534 research outputs found
High-energy gamma-ray emission from the inner jet of LS I+61 303: the hadronic contribution revisited
LS I+61 303 has been detected by the Cherenkov telescope MAGIC at very high
energies, presenting a variable flux along the orbital motion with a maximum
clearly separated from the periastron passage. In the light of the new
observational constraints, we revisit the discussion of the production of
high-energy gamma rays from particle interactions in the inner jet of this
system. The hadronic contribution could represent a major fraction of the TeV
emission detected from this source. The spectral energy distribution resulting
from p-p interactions is recalculated. Opacity effects introduced by the photon
fields of the primary star and the stellar decretion disk are shown to be
essential in shaping the high-energy gamma-ray light curve at energies close to
200 GeV. We also present results of Monte Carlo simulations of the
electromagnetic cascades developed very close to the periastron passage. We
conclude that a hadronic microquasar model for the gamma-ray emission in LS I
+61 303 can reproduce the main features of its observed high-energy gamma-ray
flux.Comment: 6 pages. Sligth improvements made. Accepted version by Astrophysics
and Space Scienc
TeV Gamma-rays from accreting magnetars in massive binaries
We consider the neutron star (NS) of the magnetar type inside the massive
binary system. We determine the conditions under which the matter from the
stellar wind can penetrate the inner magnetosphere of the magnetar. At some
distance from the NS surface, the magnetic pressure can balance the
gravitational pressure of the accreting matter creating very turbulent,
magnetized transition region. It is suggested that this region provides good
conditions for acceleration of electrons to relativistic energies. These
electrons lose energy on the synchrotron process and the Inverse Compton (IC)
scattering of the radiation from the nearby massive stellar companion,
producing high energy radiation from the X-rays up to TeV -rays.
The primary -rays can be farther absorbed in the stellar radiation
developing the IC pair cascade. We calculate the synchrotron X-ray
emission from primary electrons and secondary pairs and the IC
-ray emission from the cascade process. It is shown that the
quasi-simultaneous observations of the TeV -ray binary system LSI +61
303 in the X-ray and the TeV -ray energy ranges can be explained in
such an accreting magnetar model.Comment: 5 pages, 1 figure, 1 table, submitted to MNRA
XMM-Newton survey of the Local Group galaxy M 33
In an XMM-Newton raster observation of the bright Local Group spiral galaxy M
33 we study the population of X-ray sources (X-ray binaries, supernova
remnants) down to a 0.2--4.5 keV luminosity of 10^35 erg/s -- more than a
factor of 10 deeper than earlier ROSAT observations. EPIC hardness ratios and
optical and radio information are used to distinguish between different source
classes. The survey detects 408 sources in an area of 0.80 square degree. We
correlate these newly detected sources with earlier M 33 X-ray catalogues and
information from optical, infra-red and radio wavelengths. As M 33 sources we
detect 21 supernova remnants (SNR) and 23 SNR candidates, 5 super-soft sources,
and 2 X-ray binaries (XRBs). There are 267 sources classified as hard, which
may either be XRBs or Crab-like SNRs in M 33 or background AGN. The 44
confirmed and candidate SNRs more than double the number of X-ray detected SNRs
in M 33. 16 of these are proposed as SNR candidates from the X-ray data for the
first time. On the other hand, there are several sources not connected to M 33:
five foreground stars, 30 foreground star candidates, 12 active galactic
nucleus candidates, one background galaxy and one background galaxy candidate.
