Issue |
A&A
Volume 507, Number 1, November III 2009
|
|
---|---|---|
Page(s) | 389 - 396 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361/200912339 | |
Published online | 03 September 2009 |
A&A 507, 389-396 (2009)
Very high energy
-ray
observations of the binary PSR B1259-63/SS2883 around the 2007
Periastron
F. Aharonian1,13
- A. G. Akhperjanian2 -
G. Anton16 - U. Barres de
Almeida8,
- A. R. Bazer-Bachi3 -
Y. Becherini12 - B. Behera14
- K. Bernlöhr1,5 - A. Bochow1
- C. Boisson6 - J. Bolmont19
- V. Borrel3 - J. Brucker16
- F. Brun19 - P. Brun7 -
R. Bühler1 - T. Bulik24
- I. Büsching9 - T. Boutelier17
- P. M. Chadwick8 -
A. Charbonnier19 -
R. C. G. Chaves1 -
A. Cheesebrough8 -
L.-M. Chounet10 -
A. C. Clapson1 -
G. Coignet11 - M. Dalton5
- M. K. Daniel8 -
I. D. Davids22,9 -
B. Degrange10 - C. Deil1
- H. J. Dickinson8 -
A. Djannati-Ataï12 -
W. Domainko1 -
L. O'C. Drury13 -
F. Dubois11 - G. Dubus17
- J. Dyks24 - M. Dyrda28
- K. Egberts1 -
D. Emmanoulopoulos14 -
P. Espigat12 - C. Farnier15
- F. Feinstein15 - A. Fiasson11
- A. Förster1 - G. Fontaine10
- M. Füßling5 - S. Gabici13
- Y. A. Gallant15 -
L. Gérard12 - D. Gerbig21
- B. Giebels10 -
J. F. Glicenstein7 -
B. Glück16 - P. Goret7
- D. Göring16 - D. Hauser14
- M. Hauser14 - S. Heinz16
- G. Heinzelmann4 - G. Henri17
- G. Hermann1 -
J. A. Hinton25 -
A. Hoffmann18 - W. Hofmann1
- M. Holleran9 - S. Hoppe1
- D. Horns4 - A. Jacholkowska19
- O. C. de Jager9 -
C. Jahn16 - I. Jung16
- K. Katarzynski27 - U. Katz16
- S. Kaufmann14 -
M. Kerschhaggl5 - D. Khangulyan1
- B. Khélifi10 - D. Keogh8
- D. Klochkov18 - W. Kluzniak24
- T. Kneiske4 - Nu. Komin7
- K. Kosack1 - R. Kossakowski11
- G. Lamanna11 - J.-P. Lenain6
- T. Lohse5 - V. Marandon12
- O. Martineau-Huynh19 -
A. Marcowith15 - J. Masbou11
- D. Maurin19 -
T. J. L. McComb8 -
M. C. Medina6 -
R. Moderski24 - E. Moulin7
- M. Naumann-Godo10 -
M. de Naurois19 -
D. Nedbal20 - D. Nekrassov1
- B. Nicholas26 - J. Niemiec28
- S. J. Nolan8 -
S. Ohm1 - J.-F. Olive3
- E. de Oña Wilhelmi
1,12,29
- K. J. Orford8 -
M. Ostrowski23 - M. Panter1
- M. Paz Arribas5 -
G. Pedaletti14 - G. Pelletier17
- P.-O. Petrucci17 - S. Pita12
- G. Pühlhofer18,14 - M. Punch12
- A. Quirrenbach14 -
B. C. Raubenheimer9 -
M. Raue1,29 -
S. M. Rayner8 -
M. Renaud12,1 - F. Rieger1,29
- J. Ripken4 - L. Rob20
- S. Rosier-Lees11 - G. Rowell26
- B. Rudak24 -
C. B. Rulten8 -
J. Ruppel21 - V. Sahakian2
- A. Santangelo18 -
R. Schlickeiser21 -
F. M. Schöck16 -
U. Schwanke5 - S. Schwarzburg18
- S. Schwemmer14 - A. Shalchi21
- M. Sikora24 - J. L. Skilton25
- H. Sol6 - D. Spangler8
-
. Stawarz23
- R. Steenkamp22 - C. Stegmann16
- F. Stinzing16 - G. Superina10
- A. Szostek23,17 -
P. H. Tam14 -
J.-P. Tavernet19 - R. Terrier12
- O. Tibolla1 - M. Tluczykont4
- C. van Eldik1 -
G. Vasileiadis15 - C. Venter9
- L. Venter6 -
J. P. Vialle11 -
P. Vincent19 - M. Vivier7
- H. J. Völk1 -
F. Volpe1 -
S. J. Wagner14 -
M. Ward8 -
A. A. Zdziarski24 -
A. Zech6
1 - Max-Planck-Institut für Kernphysik, PO Box 103980, 69029
Heidelberg, Germany
2 - Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036
Yerevan,
Armenia
3 - Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 Av. du
Colonel Roche, BP
4346, 31029 Toulouse Cedex 4, France
4 - Universität Hamburg, Institut für Experimentalphysik, Luruper
