CI Cam Research Articles

CI Camelopardalis: A Shell‐shocked X‐Ray Nova

We present radio imaging observations of the 1998 outburst of the peculiar emission-line star CI Cam, taken ~1, 4, 75, 82, 93, 163, and 306 days after the beginning of the 1998 March 31.64 X-ray flare. The first two epochs show a resolved but compact (no larger than 12 mas) radio source that becomes optically thin at frequencies higher than 5 GHz. The spectrum and brightness temperatures are consistent with synchrotron self-absorption, although free-free absorption may also play a role. The later images show a large (120-350 mas) oval-shaped or double-ring remnant. The radio spectrum combined with the high brightness temperature indicates that the emission is synchrotron, while the morphology suggests that this is powered by a decelerating shock moving through dense circumstellar material produced by a strong stellar wind. The radio images of CI Cam are equally well fitted by an expanding ellipsoid or two expanding rings; the former gives Θ ≈ 4.2(t - 50,904.1)0.77, with Θ the major axis in milliaarcseconds and t the Modified Julian Date (MJD). The corresponding expansion speed in the plane of the sky was ~12,000 km s-1 over the first few days (for an assumed distance of 5 kpc), slowing by a factor of ~3 by the time of the last observation almost a year later. The radio emission from all other X-ray binary transients is either unresolved or takes the form of highly collimated relativistic jets. We suggest that CI Cam represents a rare case in which these jets were smothered early on by the unusually dense circumstellar medium. In this model, CI Cam is the analog to extragalactic supernovae formed by the collapsar mechanism, while the more usual X-ray binaries with relativistic jets are analogous to the jets that escape those supernovae to form a subset of γ-ray bursts.

Revealing the Nature of the Highly Obscured Galactic Source IGR J16318-4848

AbstractThe X-ray source IGR J16318-4848 was the first source discovered by INTEGRAL on 2003, January 29. We carried out optical and near-infrared (NIR) observations at the European Southern Observatory (ESO La Silla) in the course of a Target of Opportunity (ToO) programme. We discovered the optical counterpart and confirmed an already proposed NIR candidate. NIR spectroscopy revealed a large amount of emission lines, including forbidden iron lines and P-Cygni profiles. The spectral energy distribution of the source points towards a high luminosity and a high temperature, with an absorption greater than the interstellar absorption, but two orders of magnitude lower than the X-ray absorption. We show that the source is an High Mass X-ray binary (HMXB) at a distance between ~ 1 and ~ 6 kpc, the mass donor being an early-type star, probably a sgB[e] star, surrounded by a rich and absorbing circumstellar material. This would make the second High Mass X-ray Binary (HMXB) with a sgB[e] star after CI Cam, indicating that a new class of strongly absorbed X-ray binaries is being unveiled by INTEGRAL.

Possibility of a White Dwarf as the Accreting Compact Star in CI Cam (XTE J0421+560)

AbstractAlthough CI Cam, which showed an outburst in April 1998, has been regarded as a soft X-ray transient (SXT) harboring a, black hole, it has some characteristics that can hardly be reconciled with the SXT picture. In this paper, we propose a white dwarf as the central accreting star in CI Cam.

News from Galactic Black Holes

Present status of the possible black hole microlensing events and the present understanding of high frequency quasi-periodic oscillations in systems containing black hole candidates (BHCs) are briefly discussed. The growing evidence for the presence of the event horizons around some compact objects is reviewed. Finally, the news from four individual objects (SS 433, GX 339-4, CI Cam and Cyg X-3) are presented and the updated list of BHCs, containing 49 objects is given.

