−ray Spectrum, γ
6.3 The Hadronic Interaction Scenario
An alternative to the leptonic origin of the observedγ-rays is provided by hadronic interactions of shock-accelerated ions. The leptonic scenario, where the observed X- and γ-rays can be interpreted as synchrotron and IC emission from a single population of shock-accelerated elec-trons, seems to be rather self-consistent in terms of the estimated magnetic field strength in the radiating plasma and the corresponding flux level and spectral shape of the radiation. However, a hadronic origin of theγ-rays cannot be ruled out by the data.
There are two possibilities for the origin of shock-accelerated ions: (1) a significant fraction of the pulsar wind kinetic energy is carried by ions which are accelerated in the pulsar wind termination shock, or (2) ions from the stellar mass outflow accelerated in the Be star side shock front. The correlation between the flux variability pattern of the unpulsed radio emission and the γ-ray light curve could be interpreted as indication for case (2) if one follows the argumentation of Ball et al. [1999] about the Be star shock origin of the radio emission.
In Section 2.4.2, the model from Kawachi et al. [2004] was introduced, wherein protons are modeled to be shock-accelerated in the Be star side shock producingγ-rays via proton-proton
interactions. The expected flux level was found to be proportional to the parameter x of the stellar mass outflow in Kawachi et al. [2004],Fγ ∝ x2 and theγ-ray spectral shape reflects that of the distribution of accelerated protons which was assumed to be a power law.
10−12
Figure 6.12: Comparison of the γ-ray spectrum in the hadronic interaction model by Kawachi et al. [2004] (c.f.
Fig. 2.21) for the epoch of periastron and the H.E.S.S.
time averaged spectrum (full circles).
Figure 6.12 shows the model spec-trum for the epoch of periastron to-gether with the time-averaged spec-trum of γ-rays obtained from the H.E.S.S. data. Obviously, neither the photon index nor the flux level is in good agreement. Nevertheless, the flux level depends on the choice of the outflow parameter x and the spectral shape of the shock-accelerated protons in the model was assumed to be αp = 2. A shock-acceleration process which is able to produce a proton spectrum with n(p) ∝ p−2.7±0.3 could account for the observed shape of the γ-ray spectrum, although this is incompati-ble with first order Fermi acceleration.
Since the flux is expected to be vari-able and the periastron passage was not covered by the H.E.S.S. observa-tions, the flux normalisation from the model spectrum cannot be compared with that of the H.E.S.S. spectrum.
Several cases for the alignment of
the stellar disk of SS 2883 were considered in the model resulting in different flux variabil-ity patterns. Figure 6.13 shows the comparison of the expected and measured energy flux at 1 TeVγ-ray energy for the case of a misaligned disk with two associated crossings of the pulsar through the disk. The model light curve seems to be qualitatively similar to that of the mea-surement, although the epochs of the high flux states do not match. The peaks of the emission could be shifted due to the same reasons as discussed in the last section: by a Doppler-shift of the down-stream plasma flow in case of a “cometary tail” or by a different orientation of the stellar disk with respect to periastron. However, in the latter case where an orientation angle ωdisk> 90◦was discussed, the first flux peak would be shifted further towards periastron which might be inconsistent with the data. The model light curve for an outflow parameter x = 1500 was scaled by a factor 0.3 in order to roughly match the flux level of the data. Thus, the outflow parameter in this model can be constrained to bex≈ 800.
6.4 Outlook
With the H.E.S.S. detection of VHEγ-ray emission from PSR B1259−63/SS 2883 and the mea-surement of the energy spectrum and variability pattern, an important step in the understanding
−20 τ 20 40 60 τ
80 100
−1−2−12EF (1TeV) [10 TeV cm s ]
Time [days relative to periastron ]
0
E
2 4
Time [MJD]
Hadronic Interaction Scenario
53060 53080 53100 53120 53140 53160 53180
6 pulsar radio eclipse
H.E.S.S. data 2004, 2−day average Kawachi et al. 2004, misaligned disk (iii) x 0.3
Figure 6.13: Comparison of the VHEγ-ray light curve (energy flux at 1 TeV) in the hadronic interaction model by Kawachi et al. [2004] (c.f. Fig. 2.21) for the case of a misaligned disk and the H.E.S.S. data using flux points with a 2-day average.
of the high energy emission from this object was achieved. Nevertheless, many open questions remain to be answered by either detailed theoretical modeling of the data presented or future observations.
