2,346 research outputs found
The 2-10 keV emission properties of PSR B1937+21
We present the results of a BeppoSAX observation of the fastest pulsar known:
PSR B1937+21. The ~ 200 ks observation (78.5 (34) ks MECS (LECS) exposure
times) allowed us to investigate with high statistical significance both the
spectral properties and the pulse profile shape. The absorbed power law
spectral model gave a photon index of ~ 1.7 and N_H ~ 2.3 x 10^22 cm^-2. These
values explain both a) the ROSAT non-detection and b) the deviant estimate of a
photon index of ~ 0.8 obtained by ASCA. The pulse profile appears, for the
first time, clearly double peaked with the main component much stronger than
the other. The statistical significance is 10 sigma (main peak) and 5 sigma
(secondary peak). The 1.6-10 keV pulsed fraction is consistent with 100%; only
in the 1.6-4 keV band there is a ~ 2 sigma indication for a DC component. The
secondary peak is detected significantly only for energies above 3 / 4 keV. The
unabsorbed (2-10 keV) flux is F_2-10 = 3.7 x 10^-13 erg cm^-2 s^-1, implying a
luminosity of L_X = 4.6 x 10^31 Theta (d/3.6 kpc)^2 erg s^-1 and an X-ray
efficiency of eta = 4 x 10^-5 Theta, where Theta is the solid angle spanned by
the emission beam. These results are in agreement with those obtained by ASCA.Comment: 4 pages, 4 figures, 2 tables. To appear in the Proceedings of the
270. WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants,
Jan. 21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J.
Truemper. Proceedings are available as MPE-Report 27
On the observability of bow shocks of Galactic runaway OB stars
Massive stars that have been ejected from their parent cluster and
supersonically sailing away through the interstellar medium (ISM) are
classified as exiled. They generate circumstellar bow shock nebulae that can be
observed. We present two-dimensional, axisymmetric hydrodynamical simulations
of a representative sample of stellar wind bow shocks from Galactic OB stars in
an ambient medium of densities ranging from n_ISM=0.01 up to 10.0/cm3.
Independently of their location in the Galaxy, we confirm that the infrared is
the most appropriated waveband to search for bow shocks from massive stars.
Their spectral energy distribution is the convenient tool to analyze them since
their emission does not depend on the temporary effects which could affect
unstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks
generated by high-mass (~40 Mo) runaway stars yield H fluxes which
could be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The
brightest bow shock nebulae are produced in the denser regions of the ISM. We
predict that bow shocks in the field observed at Ha by means of
Rayleigh-sensitive facilities are formed around stars of initial mass larger
than about 20 Mo. Our models of bow shocks from OB stars have the emission
maximum in the wavelength range 3 <= lambda <= 50 micrometer which can be up to
several orders of magnitude brighter than the runaway stars themselves,
particularly for stars of initial mass larger than 20 Mo.Comment: 13 pages, 12 figures. Accepted to MNRAS (2016
BeppoSAX observation of PSR B1937+21
We present the results of a BeppoSAX observation of the fastest rotating
pulsar known: PSR B1937+21. The ~200 ks observation (78.5 ks MECS/34 ks LECS
on-source time) allowed us to investigate with high statistical significance
both the spectral properties and the pulse profile shape. The pulse profile is
clearly double peaked at energies > ~4 keV. Peak widths are compatible with the
instrumental time resolution and the second pulse lags the main pulse 0.52 in
phase, like is the case in the radio. In the 1.3-4 keV band we detect a ~45% DC
component; conversely the 4-10 keV pulsed fraction is consistent with 100%. The
on-pulse spectrum is fitted with an absorbed power-law of spectral index ~1.2,
harder than that of the total flux which is ~1.9. The total unabsorbed (2-10
keV) flux is F_{2-10} = 4.1 10^-13 cgs, implying a luminosity of L_X = 5.0
10^31 \Theta (d/3.6 kpc)^2 erg s^-1 and a X-ray efficiency of \eta = 4.5 10^-5
\Theta, where \Theta is the solid angle spanned by the emission beam. These
results are in agreement with those obtained by ASCA and a more recent
Rossi-XTE observation. The hydrogen column density N_H ~2 10^22 cm^-2 is ~10
times higher than expected from the radio dispersion measure and average
Galactic density of e-. Though it is compatible (within 2\sigma) with the
Galactic (HI derived) value of ~1 10^22 cm^-2, inspection of dust extinction