Possibility of a White Dwarf as the Accreting Compact Star in CI Camelopardalis (=XTE J0421+560)

We present results from ASCA observations of the binary CI Cam both in quiescence and in outburst in order to identify its central accreting object. The quiescence spectrum of CI Cam consists of soft and hard components, which are separated clearly at around 2-3 keV. A large equivalent width of an iron K? emission line prefers an optically thin thermal plasma emission model to a nonthermal power-law model for the hard component, which favors a white dwarf as the accreting object, since optically thin thermal hard X-ray emission is a common characteristic among cataclysmic variables (binaries including an accreting white dwarf). However, since the power-law model, which represents the X-ray spectrum of the soft X-ray transients in quiescence, provides an equally good fit to the hard component statistically, we cannot exclude the possibility of a neutron star or a black hole from the quiescence data. The outburst spectrum, on the other hand, is composed of a hard component represented by multitemperature optically thin thermal plasma emission and an independent soft X-ray component that appears below 1 keV intermittently on a decaying light curve of the hard component. The spectrum of the soft component is represented well by a blackbody with the temperature of 0.07-0.12 keV overlaid with several K edges associated with highly ionized oxygen. This, together with the luminosity as high as ~1 ? 1038 ergs s-1, is similar to a supersoft source. The outburst in the hard X-ray band followed by the appearance of the soft blackbody component reminds us of recent observations of novae in outburst. We thus assume that the outburst of CI Cam is that of a nova and obtain the distance to CI Cam of 5-17 kpc by means of the relation between the optical decay time and the absolute magnitude. This agrees well with a recent estimate of the distance of 5-9 kpc in the optical band. All of these results from the outburst data prefer a white dwarf for the central object of CI Cam.

Strongly absorbed quiescent X-ray emission from the X-ray transient XTE J0421+56 (CI Cam) observed with XMM-Newton

We have observed the X-ray transient XTE J0421+56 in quiescence with XMM-Newton. The observed spectrum is highly unusual being dominated by an emission feature at ~6.5 keV. The spectrum can be fit using a partially covered power-law and Gaussian line model, in which the emission is almost completely covered (covering fraction of 0.98_{-0.06}^{+0.02}) by neutral material and is strongly absorbed with an N_H of (5_{-2}^{+3}) x 10^{23} atom cm^{-2}. This absorption is local and not interstellar. The Gaussian has a centroid energy of 6.4 +/- 0.1 keV, a width < 0.28 keV and an equivalent width of 940 ^{+650}_{-460} eV. It can be interpreted as fluorescent emission line from iron. Using this model and assuming XTE J0421+56 is at a distance of 5 kpc, its 0.5-10 keV luminosity is 3.5 x 10^{33} erg s^{-1}. The Optical Monitor onboard XMM-Newton indicates a V magnitude of 11.86 +/- 0.03. The spectra of X-ray transients in quiescence are normally modeled using advection dominated accretion flows, power-laws, or by the thermal emission from a neutron star surface. The strongly locally absorbed X-ray emission from XTE J0421+56 is therefore highly unusual and could result from the compact object being embedded within a dense circumstellar wind emitted from the supergiant B[e] companion star. The uncovered and unabsorbed component observed below 5 keV could be due either to X-ray emission from the supergiant B[e] star itself, or to the scattering of high-energy X-ray photons in a wind or ionized corona, such as observed in some low-mass X-ray binary systems.

Spectroscopic observations of the candidate sgB[e]/X-ray binary CI Camelopardalis

We present a compilation of spectroscopic observations of the sgB(e) star CI Cam, the optical counterpart of XTE J0421+560. This includes data from before, during, and after its 1998 outburst, with quantitative results spanning 37 years. The object shows a rich emission line spectrum originating from circumstellar material, rendering it dicult to determine the nature of either star involved or the cause of the outburst. We collate all available pre-outburst data to determine the state of the system before this occurred and provide a baseline for comparison with outburst and post-outburst data. During the outburst all lines become stronger, and hydrogen and helium lines become significantly broader and asymmetric. After the outburst, spectral changes persist for at least three years, with Feii and (Nii) lines still a factor of2 above the pre-outburst level and Hei ,H eii ,a nd Nii lines suppressed by a factor of 2-10. We find that the spectral properties of CI Cam are similar to other sgB(e) stars and therefore suggest that the geometry of the circumstellar material is similar to that proposed for the other ob- jects: a two component outflow, with a fast, hot, rarefied polar wind indistinguishable from that of a normal supergiant and a dense, cooler equatorial outflow with a much lower velocity. Based on a comparison of the properties of CI Cam with the other sgB(e) stars we suggest that CI Cam is among the hotter members of the class and is viewed nearly pole-on. The nature of the compact object and the mechanism for the outburst remain uncertain, although it is likely that the compact object is a black hole or neutron star, and that the outburst was precipitated by its passage through the equatorial material. We suggest that this prompted a burst of supercritical accretion resulting in ejection of much of the material, which was later seen as an expanding radio remnant. The enhanced outburst emission most likely originated either directly from this supercritical accretion, or from the interaction of the expanding remnant with the equatorial material, or from a combination of both mechanisms.