Chaussee
149, 22761 Hamburg, Germany
5 - Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15,
12489 Berlin, Germany
6 - LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5
Place Jules Janssen, 92190 Meudon, France
7 - IRFU/DSM/CEA, CE Saclay, 91191 Gif-sur-Yvette, Cedex, France
8 - University of Durham, Department of Physics, South Road, Durham DH1
3LE, UK
9 - Unit for Space Physics, North-West University, Potchefstroom 2520,
South Africa
10 - Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3,
91128 Palaiseau, France
11 - Laboratoire d'Annecy-le-Vieux de Physique des Particules,
Université de Savoie, CNRS/IN2P3, 74941 Annecy-le-Vieux, France
12 - Astroparticule et Cosmologie (APC), CNRS, Université Paris 7 Denis
Diderot,
10 rue Alice Domon et Leonie Duquet, 75205 Paris Cedex 13, France
UMR 7164 (CNRS, Université Paris VII, CEA, Observatoire de Paris),
France
13 - Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2,
Ireland
14 - Landessternwarte, Universität Heidelberg, Königstuhl, 69117
Heidelberg, Germany
15 - Laboratoire de Physique Théorique et Astroparticules, Université
Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, 34095
Montpellier Cedex 5, France
16 - Universität Erlangen-Nürnberg, Physikalisches Institut,
Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
17 - Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université
Joseph Fourier, BP
53, 38041 Grenoble Cedex 9, France
18 - Institut für Astronomie und Astrophysik, Universität Tübingen,
Sand 1, 72076 Tübingen, Germany
19 - LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis
Diderot
Paris 7, CNRS/IN2P3, 4 Place Jussieu, 75252 Paris Cedex 5, France
20 - Charles University, Faculty of Mathematics and Physics, Institute
of Particle and Nuclear Physics, V Holesovickách 2, 180 00, Czech
Republic
21 - Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und
Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
22 - University of Namibia, Private Bag 13301, Windhoek, Namibia
23 - Obserwatorium Astronomiczne, Uniwersytet Jagiellonski, ul. Orla
171,
30-244 Kraków, Poland
24 - Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716
Warsaw,
Poland
25 - School of Physics & Astronomy, University of Leeds, Leeds
LS2 9JT, UK
26 - School of Chemistry & Physics, University of Adelaide,
Adelaide 5005, Australia
27 - Torun Centre for Astronomy, Nicolaus Copernicus University, ul.
Gagarina 11, 87-100 Torun, Poland
28 - Instytut Fizyki Jadrowej PAN, ul. Radzikowskiego 152, 31-342
Kraków,
Poland
29 - European Associated Laboratory for Gamma-Ray Astronomy, jointly
supported by CNRS and MPG, Europe
Received 16 April 2009 / Accepted 1 September 2009
Abstract
Aims. This article presents very-high-energy (VHE; )
data from the
-ray
binary PSR B1259-63 as taken during the years 2005, 2006 and
before as well as shortly after the 2007 periastron passage. These data
extend the knowledge of the lightcurve of this object to all phases of
the 3.4 year binary orbit. The lightcurve constrains physical
mechanisms present in this TeV source.
Methods. Observations of VHE -rays
with the HESS telescope array using the Imaging Atmospheric Cherenkov
Technique were performed. The HESS instrument features an
angular
resolution of <
and an energy resolution of <
.