maps reveal that the pulsar falls in a highly absorbed region. In addition, 1.4
GHz radio map shows that the nearby (likely unrelated) HII source 4C21.53W is
part of a circular emission region ~4' across.Comment: 8 pages, 5 figures; accepted for publication in A&
Discovery of hard non-thermal pulsed X-ray emission from the anomalous X-ray pulsar 1E 1841-045
We report the discovery of non-thermal pulsed X-ray/soft gamma-ray emission
up to about 150 keV from the anomalous X-ray pulsar AXP 1E 1841-045 located
near the centre of supernova remnant Kes 73 using RXTE PCA and HEXTE data. The
morphology of the double-peaked pulse profile changes rapidly with energy from
2 keV up to about 8 keV, above which the pulse shape remains more or less
stable. The pulsed spectrum is very hard, its shape above 10 keV can be
described well by a power law with a photon index of 0.94 +/- 0.16. 1E 1841-045
is the first AXP for which such very-hard pulsed emission has been detected,
which points to an origin in the magnetosphere of a magnetar.Comment: 14 pages in ApJ preprint style, 5 figures one in color, Submitted to
Ap
The Spectral Variability of Cygnus X-1 at MeV Energies
In previous work, we have used data from the first three years of the CGRO
mission to assemble a broad-band -ray spectrum of the galactic black
hole candidate Cygnus X-1. Contemporaneous data from the COMPTEL, OSSE and
BATSE experiments on CGRO were selected on the basis of the hard X-ray flux
(45--140 keV) as measured by BATSE. This provided a spectrum of Cygnus X-1 in
its canonical low X-ray state (as measured at energies below 10 keV), covering
the energy range from 50 keV to 5 MeV. Here we report on a comparison of this
spectrum to a COMPTEL-OSSE spectrum collected during a high X-ray state of
Cygnus X-1 (May, 1996). These data provide evidence for significant spectral
variability at energies above 1 MeV. In particular, whereas the hard X-ray flux
{\it decreases} during the high X-ray state, the flux at energies above 1 MeV
{\it increases}, resulting in a significantly harder high energy spectrum. This
behavior is consistent with the general picture of galactic black hole
candidates having two distinct spectral forms at soft -ray energies.
These data extend this picture, for the first time, to energies above 1 MeV.Comment: 5 pages, 4 figures, to be published in AIP Conf. Proc., "The Fifth
Compton Symposium
Accreting millisecond X-ray pulsars: 10 years of INTEGRAL observations
During the last 10 years, INTEGRAL made a unique contribution to the study of
accreting millisecond X-ray pulsars (AMXPs), discovering three of the 14
sources now known of this class. Besides increasing the number of known AMXPs,
INTEGRAL also carried out observations of these objects above 20 keV,
substantially advancing our understanding of their behaviour. We present here a
review of all the AMXPs observed with INTEGRAL and discuss the physical
interpretation of their behaviour in the X-ray domain. We focus in particular
on the lightcurve profile during outburst, as well as the timing, spectral, and
thermonuclear type-I X-ray bursts properties.Comment: 8 pages, 8 figures. Proceedings of "An INTEGRAL view of the
high-energy sky (the first 10 years)" the 9th INTEGRAL Workshop, October
15-19, 2012, Paris, Franc
COMPTEL measurements of MeV gamma-ray burst spectra
We present results from the on-going spectral analysis of gamma-ray bursts measured by the COMPTEL instrument in its main Compton “Telescope” observing mode (0.75–30 MeV). Thus far, 18 bursts have been analyzed from three years (April 1991–April 1994) of observations. The time-averaged spectra of these events above 1 MeV are all consistent with a simple power law model with spectral index in the range 1.5–3.5. Exponential, thermal bremsstrahlung and thermal synchrotron models are statistically inconsistent with the burst sample, although they can adequately describe some of the individual burst spectra. We find good agreement between burst spectra measured simultaneously by BATSE, COMPTEL and EGRET, which typically show a spectral transition or “break” in the BATSE energy range around a few hundred keV followed by simple power law emission extending to hundreds of MeV. However, the temporal relation between MeV and GeV (e.g., as measured by EGRET) burst emission is still unclear. Measurement of rapid variability at MeV energies in the stronger bursts provides evidence that either the sources are nearby (within the Galaxy) or the gamma-ray emission is relativistically beamed
- …