Properties of galactic B[e] supergiants. I. CI Camelopardalis

We present a study of the high-resolution (R = 60 000) optical spectrum of the B[e] supergiant CI Cam obtained 4 years after its all-wavelength outburst. The profiles of most of the emission lines show a triple-peaked structure, an effect previously not observed. The Na i D-lines clearly have 2 interstellar absorption components, suggesting that the system is most probably located within the Perseus arm at a distance of <3 kpc. Uncertainties of the distance toward the object, its luminosity, and physical parameters of the circumstellar disk are discussed. Simple observational tests are suggested to clarify these issues.

A photometric and spectroscopic study of the B[e]/X-ray transient CI Cam

We combined the results of UBVR photometry of CI Cam taken at Sternberg Astronomical Institute in 1998--2001, and moderate resolution spectroscopy taken at Special Astrophysical Observatory during the same time period. Photometry as well as fluxes of Balmer emissions and of some Fe II emission lines of CI Cam in quiet state reveal a cyclic variation with the period of $1100\pm50^d$. The variation like this may be due to an orbital motion in a wide pair with a giant star companion that exhibits the reflection effect on its side faced to a compact companion. The V-band photometry also confirms the pre-outburst 11.7 day period found by Miroshnichenko earlier, but with a lower amplitude of 3 per cent. The possibility of identity of this photometric period with the period of jet's rotation in the VLA radio map of the object CI Cam was investigated. The radio map modelling reveals the inclination of the jet rotation axis to the line of sight, $i = 35-40^o$, the angle between the rotation axis and the direction of ejection of the jet, $\theta = 7-10^o$, and jet's spatial velocity of 0.23--0.26c. Equivalent widths and fluxes of various spectral lines show different amplitudes of changes during the outburst, and essentially distinct behaviour in quiescence. Five types of such behaviour were revealed, that indicates the strong stratification of a gas and dust envelope round the system . The time lag of strengthening of 50--250$^d$ in the forbidden line of nitrogen [N II] was found relatively to the X-ray outburst maximum.

High‐Dispersion Spectroscopy of the X‐Ray Transient RXTE J0421+560 (=CI Camelopardalis) during Outburst

We obtained high-dispersion spectroscopy of CI Cam, the optical counterpart of XTE J0421+560, 2 weeks after the peak of its short outburst in 1998 April. The optical counterpart is a supergiant B[e] star that is emitting a two-component wind, a cool, low-velocity wind and a hot, high-velocity wind. The cool wind, which is the source of narrow emission lines of neutral and ionized metals, has a velocity of 32 km s-1 and a temperature near 8000 K. It is dense, roughly spherical, fills the space around the sgB[e] star, and, based on the size of an infrared-emitting dust shell around the system, extends to a radius between 13 and 50 AU. It carries away mass at a high rate, > 10-6 M☉ yr-1. The hot wind has a velocity in excess of 2500 km s-1 and a temperature of 1.7 ± 0.3 × 104 K. From an ultraviolet spectrogram of CI Cam obtained in 2000 March with Hubble Space Telescope, we derive a differential extinction E(B-V) = 0.85 ± 0.05. We show that the distance to CI Cam is greater than 5 kpc. Based on this revised distance, the X-ray luminosity at the peak of the outburst was L(2-25 keV) > 3.0 × 1038 ergs s-1, making CI Cam one of the most luminous X-ray transients. The ratio of quiescent luminosity to peak luminosity in the 2-25 keV band is Lq/Lp < 1.7 × 10-6. The compact star in CI Cam is immersed in the dense circumstellar wind from the sgB[e] star and burrows through the wind, producing little X-ray emission except for rare transient outbursts. This picture, a compact star traveling in a wide orbit through the dense circumstellar envelope of a sgB[e] star, occasionally producing transient X-ray outbursts, makes CI Cam unique among the known X-ray binaries. There is strong circumstantial evidence that the compact object is a black hole, not a neutron star. We speculate that the X-ray outburst was short because the accretion disk around the compact star is fed from a stellar wind and is smaller than disks fed by Roche lobe overflow.