Further monitoring of the source in the VHE γ-ray energy band, also far away from pe-riastron, seems desirable for two reasons: Firstly, the distance of the pulsar wind termination shock is expected to drastically increase while the pulsar travels away from its companion and this would result in a lower magnetic field strength in the down-stream region of the shock.
PSR B1259−63 was detected in X-rays near apastron at a flux level roughly one order of mag-nitude lower than near periastron. In the scenario of IC emission from shock-accelerated elec-trons, the low magnetic field would result in a higher γ-ray emission in relation to the syn-chrotron X-ray emission, enabling H.E.S.S. to detect the source at apastron. Secondly, VHE γ-ray observations during future periastron passages with deep exposures of H.E.S.S. and other existing or even with more sensitive instruments would be useful in order to improve the time coverage of the observations and derive γ-ray spectra on a daily basis providing further con-strains on the particle acceleration and radiation mechanisms at work.
Furthermore, the question of the leptonic or hadronic origin of the the VHEγ-rays emitted near periastron could be answered by sensitive detectors covering the energy range between several MeV and 100 GeV, like satellite detectors and IACT experiments with a low energy threshold. The shape of the spectral energy distribution in this energy range has to be resolved, where either a local minimum between the peak of the synchrotron and IC radiation of elec-trons or a rather flat distribution due to the emission resulting from hadronic interactions is expected. Additionally, with these instruments it may be possible to see the IC γ-ray emission from electrons in the unshocked pulsar wind.
The high variability of the emission near periastron in all energy bands will require more simultaneous observations in dedicated multi-wavelength campaigns, especially near periastron where many instruments provide a sufficient sensitivity in order to detect the high flux states induced by the interaction of the highly energetic winds of PSR B1259−63 and SS 2883.
Summary
This work represents the first comprehensive analysis of VHE γ-rays discovered to be emitted from the binary system of PSR B1259−63/SS 2883 using data obtained with the H.E.S.S. array of imaging atmospheric Cherenkov telescopes.
In the experimental part of the work, firstly an overview of the detector components was given with special emphasis on the design and implementation of the H.E.S.S. central data acquisition system (DAQ). The DAQ hardware is represented by a distributed computer cluster connected via a local area network on which a system of distributed processes controls and monitors all detector components. The processes are implemented using an object oriented software design which can be easily configured to control additional detector components such as new telescopes.
Subsequently, the data analysis was described in detail and the results were presented. The detector calibration, data reduction, and γ-ray reconstruction are well understood, which was demonstrated by the good agreement between γ-ray simulations and data. It was shown that H.E.S.S. is able to reconstructγ-ray air showers with an angular resolution of better than 0.1◦ and an energy resolution of < 20%. The analysis routines used for the extraction of the char-acteristics of a VHEγ-ray signal such as energy spectrum and light curve were tested with the data from the Crab Nebula – the standard candle of VHEγ-ray astronomy – and the results were found to be in good agreement with results from earlier IACT instruments.
Finally, the results from the analysis of the data set recorded in the observations from Febru-ary to June 2004 near the periastron passage of PSR B1259−63 were presented. The VHEγ-ray signal measured at the direction of the system was found to have a total significance of more than 13 standard deviations for the complete data set. It was shown, that the center of gravity of the emission coincides with the position of PSR B1259−63 within errors and that the source has a point source character allowing to constrain the intrinsic source size to < 3300 at 95%
confidence level.
The time averagedγ-ray spectrum can be described by a power law dN/dE = F0E−Γ with the photon indexΓ =2.7±0.2stat±0.2sysand the flux normalisationF0 =(1.3±0.1stat±0.4sys)× 10−12cm−2s−1TeV−1in theγ-ray energy range between 0.4 TeV and 3 TeV corresponding to an average of roughly 4% of the flux of the Crab Nebula. Within statistical errors, no variation of the photon index was seen in the data set.