Gamma-ray events in an energy range of
were recorded. From these data, energy spectra and lightcurve with a
monthly time sampling were extracted.
Results. VHE -ray emission from
PSR B1259-63 was detected with an overall
significance of 9.5 standard deviations using
of exposure, obtained from April to August 2007. The monthly flux of
-rays
during the observation period was measured, yielding VHE lightcurve
data for the early pre-periastron phase of the system for the first
time. No spectral variability was found on timescales of months. The
spectrum is described by a power law with a photon index of
and flux normalisation
TeV-1 cm-2 s-1.
PSR B1259-63 was also monitored in 2005 and 2006, far
from
periastron passage, comprising 8.9 h and 7.5 h of
exposure,
respectively. No significant excess of -rays is seen in those
observations.
Conclusions. PSR B1259-63 has
been re-confirmed as a variable TeV -ray emitter. The firm
detection of VHE photons emitted at a true anomaly
of the pulsar orbit, i.e. already
50 days
prior to the periastron passage, disfavors the stellar disc target
scenario as a primary emission mechanism, based on current knowledge
about the companion star's disc inclination, extension, and density
profile.
Key words: radiation mechanisms: non-thermal - methods: observational - stars: binaries: general - stars: neutron - gamma rays: observations - telescopes
1 Introduction
The binary system PSR B1259-63/SS2883 is known since
its
discovery at radio wavelengths by Johnston and collaborators in 1991 (Johnston et al.
1992) (see Table 1).
It consists of a 48 ms pulsar with a Crab-like pulse profile
(double peaked, showing a main pulse and an interpulse) in a
3.4 year eccentric orbit (e=0.87) around a
massive Be star.
The latter feeds a dense circumstellar disc, as indicated by its
optical spectrum and by the eclipse of the pulsed radio signal around
periastron (Johnston et al.
1999). This circumstellar disc is likely to be misaligned
with respect to the pulsar orbit (Bogomazov
2005; Johnston
et al. 2005; Melatos
et al. 1995).
PSR B1259-63 has been observed in various energy bands from
radio to -rays
throughout the years since its discovery. Most observations took place
around periastron passage where the distance between the pulsar and the
massive star is at its minimum of
0.7 AU and the interactions
between the two objects are believed to be most intense.
Table 1: System parameters of the binary system PSR B1259-63/SS2883 (Tavani & Arons 1997).
X-ray observations of PSR B1259-63 show unpulsed emission with
a variable flux and spectral index (Greiner
et al. 1995; Shaw
et al. 2004; Nicastro
et al. 1999; Hirayama
et al. 1999; Chernyakova
et al. 2006). The X-ray luminosity in the
1-10 keV band evolves significantly with orbital phase (mean
anomaly), with
at apastron but reaching
some
10 days prior to and during periastron passage. The X-ray luminosity
then decreases. The spectral index also shows orbit-related variability
with softer indices at periastron just like the unpulsed radio flux
which is enhanced during the passage (Neronov &
Chernyakova 2007). Recently, evidence for the presence of a
spectral break around
during the hard-spectrum state of the 2007 periastron passage was
presented (Uchiyama
et al. 2009).
PSR B1259-63 was first detected in TeV -rays
around its periastron passage in 2004 by HESS, making it the first
known binary to emit at very high energies and the first variable VHE
source in our Galaxy (Aharonian et al.
2005b). The HESS observations showed that the flux
of VHE
-rays
from this source is significantly variable on timescales of days. The
overall spectrum of the emission extracted from
(livetime) of data followed a simple power law with photon index
and flux normalisation
showing no indication for index variability on timescales of months.
Unlike the three other binary systems LS5039 (Aharonian et al.
2005a), LSI+61 303 (Albert et al. 2006)
and Cygnus X-1 (Albert et al.
2007)
seen in the VHE domain, PSR B1259-63 represents the
only
known system where the compact object is unambiguously identified as a
neutron star (NS) (Johnston
et al. 1992). The relatively short pulsar spin
period and high spin-down luminosity (see Table 1)
are sufficient to generate a relativistic pulsar wind (PW) which
prevents accretion onto the neutron star.
PSR B1259-63 can
be classified as a binary system with a plerionic component (Tavani &
Arons 1997),
i.e. as a unique system for the study of PW interactions with
ambient radiation and matter outflow originating from a companion star.
