HD 306414 is the optical counterpart to the transient X-ray source IGR J11215–5952, characterised by short flares occurring every 165 days. In order to improve our understanding of this source, we obtained high-resolution spectroscopy of HD 306414 around the time of the February 2007 X-ray outburst. Large variations in the shape of the H? emission feature and the radial velocities of metallic lines are seen around the time of the X-ray emission

Lorenzo, Javier

Negueruela, Ignacio

Norton, Andrew J .

English

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Open Research OnlineThe Open University’s repository of research publicationsand other research outputsHD 306414, the optical counterpart to the Peculiar X-Ray Transient IGR J11215-5952Conference ItemHow to cite:Lorenzo, Javier; Negueruela, Ignacio and Norton, Andrew J . (2010). HD 306414, the optical counter-part to the Peculiar X-Ray Transient IGR J11215-5952. In: High Energy Phenomena in Massive Stars, 2-5Feb 2009, Ja Spain.For guidance on citations see FAQs.c© 2009 The AuthorsVersion: Accepted ManuscriptCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copy-right owners. For more information on Open Research Online’s data policy on reuse of materials please consultthe policies page.oro.open.ac.uk**FULL TITLE**ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION****NAMES OF EDITORS**HD 306414, the optical counterpart to the peculiarX-ray transient IGR J11215-5952J. Lorenzo and I. NegueruelaAlicante University, SpainA.J. NortonOpen University, United KingdomAbstract. HD 306414 is the optical counterpart to the transient X-ray sourceIGR J11215−5952, characterised by short ﬂares occurring every 165 days. Inorder to improve our understanding of this source, we obtained high-resolutionspectroscopy of HD 306414 around the time of the February 2007 X-ray outburst.Large variations in the shape of the Hα emission feature and the radial velocitiesof metallic lines are seen around the time of the X-ray emission.1. Introduction and ObservationsThe hard X-ray source IGR J11215−5952 is a peculiar transient displaying veryshort X-ray outbursts every 165 days (Sidoli et al 2007). The outbursts lastonly a few days, and strong emission (LX > 1035erg s−1) is only seen for 2–3 dayseach cycle. The source is associated with the luminous supergiant HD 306414(Steeghs et al 2006) and shows characteristics typical of wind accretors. Someauthors have considered IGR J11215-5952 an example (even a paradigm) of aSupergiant Fast X-ray Transient, while others doubt this association. Here wepresent the ﬁrst study of the optical counterpart in order to shed light on itsproperties.Spectra of HD 306414 were obtained with FEROS at the ESO/MPG 2.2-m telescope, located at the European Southern Observatory in La Silla. Theobservations were conducted between December 2006 and February 2007. Theresolving power of the spectra is R ∼ 48000. The spectra cover a wide spectralrange, from ∼ 3500A˚ to ∼ 9200A˚ spread over 39 e´chelle orders. A total ofeleven spectra were bias subtracted, ﬂat-ﬁeld corrected, optimally extracted andwavelength calibrated using the MIDAS data reduction package.The extracted spectrum is typical of a luminous supergiant. The spectraltype is between B0.5 and B0.7, while all luminosity criteria support a veryluminous supergiant of class Ia. The spectrum contains several emission lines,most notably Hα, which presents a P-Cygni-type proﬁle in emission. We alsonote several Fe iii emission lines between 5920 and 6032A˚. Wolf & Stahl (1985)proposed that the Fe iii lines of multiplets 115 and 117 in emission discriminatebetween normal B supergiants and B hypergiants. This is actually not the case.We have checked that several early-B Ia supergiants show these lines in emission.HD 306414 is a B0.5 Ia supergiant.12 Lorenzo, Negueruela and NortonWe display the evolution of Hα in Figure 1. We notice a very strong varia-tion in radial velocity around the time of the X-ray outburst, which started onFebruary 6th and peaked on February 8th. A blue absorption feature is clearlyseen on the February 5th spectrum, suggesting enhanced mass loss. Unfortu-nately, our coverage of the outburst is not complete. We note, however, that abig excursion in radial velocity was also seen on December 22nd, far away fromperiastron, though no absorption feature was seen then.6555 6560 6565 6570 6575Wavelength (A)o012Flux (Normalized)03-12-200611-12-200615-12-200622-12-200629-12-200617-01-200705-02-200711-02-200713-02-200716-02-2007Figure 1. Hα proﬁles normalized and displayed in temporal order (day-month-year).2. Radial Velocities (RVs)First, we calculate radial velocities by