One Year Later - LS 5039 Revisited

August 2006

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Orbital geometry of LS 5039, where a compact object - presumable a black hole -  is in a tight orbit around a massive star  (Casares et al. 2005). The system is viewed from above. Orbit geometry and the 3.906 day orbital period are derived from radial velocity measurements. Indicated are the points of closest approach of the compact object (Periastron), with a separation of about 2 stellar radii, and the most distant point on the orbit (Apastron), as well as the superior and inferior conjunction, where the objects line up along the line of sight towards the observer. The orbit plane must actually be somewhat inclined since X-ray emission from the compact object is not eclipsed by the star.

One year ago, in August 2005, the microquasar LS 5039 (top figure) was featured as source of the month. Novel exciting results on this source merit a renewed discussion, despite the many other H.E.S.S. sources which await presentation. LS 5039 was the first microquasar established as a high-energy gamma-ray source (Aharonian et al. 2005), more recently followed by LS I +61 303  (Albert et al. 2006) as a second example. Such binary systems with eccentric orbits provide a unique laboratory to study how particle acceleration and propagation vary with the changing environment along the orbit. Orbital X-ray variability of LS 5039 was indeed reported byBosch-Ramon et al. (2005). Variation of high-energy gamma-ray emission from binaries has been observed on sections of the 3.4-year orbit of the binary pulsar PSR B1259-63/SS 2883 by H.E.S.S. (April 2005) and along the 26 day orbit of LS I +61 303 by MAGIC (Albert et al. 2006). With its 3.9 day orbit, LS 5039 now allows for the first time recording many periods and establishing periodic variation of the gamma-ray flux along the orbit.

In 2004 and 2005, H.E.S.S. accumulated 69 hours of data on LS 5039, boosting the gamma-ray signal to 40 standard deviations above background. In the 160 observation runs, significant variability of the gamma-ray emission was immediately obvious. A periodicity analysis using the Lomb-Scargle test (Scargle 1982)- similar to a Fourier decomposition of the time-variable flux into its frequency components - shows a strong frequency component at a period of 3.908 days (Fig. 1), within errors consistent with the know orbital period of 3.906 days (Casares et al. 2005). The nearby source HESS J1825-137, observed simultaneously, showed no modulation, excluding any kind of instrumental artifact. Folding the measured light curve with the period results in Fig. 2, showing gamma-ray flux as a function of orbital phase. Orbital modulation is clearly visible. However, contrary to naive expectations, the maximum emission is not at periastron, but rather around the inferior conjunction, where the compact object and the star line up along the line of sight. Periastron - the closest approach - should provide the highest accretion rate onto the compact object, and the highest magnetic and photon fields, which should boost particle acceleration and gamma-ray production (unless radiative energy losses are so high that they prevent further acceleration). However, if gamma rays are created close to the compact object, then another effect becomes important: absorption of gamma rays in collisions with stellar photons (e.g.Böttcher and Dermer 2005,Dubus 2006,Bednarek 2006). When the compact object is "behind" the star, gamma rays and stellar photons collide head-on and can easily surpass the threshold for electron-positron pair production, resulting in a nearly 100% loss of high-energy photons (top figure; stellar photons are shown in blue and high-energy gamma rays in black). At the inferior conjunction, when the compact object is "in front", gamma rays and stellar photons travel on parallel paths and cannot interact, therefore the flux maximum. However, this cannot be the full story. The modulation of the gamma-ray spectrum is strongly energy-dependent (Fig. 3): around 200 GeV and 30 TeV, the flux does not vary strongly with orbital phase, whereas at a few TeV, variation is as large as a factor 5. Absorption alone, however, should generate a significant modulation already at 200 GeV. It seems that - not quite unexpectedly - orbital variation of the particle acceleration partly compensates for the absorption and results in a complex, energy-dependent flux modulation. Recent articles which discuss gamma-ray production mechanism and give further references are e.g.Bosch-Ramon et al. (2006),Paredes et al. (2006),Gupta et al. (2006) andDermer and Böttcher (2006). Besides the leptonic origin discussed in these papers, also hadron acceleration has been considered (e.g. Aharonian et al. 2006, in press, Bosch-Ramon et al. 2006)

References:

3.9 day orbital modulation in the TeV gamma-ray flux and spectrum from the X-ray binary LS 5039, H.E.S.S. collaboration, F. Aharonian et al., astro-ph/0607192 (2006).

Fig. 1: Periodogram of the gamma-ray emission from LS 5039 (yellow), where the intensity of flux modulation at a given frequency is expressed in terms of the probability that this intensity results from random fluctuations of the measured flux. The peak at an orbital period of 3.908(2) days is highly significant, and the frequency of the peak agrees well with the orbital frequency of 3.906 days. The other peaks in the periodogram result from beats between the orbital frequency and semi-periodic gaps in the data,  such as full moon periods. These additional peaks disappear when a sine wave with the fundamental frequency is subtracted from the measured gamma-ray rates before calculating the periodogram (middle diagram). The source HESS J1825-137, located in the same field of view and observed at the same time, shows no modulation (green); its periodogram is consistent with statistical fluctuations in the gamma-ray rate.
Fig. 2: Gamma-ray rate as a function of orbital phase, averaged over many orbital periods. The rate is repeated for two orbits. The maximum emission is centered around the inferior conjunction (see top figure).
Fig. 3: Spectral energy distribution of gamma rays in the orbit sections with high gamma ray flux, around the inferior conjunction (red), and with low flux, around the superior conjunction (blue) .