On the peak radio and X-ray emission from neutron star and black hole candidate X-ray transients

We have compiled and analysed reports from the literature of (quasi-)simultaneous observations of X-ray transients at radio and X-ray wavelengths and compared them with each other and with more unusual radio-bright sources such as Cygnus X-3, GRS 1915+105 and Circinus X-1. There exists a significant (>97 per cent likelihood) positive (rank) correlation between the peak X-ray flux PX and radio flux density PR for the black hole candidate (BHC) systems, and a marginally significant positive (rank) correlation for the neutron star (NS) systems. This is further evidence for a coupling between accretion and outflows in X-ray binary systems, in this case implying a relation between peak disc-accretion rate and the number of synchrotron-emitting electrons ejected. However, we also show that the distribution of ‘radio loudness’, PR/PX, is significantly different for the two samples, in the sense that the BHCs generally have a higher PR/PX ratio. The origin of this discrepancy is uncertain, but probably reflects differences in the energetics and/or radiative efficiency of flows around the neutron stars and black holes; we briefly discuss some of these possibilities. Furthermore, the data for the two recently discovered ‘fast transients’ (FTs), XTE J0421+560/CI Cam and SAX J1819.3−2525/V4641 Sgr are entirely compatible with the distribution of BHCs. As at least three of the BHCs and both FTs have been directly resolved into mildly relativistic jets, it seems likely that such outflows are the origin of the radio emission in all BHC and FT transients, and probably also for the NS transients. We further note that the range of X-ray and radio fluxes observed from the unusual superluminal source GRS 1915+105 is also entirely compatible with the distribution for transients, implying that there is nothing special about the physics of jet formation in that system. We conclude that these data point to the formation of a mildly relativistic jet whose luminosity is a function of the accretion rate in the majority, if not all, X-ray transient outbursts, but whose relation to the observed X-ray emission is dependent on the nature of the accreting compact object.

The B[e] Star MWC 84 (CI Cam)

The star MWC 84 (CI Cam) was first discovered to be an emission line object classified Bep (Merrill &amp; Burwell, 1933), then classified as a symbiotic star and finally identified as the possible optical counterpart of the X-ray transient XTE J0421+560 (Wagner &amp; Starrfield, 1998)We have observed MWC 84 at the Haute Provence Observatory (CNRS) with a resolution of about 1.3 Å in 1992 (wavelength regions Hα and near infrared), 1998 and 1999 (from 4080 to 8880 Å).

On the Nature of XTE J0421+560/CI Camelopardalis

We present the results of an analysis of RXTE, BATSE, and optical/infrared data of the 1998 outburst of the X-ray transient system XTE J0421+560 (CI Cam). The X-ray outburst shows a very fast decay (initial e-folding time ~0.5 days, slowing down to ~2.3 days). The X-ray spectrum in the 2-25 keV band is complex, softening considerably during decay and with strongly variable intrinsic absorption. A strong iron emission line is observed. No fast time variability is detected (<0.5% rms in the 1-4096 Hz band at the outburst peak). The analysis of the optical/IR data suggests that the secondary is a B[e] star surrounded by cool dust, and places the system at a distance of 2 kpc. At this distance, the peak at 2-25 keV luminosity is ~4 × 1037 ergs s-1. We compare the properties of this peculiar system with those of the Be/NS LMC transient A 0538-66 and suggest that CI Cam is of similar nature. The presence of strong radio emission during outburst indicates that the compact object is likely to be a black hole or a weakly magnetized neutron star.