Extrapolating from deep field observations we would expect 175 to 210
background sources in this field. This indicates that about half of the sources
detected are sources within M 33.Comment: 14 pages, 6 figures, accepted for publication in A&A, the images of
Figs. 1,2,3,4,6 are available in jpg format, a full version of the paper is
available at ftp://ftp.xray.mpe.mpg.de/people/fwh/docs/M33_AA0068.p
A Spectacular VHE Gamma-Ray Outburst from PKS 2155-304 in 2006
Since 2002 the VHE (>100 GeV) gamma-ray flux of the high-frequency peaked BL
Lac PKS 2155-304 has been monitored with the High Energy Stereoscopic System
(HESS). An extreme gamma-ray outburst was detected in the early hours of July
28, 2006 (MJD 53944). The average flux above 200 GeV observed during this
outburst is ~7 times the flux observed from the Crab Nebula above the same
threshold. Peak fluxes are measured with one-minute time scale resolution at
more than twice this average value. Variability is seen up to ~600 s in the
Fourier power spectrum, and well-resolved bursts varying on time scales of ~200
seconds are observed. There are no strong indications for spectral variability
within the data. Assuming the emission region has a size comparable to the
Schwarzschild radius of a ~10^9 solar mass black hole, Doppler factors greater
than 100 are required to accommodate the observed variability time scales.Comment: 4 pages, 3 figures; To appear in the Proceedings of the 30th ICRC
(Merida, Mexico
GeV-TeV gamma-ray light curves expected in the IC electron-positron pair cascade model for massive binaries: Application to LS 5039
TeV gamma-ray emission from two massive binaries of the microquasar type, LS
5039 and LS I +61 303, show clear variability with their orbital
periods. Our purpose is to calculate the GeV and TeV -ray light curves
from the massive binary LS 5039 which are expected in the specific Inverse
Compton pair cascade model. This model successfully predicted the basic
features of the high energy -ray emission from LS 5039 and LS I +61
303. In the calculations we apply the Monte Carlo code which follows the IC
pair cascade in the anisotropic radiation of the massive star. The
-ray light curves and spectra are obtained for different parameters of
the acceleration scenario and the inclination angles of the binary system. It
is found that the GeV and TeV -ray light curves should be
anti-correlated. This feature can be tested in the near future by the
simultaneous observations of LS 5039 with the AGILE and GLAST telescopes in GeV
energies and the Cherenkov telescopes in the TeV energies. Considered model
also predicts a broad maximum in the TeV -ray light curve between the
phases consistently with the observations of LS 5039 by the HESS
telescopes. Moreover, we predict additional dip in the TeV light curve for
large inclination angles . This feature could serve as a
diagnostic for independent measuring of the inclination angle of this binary
system indicating also on the presence of a neutron star in LS 5039.Comment: 5 pages, 4 figures, A&A accepte
Photometric Analysis of the Optical Counterpart of the Black Hole HMXB M33 X-7
Aims: Study the high-mass X-ray binary X-7 in M33 using broad-band optical
data.
Methods: We used recently published CFHT r' and i' data for variable stars in
M33 to extract the light curve of the optical counterpart of X-7. We combined
these data with DIRECT B and V measurements in order to search for an
independent optical modulation with the X-ray periodicity. The periodic
modulation is modelled with the ellipsoidal effect. We used UBVRr'i' magnitudes
of the system to constrain the temperature and radius of the optical component.
Results: The optical data revealed a periodicity of 3.4530 +- 0.0014 days,
which is consistent with the known X-ray period. Double modulation, which we
attributed to ellipsoidal modulation, is clearly seen in four different optical
bands. The absolute magnitude in six optical bands is most consistent with a
stellar counterpart with 33000 < T_{eff} < 47000 K and 15 < R < 20 R_{\sun}. We
modelled the optical periodic modulation and derived the masses of the two
components as a function of the orbital inclination and the radius of the
stellar component. The resulting mass range for the compact object is 1.3 < M <
23 M_{\sun}.
Conclusions: The system is probably a black hole HMXB, similar to Cyg X-1,
LMC X-1 and LMC X-3.Comment: Accepted for publication in A&
The missing GeV {\gamma}-ray binary: Searching for HESS J0632+057 with Fermi-LAT
The very high energy (VHE; >100 GeV) source HESS J0632+057 has been recently
confirmed as a \gamma-ray binary, a subclass of the high mass X-ray binary
(HMXB) population, through the detection of an orbital period of 321 days. We
performed a deep search for the emission of HESS J0632+057 in the GeV energy
range using data from the Fermi Large Area Telescope (LAT). The analysis was
challenging due to the source being located in close proximity to the bright
\gamma-ray pulsar PSR J0633+0632 and lying in a crowded region of the Galactic
plane where there is prominent diffuse emission. We formulated a Bayesian block
algorithm adapted to work with weighted photon counts, in order to define the
off-pulse phases of PSR J0633+0632. A detailed spectral-spatial model of a 5
deg circular region centred on the known location of HESS J0632+057 was
generated to accurately model the LAT data. No significant emission from the
location of HESS J0632+057 was detected in the 0.1-100 GeV energy range