The peculiar double humped shape of the VHE flux in 2004 as
well as
the correlated rise and fall in the lightcurves in other wavebands
around periastron have been taken as hints for the causal influence of
the Be star disc on the emission process (Kawachi
et al. 2004; Chernyakova et al.
2006).
The dense equatorial matter outflow is an ideal source of target
material within the framework of a hadronic scenario where the
ultrarelativistic PW particles would produce
mesons and hence TeV
-rays.
However, the situation is ambiguous and such a scenario cannot be
proven yet as the combined spectral and lightcurve data allow for
different model explanations (Neronov
& Chernyakova 2007). The generation of TeV
-rays within
an Inverse Compton (IC) scenario is another explanation for the data (Kirk et al. 1999;
Sierpowska-Bartosik
& Bednarek 2008; Ball
& Kirk 2000; Dubus
2006a; Khangulyan
et al. 2007).
In such models PW electrons moving with highly relativistic energies
upscatter soft UV photons stemming from the stellar radiation field
into the VHE regime. Strong constraints on the models can be obtained
from phase dependent observations. Most relevant physical quantities
such as the magnetic field, the radiation and matter densities and the
binary separation are a function of the pulsar orbital phase. The
changing geometry of the system as seen by the observer can also cause
variations in the measured TeV flux (e.g. because of the anisotropic
nature of IC scattering).
The previously published VHE lightcurve lacks data from the
early
phases prior to periastron. This prompted observation of
PSR B1259-63 around the 2007 periastron (27th of
July), the
results of which are described here. A specific campaign was carried
out from April to August 2007, resulting in
(livetime) of data. Note that right at periastron passage
PSR B1259-63 was not visible for HESS during
night time.
In addition, monitoring observations performed during the years 2005 (
livetime from March to April) and 2006 (
livetime from April to May) are also reported here.
Table 2: Datasets for the HESS campaigns on PSR B1259-63/SS2883 in 2005, 2006 and 2007.
2 Observations and analysis
The High Energy Stereoscopic System HESS is an array
of four
imaging air Cherenkov telescopes located in the Khomas Highland in
Namibia (
)
at an altitude of
above sea level. Each telescope consists of a spherical dish
in diameter, hosting 380 individual mirrors giving an overall
reflective area of
.
Cherenkov radiation as generated in extended air showers is collected
by the mirrors and focused onto a camera consisting of 960
photomultipliers with a pixel size of
resulting in a Field of View (FoV) of
.
Following the usual trigger criterion with respect to telescope
multiplicity for coincident operation, a shower image will be recorded
once at least two out of four telescopes trigger (Funk et al. 2004).
Determination of shower parameters and consequent evaluation of the
primary particle type, energy and direction is done using an image
reconstruction technique based on Hillas moments (Aharonian et al.
2006). The HESS instrument has a trigger threshold
for the photon energy of
for observations at zenith given an optical mirror efficiency of
>
.
Above this ideal threshold a point source at zenith with a photon flux
of
1% of that
of the Crab nebula, i.e. <
,
can be detected in a
observation at a significance level of
.
Table 2
summarizes the dates
and livetimes of the dataset used here. The data were taken in
wobble-mode, i.e. with the pointing position slightly offset from the
target position. Due to the radial acceptance profile of the detector
this mode allows a simple simultaneous background estimation using the
Reflected-Region method (Berge et al.
2007).
For the observation campaign in 2007 this technique has been applied
only with respect to right ascension (RA) at an offset of
so as not to interfere with a second source in the FoV
HESS J1303-631 (Aharonian et al.
2005c) which is located
to the north of PSR B1259-63. The 2007 observations were
carried out at zenith angles (ZA) ranging from
to
.
The mean ZA is
leading to an analysis energy threshold of 620 GeV. The
datasets
from 2005 and 2006 contain exposures taken during the HESS galactic
plane scan and dedicated observation runs on
HESS J1303-631 with a positive offset of
in declination (Dec.). This leads to wobble offsets with respect to the
position of PSR B1259-63 for the analysis presented
here both
in RA and Dec., ranging between
and
.
The mean ZA for these observations were
in 2005 and
in 2006. The usual quality criteria with respect to weather conditions
and fully functional array hardware are applied to the data (Aharonian et al.