estimating by hand the shift with respectto the position of the spectral lines at rest, in order to decide which set of spectrallines would be more suitable for deriving a radial velocity curve. We triedthe following lines: Si iii triplet (λλ4552.66, 4567.87, 4574.77), O ii (λλ4590.97,4596.17) and N ii (λλ4601.48, 4607.15, 4613.87, 4621.39). We conclude that thebest behavior corresponds to the Si iii triplet. This multiplet has three well-resolved lines covering a substantial range in line strength. When we displaythe radial velocity as a function of time for the three components of the Si iiitriplet, we estimate a maximum dispersion of 11.7 km s−1. For the other lines3used, we ﬁnd that the dispersion is 2.5 times larger than the O ii lines and 3.3times larger for the N ii lines.After this preliminary analysis, we used the cross-correlation technique, asimplemented in the Starlink/FIGARO HCROSS command, on the Si iii tripletlines. A spectrum of the B0.7 Iab supergiant HR 4806 was used as a template.We cross-correlate every individual spectrum using the region of the Si iii triplet.The resulting velocities corrected for heliocentric motion are displayed in Fig-ure 2.2.1. Radial Velocity CurveIn Fig. 2, we fold the radial velocities using the period of 165 days from Sidoli et al(2007). We have tried to match their distribution with a model radial velocitycurve. The model assumes Mx = 1.44 M and M∗ = 35 M, typical of thespectral type. Even though the model does not reproduce well the distributionof the radial velocities, we favour the set of orbital parameters shown in Table 1.0 0.5 1 1.5 2Phase253035404550RV (km/s)Figure 2. Measured radial velocities (crosses) and model radial velocitycurve for the optical component. Two whole periods are displayed.Table 1. Possible orbital solutions. The semi-amplitude of the velocitycurve and the major semi-axes for both components are calculated only forthe maximum values of eccentricity, inclination and systemic velocity allowed.Parameter ValueEccentricity 0.5 < e < 0.88Inclination 60◦ < i < 90◦Systemic velocity [km s−1] 36 < γ < 42Optical component mass [M] 35 (assumed)Kx [km s−1] 260.25Ko [km s−1] 10.70aX [R] 403a∗ [R] 16.64 Lorenzo, Negueruela and NortonThe measured errors on the radial velocities are negligible on this scale andmuch smaller than the excursions seen. These excursions are also observed inother systems containing very luminous supergiants, such as GX301−2 (Kaperet al 2006), and their origin is yet unknown.3. Conclusions• Our 11 high resolution spectra cover ∼ 75 days. This is less than halfthe 165-d orbital period and therefore we cannot derive a true orbitalsolution. We have only checked that the observed radial velocities seemcompatible with the expected curve for a high-eccentricity system. Weneed further spectroscopy of HD 306414 to improve on these results, asthe large excursions seen in the radial velocity curve are preventing usfrom ﬁnding an orbital solution.• We are working on an atmosphere model ﬁt for HD 306414, using theFASTWIND code. This will be used to derive stellar parameters andabundances. The synthetic spectrum will also be used as a template formore accurate RV measurements, expanding our set of spectral lines.• Two models have been proposed to explain the behaviour of IGR J11215−5952.Sidoli et al. (2007) speculate on the presence of an equatorial disk aroundthe supergiant. The optical spectra do not seem to present evidence fora sizable disk. A very thin disk is still a possibility, but parameters needto be ﬁnely tuned for the model to work. Negueruela et al. (2008) havesuggested that the wind is clumpy and accretion becomes ineﬃcient whenthe neutron star is more than ∼ 3R∗ away from the supergiant. Our simu-lations show that this scenario is viable if the eccentricity is very high. Foran assumed R∗ = 30 R, we must have e < 0.89 (otherwise the neutronstar hits the supergiant). For eccentricities 0.85 < e < 0.88, the neutronstar is at a distance r < 3R∗ for ∼ 5 days. For eccentricities smaller than0.73, the neutron star is never at r < 3R∗ (note that this result dependsfundamentally on the assumed R∗).ReferencesKaper, L., van der Meer, A.& Najarro, F.(2006) A&A 457, 595-610.Negueruela, I., Torrejo´n, J.M., Reig, P., Ribo´, M. & Smith, D.M. (2008) AIP Conf.Proc., vol.1010, p.252.Sidoli, L., Romano, P., Mereghetti, S., Paizis, A., Vercellone, S., Mangano, V. & Go¨tz.(2007) A&A 476, 1307.Steeghs, D., Torres, M.A.P, Jonker, P.G. (2006) ATEL 768.Wolf, B. & Stahl, O. (1985) A&A 148, 412.

HD 306414, the optical counterpart to the Peculiar X-Ray Transient IGR J11215-5952

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