integrating over ~3.5 years of data; with a 95% flux upper limit of F_{0.1-100
GeV} < 3 x 10-8 ph cm-2 s-1. A search for emission over different phases of the
orbit also yielded no significant detection. A search for source emission on
shorter timescales (days--months) did not yield any significant detections. We
also report the results of a search for radio pulsations using the 100-m Green
Bank Telescope (GBT). No periodic signals or individual dispersed bursts of a
likely astronomical origin were detected. We estimated the flux density limit
of < 90/40 \mu Jy at 2/9 GHz. The LAT flux upper limits combined with the
detection of HESS J0632+057 in the 136-400 TeV energy band by the MAGIC
collaboration imply that the VHE spectrum must turn over at energies <136 GeV
placing constraints on any theoretical models invoked to explain the \gamma-ray
emission.Comment: 11 pages, 4 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS) Main Journa
Optical depths for gamma-rays in the radiation field of a star heated by external X-ray source in LMXBs: Application to Her X-1 and Sco X-1
The surface of a low mass star inside a compact low mass X-ray binary system
(LMXB) can be heated by the external X-ray source which may appear due to the
accretion process onto a companion compact object (a neutron star or a black
hole). As a result, the surface temperature of the star can become
significantly higher than it is in the normal state resulting from
thermonuclear burning. We wonder whether high energy electrons and gamma-rays,
injected within the binary system, can efficiently interact with this enhanced
radiation field. To decide this, we calculate the optical depths for the
gamma-ray photons in the radiation field of such irradiated star as a function
of the phase of the binary system. Based on these calculations, we conclude
that compact low mass X-ray binary systems may also become sources of high
energy gamma-rays since conditions for interaction of electrons and gamma-rays
are quite similar to these ones observed within the high mass TeV gamma-ray
binaries such as LS 5039 and LSI 303 +61. However, due to differences in the
soft radiation field, the expected gamma-ray light curves can significantly
differ between low mass and high mass X-ray binaries. As an example, we apply
such calculations to two well known LMXBs: Her X-1 and Sco X-1. It is concluded
that electrons accelerated to high energies inside these binaries should find
enough soft photon target from the companion star for efficient gamma-ray
production.Comment: 10 pages, 8 figures, accepted to A&
BeppoSAX spectroscopy of the luminous X-ray sources in M33
The nearby galaxy M33 was observed by the imaging X-ray instruments on-board
BeppoSAX. Two observations at different phases of the 105.9 day intensity cycle
of the luminous central source X-8 failed to reveal the expected modulation,
suggesting that it is probably transitory. Similar behavior has been observed
from several X-ray binary sources. This strengthens somewhat the idea that M33
X-8 is a black hole accreting from a binary companion. The 0.2-10 keV spectrum
of M33 X-8 can best be modeled by an absorbed power-law with a photon index of
1.89 +0.40 -0.79 and a disk-blackbody with a temperature, kT, of 1.10 +/0 0.05
keV and a projected inner-disk radius of 55.4 +6.0 -7.7 km. This spectral shape
is in good agreement with earlier ASCA results. The 2-10 keV spectra of M33
X-4, X-5, X-7, X-9 and X-10 are all consistent with power-law or bremsstrahlung
models, whilst that of X-6 appears to be significantly more complex and may be
reasonably well modeled with a disk-blackbody with kT = 1.7 +/- 0.2 keV and a
projected inner-disk radius of 7 +/- 2 km. The spectrum of M33 X-9 is rather
hard with a photon index of 1.3. Compared to earlier Einstein and ROSAT
observations, M33 X-7 and X-9 varied in intensity, M33 X-4, X-6, and X-10 may
have varied and M33 X-5 remained constant.Comment: 11 pages. To appear in A&
Can angular momentum loss cause the period change of NN Ser?
NN Ser is a non mass-transferring pre-cataclysmic variable containing a white
dwarf with a mass of and an M dwarf secondary star with a
mass of . Based on the data detected by the high-speed CCD
camera ULTRACAM, it was observed that the orbital period of NN Ser is
decreasing, which may be caused by a genuine angular momentum loss or the
presence of a third body. However, neither gravitational radiation and magnetic
braking can ideally account for the period change of NN Ser. In this Letter, we
attempt to examine a feasible mechanism which can drain the angular momentum
from NN Ser. We propose that a fossil circumbinary disk (CB disk) around the
binary may have been established at the end of the common envelope phase, and
the tidal torques caused by the gravitational interaction between the disk and
the binary can efficiently extract the orbital angular momentum from the
system. We find that only if M dwarf has an ultra-high wind loss rates of , and a large fraction () of
wind loss is fed into the CB disk, the loss rates of angular momentum via the
CB disk can interpret the period change observed in NN Ser. Such a wind loss
rate and -value seem to be incredible. Hence it seems that the presence
of a third body in a long orbit around the binary might account for the
changing period of NN Ser.Comment: 4 pages, accepted for publication in A&A Letter
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