. 1986). Models show thatcontinuous heating of the clumps is required to explain opticalemission on a scale of 10 cm (Brinkmann et al. 1988). Futurecomparison of observed and theoretical intensity profiles mayshed light on this heating process (Brinkmann & Kawai 2000).The stationary Br γ line shows a broad and double peakedprofile and the interferometric observables suggest a geometrydominated by an East-West component, in the direction of thejet, like a (rather polar) disk wind. Then, both receding and ap-proaching components must be present to explain the changeof sign of the phases across the line. Taking the wavelength ofthe phase extrema as the corresponding blue and redshifted lineproduced by the wind, we infer a velocity of ∼
600 km s − and ∼ − for the approaching and receding flow respec-tively. We also observe a similar dissymmetry in the phases,which are significantly smaller in the blue part of the line.Absorption e ff ects could play a role here. Indeed at the preces-sional phase of the GRAVITY observation, the accretion disk hasa near edge-on orientation and strong absorption from the windcould a ff ect the line profile (like the P Cygni profile of He i ) es-pecially in its blue part.The results presented here demonstrate the potential ofspectro-interferometry to dissect the super-Eddington outflowsand jets of SS 433. Additional insights will be gained in the fu-ture by monitoring this source at di ff erent precession and orbitalphases with GRAVITY to obtain spectro-interferometric con-strains on the stationary and jet line variability. Acknowledgments
This work is based on observations made with ESO Telescopesat the La Silla Paranal Observatory, programme ID 60.A-9102.It has been supported by a grant from LabEx OSUG@2020(Investissements d’avenir – ANR10LABX5) and in part bythe National Science Foundation under Grant No. NSF PHY-1125915. It has made use of the Jean-Marie Mariotti Center
Aspro2 and
SearchCal services . POP and GD acknowledgefinancial support from CNES and the French PNHE. JD wassupported by a Sofja Kovalevskaja Award from the HumboldtFoundation of Germany. E.C. is supported by NASA through a Available at http: // \authorrunning and / or \titilerunning prior to \maketitle Hubble Fellowship grant HST-HF2-51355 awarded by STScI,operated by AURA, Inc. We thank the ESO / VLTI team for theirconstant support. We also thank the technical, administrative andscientific sta ff of the participating institutes and the observatoryfor their extraordinary support during the development, installa-tion and commissioning of GRAVITY. References
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Appendix A: Jet lines spectral and interferometricsignature modeling
We decompose the contribution from the jet1 (approaching) andjet2 (receding) components separately as follows: F Norm ( λ ) = F jet1 ( λ ) + F jet2 ( λ ) + F jet1 ( λ ) = He i jet1 + Br γ jet1 (A.2) F jet2 ( λ ) = He i jet2 + Br γ jet2 + Br δ jet2 (A.3)where He i jet1 / jet2 , Br γ jet1 / jet2 and Br δ jet2 are the flux ratios (fixedusing the average spectrum) between the lines and the contin-uum. Assuming that the Br γ , Br δ and He I lines are emitted fromthe same regions, the interferometric di ff erential observables aregiven then by Eq. 2. The characteristics of the di ff erent jet lines(redshift, FWHM, EW) are reported in Table A.1. Name Rest wavelength redshift FWHM EW( µ m) (km s − ) (Å)Br γ jet γ jet δ jet i jet i jet Table A.1.
Properties of each jet line.
Appendix B: Normalized visibility amplitudes andphases of the six baselines
The uv -plane at the time of the observation, the K-bandGRAVITY spectrum as well as the visibility amplitudes andphases for the 6 baselines are plotted in Fig. B.1. Univ. Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France Max Planck Institute for extraterrestrial Physics, Giessenbachstr.,85748 Garching, Germany LESIA, Observatoire de Paris, PSL Research University, CNRS,Sorbonne Universit´es, UPMC Univ. Paris 06, Univ. Paris Diderot,Sorbonne Paris Cit´e, France Unidad Mixta Internacional Franco-Chilena de Astronom´ıa (CNRSUMI 3386), Departamento de Astronom´ıa, Universidad de Chile,Camino El Observatorio 1515, Las Condes, Santiago, Chile European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748Garching, Germany CENTRA and Universidade de Lisboa - Faculdade de Ciˆencias,Campo Grande, 1749-016 Lisboa, Portugal CENTRA and Universidade do Porto - Faculdade de Engenharia,4200-465 Porto, Portugal Observatoire de Gen`eve, Universit´e de Gen`eve, 51 ch. desMaillettes, 1290 Versoix, Switzerland Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl 17, 69117Heidelberg, Germany Hubble Fellow, Jet Propulsion Laboratory, California Institute ofTechnology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA European Southern Observatory, Casilla 19001, Santiago 19, Chile
1. Physikalisches Institut, Universit¨at zu K¨oln, Z¨ulpicher Str. 77,50937 K¨oln, Germany Max-Planck-Institute for Radio Astronomy, Auf dem H¨ugel 69,53121 Bonn, Germany Department of Physics, Le Conte Hall, University of California,Berkeley, CA 94720, USA 5lease give a shorter version with: \authorrunning and / or \titilerunning prior to \maketitle Fig. B.1.
Top) uv -plane at the time of the observation (average over the full exposure) with the di ff erent baselines indicated by col-ored points. The grey line represents the expected jet PA. Bottom) K-band GRAVITY spectrum (the solid blue / red line correspondsto the emission of the approaching / receding jet components) as well as the visibility amplitudes (left) and phases (right) for the 6baselines. The solid line corresponds to the best fit model of the jet lines (see Sect. 3.2.2). The symbol colors correspond to thebaseline colors indicated in the uv -plane.-plane.