2006).
![]() |
Figure 1:
Correlated significance map for the PSR B1259-63 FoV
between May and August 2007. There is a clear signal in TeV |
Open with DEXTER |
2.1 Detection
The Hillas analysis was applied, incorporating so-called
standard
cuts on image quality (image amplitude 80 p.e.) and an angular cut of
for
defined as angular distance between a
-ray-like
event and the nominal target position. In order to measure the hadronic
background in the FoV simultaneously, the Reflected-Region background
model was used. Using this technique, the level of OFF events is taken
from regions within the same FoV which are located at the same distance
with respect to the camera center as the ON region in order to account
for the radial acceptance of the detector. OFF regions that overlapped
with a circular region of radius
at the position of HESS J1303-631 were omitted in the
background calculation. A correction due to a reduced reflectivity in
the instrument's mirrors has been taken into account as described in Aharonian et al.
(2006).
This standard HESS point source analysis for the 2007 dataset
resulted in a clear excess of 450 photons coming from the direction of
PSR B1259-63 resulting in a statistical significance
following Li & Ma (1983)
of 9.5 standard deviations for this detection (see Table 2).
Figure 1
shows the correlated significance map from May to August 2007 for the
PSR B1259-63 FoV using the Ring Background model with
a
correlation radius of
as described in Berge
et al. (2007).
A fit of the signal with the instrument's point spread function,
i.e. a two dimensional Gaussian, gives (J2000) RA
,
Dec.
for
the excess center position. This is within errors compatible with the
nominal position of PSR B1259-63 (Wang et al. 2004)
and the TeV signal detected in 2004 (Aharonian et al.
2005b).
Table 3: Spectral parameters for a power law fit to the monthly HESS data.
The analysis of the 2005 and 2006 datasets showed no significant excess
in -ray events
(Table 2).
A calculation of upper limits at the
confidence level according to Feldman
& Cousins (1998) on the integrated photon flux above
1 TeV yields
and
for these measurements, respectively.
A cross check analysis which is based on a semi-analytical model approach for air showers in order to predict the expected intensity in each pixel of the camera as described in de Naurois (2005) was also performed using an independent chain of rawdata processing. This method shows a similar signal efficiency but superior background rejection compared to the Hillas analysis. The latter has been used in the field for over 20 years now and is also more robust against systematics. Throughout this article the Hillas analysis method is used.
2.2 Energy spectra
A spectral analysis of the detected excess events from within
the ON
region for the whole 2007 dataset using the Hillas analysis shows that
the differential energy spectrum of the collected photons as a function
of particle energy follows a simple power law of the form
with flux normalisation at










![]() |
Figure 2:
Overall differential energy spectrum dN/dE
for |
Open with DEXTER |
![]() |
Figure 3:
( Top) PSR B1259-63 differential
energy spectra dN/dE
for the monthly darkness periods May, June and July 2007. For the data
subsets taken in April and August, no spectra could be derived due to
insufficient statistics. ( Bottom) |
Open with DEXTER |
2.3 Lightcurve
The integral VHE flux of photons
above an energy of
has been calculated by integrating Eq. (1) above
this threshold, assuming an average photon index of
taken from the overall spectrum (Fig. 2). The flux
normalisation
has been determined as outlined in Aharonian et al.
(2005b). There was no significant excess in TeV
-rays in
2005, 2006 and during the first exposures in 2007 taken in April (
;
being
the periastron passage) which re-confirms the variable character of
this source as already established from the night by night 2004
HESS data. In 2007 the total integrated photon flux above
is
corresponding to 2.7% of that of the Crab nebula above the same
threshold. This translates into a mean energy flux of the VHE emission
of
implying a
-ray
luminosity of
at a distance of 1.5 kpc. We note this luminosity
of the pulsar spin-down power. The average flux levels for each
observation period in 2007 are shown in the monthly lightcurve in
Fig. 4.
As shown in Table 2
the emission of TeV photons from PSR B1259-63 where a
significant signal (>
)
was seen under both analyses started in June 2007, i.e.
50 days
prior to the periastron passage (
). A fit with a constant to
the monthly lightcurve (April-August 2007) leads to a goodness of fit
of
resulting in a probability of
.
