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High energy astrophysics

High energy radiations are those whose energies exceed 100 eV, i.e. an energy 50 times larger than that of the visible radiation. High energies are divided into X-rays (100 eV to 100 keV, roughly) and gamma-rays (> 100 keV). The emission mechanism can be either thermal or non-thermal, albeit it is always linked to particularly violent phenomena. For example, a hot body may emit a thermal X-ray emission, if its temperature exceeds... one million degrees! Among the non thermal processes, one finds the direct or inverse Compton effects, as well as synchrotron or Bremsstrahlung radiations associated to very energetic charged particles and to intense magnetic or electric fields. Emission lines are also produced due to the ejection of an electron from the internal atomic layers (X-ray domain), to the excitation of a nucleus, or to matter-antimatter annihilation (gamma-ray domain).

The GAPHE is particularly interested by the observation of X-ray and gamma-ray emissions of massive stars, be them single or within clusters. Such data indeed reveal the most extreme phenomena associated to those stars and thus deliver unique pieces of information about them.

GAPHE X-ray researches

Since 1999 two new generation X-ray observatories produce exceptional results: the European XMM-Newton and the US Chandra satellites. The GAPHE researchers use both satellites, although with an emphasis on XMM: this observatory distinguishes itself by an unequaled sensitivity and by the parallel use of 6 instruments (EPIC-pn, EPIC MOS 1 and 2, RGS 1 and 2, OM). Thanks to its participation to XMM's Optical Monitor (OM), the Liège team has benefitted from a privileged access to that satellite. Besides guaranteed time, GAPHE has successfully obtained an appreciable amount of open time following calls for proposals published by ESA since 1999: the subsequent observations have enabled the team to obtain important results in several domains of stellar astrophysics.

Observing open clusters in the X-ray domain reveals two types of populations: adult massive stars and young low-mass stars that are still forming. The GAPHE data notably on the clusters NGC6231, 6530 and 6383 indeed reveal an appreciable number of hard X-ray sources, often showing high-amplitude eruptions, which are associated to pre-main sequence (PMS) stars of a T Tauri type. In the side image, the X emissions (shown in blue) which are superimposed on an optical picture of the NGC 6530 cluster (yellow-red) indicate that some of the brightest X-ray sources correspond to fairly weak optical sources. The GAPHE data also enable to characterize the massive stars, not numerous but brighter, thus clarifying the nature of their emission, the temperature of the hot plasma and the ratio between the X-ray luminosity and the total luminosity of the star. The analysis of a large number of objects seems in fact to indicate that this Lx vs. Lbol value may vary from one cluster to the next (tight Lx/Lbol = - 6.9 for NGC 6231, but with a larger dispersion and a slightly stronger X-ray emission for the Carina region).

The high-resolution X-ray spectra are revolutionizing high-energy astrophysics, particularly in the massive stars' domain. Many surprises indeed occurred: sharper and more symmetrical lines than expected, less important absorption of the stellar wind than announced, formation of the hot plasma near the stellar surface (1 stellar radius instead of 5). The GAPHE participates to the modeling effort of these X-ray lines, as well as to the long-term observing campaigns of several objects (e.g. 9 Sgr and HD 159176). Furthermore, the magnetic objects should be the most extreme, with harder X-ray emissions and extremely narrow lines. A few questions nevertheless remain concerning this topic. The study of theta Car, non magnetic a priori, shows very narrow lines whereas the studies and analyses of the magnetic Of?p stars show rather wide emission lines...

X-rays enable us to "see" violent phenomena in the Universe: they thus are particularly well suited when one wishes to detect wind collisions in massive binary systems (see also this page for further information on this subject). The GAPHE, which specializes in such observations, was able, and fortunate, to follow the X-ray variations with time of several binaries (notably HD 93403, HD 152248, and Cyg OB2#8A). The observed changes are directly linked to the collisions and therefore make it possible to derive precious information on the shape of the collision zone, the strengths of the winds, etc.