The observed flux was found to vary significantly on timescales of days. The measured light 112
curve shows strong signals to be emitted in pre- and post-periastron phases with a flux mini-mum around periastron, followed by a gradual flux decrease in the months after. The analysis of the variability was cross-checked using two independent methods for which a good agree-ment was found giving confidence in the obtained result. The observed flux variability makes PSR B1259−63/SS 2883 the first variable galactic source of VHEγ-rays observed so far.
The experimental results were discussed and interpreted within the context of the current theoretical understanding of the astrophysical processes associated with the production of VHE γ-rays in this source and were compared to several existingγ-ray emission models.
First of all, the discovery of VHEγ-rays from the binary system PSR B1259−63/SS 2883 by H.E.S.S. provides the first unambiguous evidence for acceleration of particles to energies above 1 TeV in this object.
The γ-ray spectrum was found to be consistent with expectations for the γ-ray emission from a pulsar wind nebula formed due to the interaction of the pulsar wind of PSR B1259−63 with the stellar outflows of its companion. In this case, the γ-ray emission is produced by particles accelerated in the termination shocks of the colliding winds. Assuming that the γ-rays originate from inverse Compton scattering of shock-accelerated electrons with the intense thermal photon field of SS 2883, several conclusions can be drawn from the γ-ray spectrum.
The assumed power-law energy spectrum of the radiating electrons was constrained to have a spectral indexαe = 2.2±0.3. Since the acceleration spectrum is expected to have a similar shape, adiabatic expansion of the electrons in the down-stream region of the shock was concluded to be the dominant energy loss mechanism. Furthermore, under the assumption that the observed X-ray emission of the system results from synchrotron emission of the same electrons, the magnetic field strength in the radiating plasma was estimated to be of the order of 1 G. Indeed, the measured pre-periastron spectrum agrees well with that of the model of Kirk et al. [1999]
which was matched to archival X-ray data for a similar magnetic field strength.
A hadronic origin of theγ-ray emission cannot be excluded, but would require a soft accel-eration spectrum with a spectral index ofαp = 2.7±0.3 which is incompatible with first order Fermi acceleration.
The measured light curve does not agree with any of the considered theoretical models which may be a result of the high level of simplifications which where made. The observed variability pattern strongly suggests theγ-ray flux to be enhanced near the orbital phases where PSR B1259−63 is thought to cross the equatorial disk of SS 2883. This would imply that the disk is inclined with respect to the orbital plane as suggested by several other measurements at various wavelengths. This indication is confirmed by the correlation found between the flux evolution of the detected γ-ray emission and that of the transient unpulsed radio emission of PSR B1259−63/SS 2883 near periastron. However, the indicated post-periastron flux maxi-mum occurs several days after the expected disk crossing, which was discussed to be possibly related to a Doppler boost of the accelerated particles or a slightly different disk orientation with respect to periastron.
In the IC scenario and considering dominant adiabatic cooling of the shocked plasma as indicated by the measured spectrum, the observed variability pattern can be qualitatively ex-plained by a varying confinement of the pulsar wind by the stellar mass outflow which results in different plasma flow velocities and therefore variable energy loss rates of the accelerated electrons. An alternative explanation, especially for the low flux state around periastron, is the
deceleration of the unshocked pulsar wind by the increased rate of IC scattering near periastron.
However, since the observed VHEγ-rays can be excluded to originate from the unshocked pul-sar wind, the wind particles would have to have Lorentz factors below 106 or above 108 such that the resulting ICγ-rays are emitted at energies undetectable by H.E.S.S.
A leptonic origin of the emission should be favoured against a possible hadronic origin since it provides a rather consistent picture considering the broad band non-thermal emission of the source, especially at X-ray energies. Nevertheless, a detailed theoretical modeling of the presented results is needed and may provide further answers on the origin of the high energy emission of PSR B1259−63/SS 2883. With the detection of PSR B1259−63/SS 2883 as an emitter of VHEγ-rays, a new type of object is established in VHEγ-ray astronomy. The results clearly demonstrate the power of γ-ray observations for the study of the properties and the nature of high energy processes in this unique cosmic accelerator.
Central Data Acquisition System
Each telescope in the H.E.S.S. array is a heterogeneous system with several subsystems, as described in Chapter 2, that must be controlled and read out. The central DAQ system provides the connectivity and readout of all these systems. It takes over run control, the recording of event and slow control data, error handling, and monitoring of all subsystems.