![]() |
Figure 4:
Integrated photon flux above |
Open with DEXTER |
The combination of the monthly integrated fluxes presented in
Fig. 4
with the daily lightcurve as extracted from the 2004 HESS campaign (Aharonian et al.
2005b) together with the measurements from 2005 and 2006 in a
single plot is shown in Fig. 5. In this
representation the flux is shown as a function of the true anomaly
(lower axis) and orbital phase
(upper axis), respectively. It can be seen that the overall flux level
in 2007 is comparable to that measured in 2004, while they correspond
to different orbital phases. All in all the lightcurve could be not
only asymmetric with respect to periastron, also the two humps appear
to exhibit a different shape with the pre-periastron hump being
broader.
![]() |
Figure 5:
VHE integrated flux from PSR B1259-63 above |
Open with DEXTER |
Between the datasets of 2004 and 2007 there is only a marginal
overlap in orbital phase, i.e. the exposures taken in July and
August 2007 match each one night of observations of the 2004 campaign
(at a true anomaly
and
,
respectively). The measurements are not statistically different (1.8
and 0.6 standard deviations, respectively). However, the 2007
measurements suffer from low statistics. It is not possible at present
to test whether the source shows orbit-to-orbit variability and more
overlapping data would be desirable.
3 Discussion
As with other VHE sources, the production of VHE gamma-rays in this binary requires a population of particles with multi-TeV energies. One possibility is shock acceleration within the termination zone of colliding winds (Maraschi & Treves 1981; Tavani et al. 1994; Tavani & Arons 1997; Bogovalov et al. 2008) which applies for PSR B1259-63/SS2883 where the PW is shocked by the stellar wind.
The modeling of the observed variability in TeV photon flux is based on IC scattering (Khangulyan et al. 2007) as well as hadronic interactions taking place when the pulsar interferes with the equatorial matter outflow. The latter simply explain the VHE flux variability with the phases when the pulsar crosses the dense circumstellar disc (Chernyakova et al. 2006).
3.1 IC scenarios
In the IC scenario many parameters contribute to the complexity of the problem. The overall system geometry significantly influences predictions for the VHE emission as shown by Kirk et al. (1999). The IC cross section varies with orbital phase as the angle between the line of sight and the vector connecting the stars changes. Also the magnetic field strength B will be a function of the separation d of the two objects due to changing magneto hydrodynamic conditions at the PW shock where the field lines from the wind get compressed (Kirk et al. 1999; Tavani & Arons 1997). The B-field should become stronger towards periastron, resulting in faster synchrotron losses. This in turn means a shift in the ratio of radiation timescales, affecting the efficiency of IC cooling for VHE electrons in the Klein-Nishina regime and hence the TeV photon index. Moreover non-radiative cooling mechanisms such as adiabatic expansion of the shock region or particle escape can be important effects as demonstrated by Khangulyan et al. (2007). In Bogovalov et al. (2008) it was pointed out that the interaction of pulsar and stellar winds leads to relativistic motion of matter at the termination shock. The corresponding Doppler factor with respect to the line of sight will strongly depend on the position of the pulsar along its orbit causing modulations of the non-thermal radiation of electrons.
Finally, Bednarek (1997), Sierpowska-Bartosik & Bednarek (2008) and Bosch-Ramon et al. (2008) have discussed the possibility of radiating secondaries stemming from pair cascades forming close to the star where the photon field density is high enough.
3.2 Hadronic disc scenarios
Concerning hadronic scenarios, the stellar disc seems to be an
ideal
reservoir for target material interacting with the PW. In this regard
parameters like the disc's density, thickness, extension and
orientation with respect to the pulsar orbit are crucial. Currently,
however, the knowledge about these quantities is limited and depends on
model interpretations of the available radio to X-ray data as discussed
in Johnston
et al. (1994,2005,1996), Melatos et al.
(1995), Tavani
& Arons (1997) and Bogomazov (2005).
According to these studies the disc appears to be inclined by an angle
in the range of
compared to the pulsar ecliptic. Moreover the vanishing of the pulsed
radio signal between 16 days before and 15 days after periastron (Johnston et al.