Some massive binaries possess a unique characteristic, the presence of a non thermal radio-emission. This is a synchrotron emission associated with electrons accelerated in a violent shock, which, in fact, is the collision between the stellar winds. In addition to detecting the binarity of several non-thermal emitters (notably Cyg OB2#8A and #9) in the optical domain, the GAPHE also searches for the "high energy" counterpart of this non-thermal emission. In the X-ray domain covered by XMM-Newton or Chandra, it was shown that these objects, e.g. 9 Sgr and Cyg OB2#8A, do show a thermal emission which is typical of binaries exhibiting wind-wind collisions. On the contrary, the X-ray data collected for HD 159176 reveal a non-thermal emission although this object does not show any non-thermal radio emissions: the two phenomena are therefore not necessarily correlated.

The GAPHE researches have also been devoted to the analysis of the X-ray emission of WR stars (see also this page for further information on this subject): the results have been surprising! Indeed, despite the high sensitivity of the EPIC instruments onboard XMM, some WRs (incl. WR 40) have yet not been detected in the X-rays, which may probably be due to the high opacity of the atmosphere of these stars, whereas other exhibit abnormal characteristics (e.g. short-term variations, of unknown origin, in WR 46). The data have also enabled to point out for WR 22 and WR 20a the non-zero dimension of the X-ray emitting region, which reinforces the wind-wind collision scenario.

Finally, the GAPHE also analyses diffuse emissions, i.e. emissions not associated to a particular star, notably in the N11 and Carina regions. Such emissions are often the trace of the interaction between the stellar winds and the ambient medium, which generates a gigantic collision heating the gas up to several mission degrees.

GAPHE gamma-ray researches

Since 2001, ESA's INTEGRAL satellite provides an unprecedented angular resolution in the "low energy" gamma-ray domain thanks to the coded masks of two of its instrument, IBIS (an imager) and SPI (a spectrograph). These instruments are complemented by two monitoring instruments, one in the hard X-rays (JEM-X) and one in the optical domain (OMC). Therefore INTEGRAL is presently the best observatory to identify the optical counterparts of a large number of unidentified gamma-ray sources. It is thus one of the best suited tools at the astrophysicists' disposal to help understanding the high-energy phenomena taking place in the Universe.

Thanks to the participation of Liège in the INTEGRAL Science Data Center (ISDC, Versoix, CH) as well as in the construction of the OMC, the GAPHE has a privileged access (guaranteed time) to the INTEGRAL "core programme". Its aim is to identify the optical counterparts of some gamma-ray sources detected with other space observatories in open clusters rich in massive stars. In fact, the INTEGRAL precursors did not have a sufficiently good angular resolution to obtain a trustworthy identification in the very crowded fields of these star clusters.

The GAPHE's systematic search for gamma-ray sources in OB associations and in very young open clusters led to the discovery of an IBIS source in Carina OB1. This source is located near the luminous blue variable (LBV) eta Carinae, and its origin could be linked to the interaction between the stellar winds in this binary system. Eta Carinae could thus be the first example of a massive binary exhibiting a non-thermal gamma-ray emission produced by an inverse Compton effect. Studies are presently underway in order to search for the orbital variations of that emission that are predicted by theoretical models.

In addition to receiving guaranteed time, the GAPHE has been successful in obtaining observations through ESA's open calls for proposals. This has enabled to study the Cyg OB2 cluster in order to try to detect the gamma-ray emission that should go with the non-thermal radio emission of O stars in this cluster. This group of stars contains at least three non-thermal radio-sources (incl. several binaries with stellar wind collisions) and the relativistic particles generating the synchrotron radiation should also produce a gamma-ray emission through inverse Compton diffusion of the UV photons coming from the O stars. Furthermore Cyg OB2 seems to be associated with a gamma-ray source detected by the EGRET onboard NASA's Compton Gamma Ray Observatory (launched in 1991). The GAPHE's detailed INTEGRAL observations, however, have not revealed any high-energy emission coming from the Wolf-Rayet stars WR 140, WR 146, WR 147, nor the EGRET source 3EG J2033+4118, nor from the very high-energy source TeV J2032+4130. Such a negative result implies stringent constraints on the relativistic particles acceleration mechanisms occurring in the Cyg OB2 massive stars and/or in the WR stars of the surrounding area.