2005)
accounts for the asymmetric position of the disc's line of intersection
with the orbital plane with respect to the orbital semimajor axis. In Chernyakova et al.
(2006), the disc
position is inferred from the HESS 2004 data by assuming that
the
peak VHE emission corresponds to orbital phases of maximum
circumstellar density. According to this, the disc intersects the
pulsar orbit at a true anomaly of
and
,
respectively (
)
with respect to periastron and covers the pulsar orbit over an angle
band of
(
).
3.3 Interpretation of the 2007 VHE data
Looking at the above estimates for the disc location of
SS2883, the
integrated flux data presented here are difficult to understand within
a purely hadronic disc scenario, considering a simple symmetry
argument. According to this the excess in June (Table 2),
corresponding to 47 days prior to periastron passage, occurs at
unexpectedly small values for the true anomaly of
(
off the expected disc density maximum; see Fig. 6),
inconsistent with the position of the Be disc as computed by Chernyakova et al.
(2006)
from the 2004 TeV data alone (see their Fig. 4). Following their
approach and fitting the complete data set including the measurements
in 2007 with a Gaussian gives a very poor
of 118.2/39, in contrast to 40.0/34 for a fit omitting the 2007 data as
shown in Fig. 7.
In this approach, flux data gathered after periastron passage has been
shifted with respect to periastron by 0.5 and added to the
pre-periastron phase. This is based on the simple assumption that the
position of the second crossing should be shifted by
relative to the first entrance of the pulsar into the disc. Note,
however, that in this approach it has not been considered that the
binary separation is different for the corresponding pulsar positions
when being shifted, translating into different densities in the stellar
disc.
The above argument is also supported by the discrepancy between the disc parameters resulting from this approach and the eclipse of the pulsed radio emission. The pulsed signal emerges 10 days earlier from the eclipse than would be expected (Dubus 2006b) and the eclipse itself is slightly shifted towards pre-periastron phases which in addition contradicts the above disc location. Altogether this would either suggest a complicated disc morphology, e.g., propagating bubbles of disc material, or reflecting the fact that the disc cannot be the only explanation for the onset of VHE radiation, thus favoring disc independent scenarios such as the IC upscattering of stellar photons at least as additional origins for the TeV emission.
![]() |
Figure 6: Top
view model
sketch for the PSR B1259-63/SS2883 orbit. The black
dot
represents SS2882 and the circumstellar disc is depicted in green. The
pulsar size is exaggerated by a huge factor. The circumstellar disc is
assumed to be extended out to 20 stellar radii |
Open with DEXTER |
![]() |
Figure 7: Fit of the 2004 (black points) integrated flux data above 1 TeV with a Gaussian following Fig. 4 in Chernyakova et al. (2006) in order to determine the location of the circumstellar disc in comparison with fluxes measured in 2007 (blue squares). The 2007 data deviates considerably from the suggested disc density model. |
Open with DEXTER |
4 Conclusions
The second HESS campaign on the binary system PSR B1259-63/SS2883 around periastron in 2007 (


















![[*]](/icons/foot_motif.png)

PSR B1259-63 remains a fascinating TeV binary system featuring complex PW dynamics with many questions still open. More sensitive observations in the VHE regime using future observatories that would allow for phase resolved spectra with time apertures on time scales of days, could contribute to a better understanding of this source.
AcknowledgementsThe support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of HESS is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the UK Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment. We would like to thank Masha Chernyakova for fruitful discussions.
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Footnotes
- ...
- Supported by CAPES Foundation, Ministry of Education of Brazil.
- ...
- Higher wobble offsets (>
) additionally increase the energy threshold due to the radial acceptance profile of the camera.
- ... phase
- The true anomaly
and orbital phase
(mean anomaly) of the system vary between -0.5 and 0.5 with periastron passage defined as
.
- ...(Albert et al. 2009)
- Here we quote phases subtracting the presently defined
periastron phase at
(Aragona et al. 2009).
All Tables
Table 1: System parameters of the binary system PSR B1259-63/SS2883 (Tavani & Arons 1997).
Table 2: Datasets for the HESS campaigns on PSR B1259-63/SS2883 in 2005, 2006 and 2007.
Table 3: Spectral parameters for a power law fit to the monthly HESS data.
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