2,484 research outputs found
Probing the Nature of the Vela X Cocoon
Vela X is a pulsar wind nebula (PWN) associated with the active pulsar
B0833-45 and contained within the Vela supernova remnant (SNR). A collimated
X-ray filament ("cocoon") extends south-southwest from the pulsar to the center
of Vela X. VLA observations uncovered radio emission coincident with the
eastern edge of the cocoon and H.E.S.S. has detected TeV -ray emission
from this region as well. Using XMM-\textit{Newton} archival data, covering the
southern portion of this feature, we analyze the X-ray properties of the
cocoon. The X-ray data are best fit by an absorbed nonequilibrium plasma model
with a powerlaw component. Our analysis of the thermal emission shows enhanced
abundances of O, Ne, and Mg within the cocoon, indicating the presence of
ejecta-rich material from the propagation of the SNR reverse shock, consistent
with Vela X being a disrupted PWN. We investigate the physical processes that
excite the electrons in the PWN to emit in the radio, X-ray and -ray
bands. The radio and non-thermal X-ray emission can be explained by synchrotron
emission. We model the -ray emission by Inverse Compton scattering of
electrons off of cosmic microwave background (CMB) photons. We use a
3-component broken power law to model the synchrotron emission, finding an
intrinsic break in the electron spectrum at keV and a
cooling break at 5.5 keV. This cooling break along with
a magnetic field strength of 5 G indicate that the synchrotron
break occurs at 1 keV.Comment: accepted for publication to ApJ
Periodic Modulations in an X-ray Flare from Sagittarius A*
We present the highly significant detection of a quasi-periodic flux
modulation with a period of 22.2 min seen in the X-ray data of the Sgr A* flare
of 2004 August 31. This flaring event, which lasted a total of about three
hours, was detected simultaneously by EPIC on XMM-Newton and the NICMOS
near-infrared camera on the HST. Given the inherent difficulty in, and the lack
of readily available methods for quantifying the probability of a periodic
signal detected over only several cycles in a data set where red noise can be
important, we developed a general method for quantifying the likelihood that
such a modulation is indeed intrinsic to the source and does not arise from
background fluctuations. We here describe this Monte Carlo based method, and
discuss the results obtained by its application to a other XMM-Newton data
sets. Under the simplest hypothesis that we witnessed a transient event that
evolved, peaked and decayed near the marginally stable orbit of the
supermassive black hole, this result implies that for a mass of 3.5 x 10^{6}
Msun, the central object must have an angular momentum corresponding to a spin
parameter of a=0.22.Comment: 4 pages, 6 figures, submitted to ApJ
Unidentified Galactic High-Energy Sources as Ancient Pulsar Wind Nebulae in the light of new high energy observations and the new code
In a Pulsar Wind Nebula (PWN), the lifetime of inverse Compton (IC) emitting
electrons exceeds the lifetime of its progenitor pulsar (as well as its
shell-type remnant), but it also exceeds the age of those that emit via
synchrotron radiation. Therefore, during its evolution, the PWN can remain
bright in IC so that its GeV-TeV gamma-ray flux remains high for timescales
much larger (for 10^5 - 10^6 yrs) than the pulsar lifetime and the X-ray PWN
lifetime. In this scenario, the magnetic field in the cavity induced by the
wind of the progenitor star plays a crucial role. This scenario is in line with
the discovery of several unidentified or "dark" sources in the TeV gamma-ray
band without X-ray counterparts; and it is also finding confirmation in the
recent discoveries at GeV gamma rays. Moreover, these consequences could be
also important for reinterpreting the detection of starburst galaxies in the
TeV gamma-ray band when considering a leptonic origin of the gamma-ray signal.
Both theoretical aspects and their observational proofs will be discussed, as
well as the first results of our new modeling code.Comment: Proceedings of the 5th International Symposium on High-Energy
Gamma-Ray Astronomy (Gamma2012
A Cosmic Ray Positron Anisotropy due to Two Middle-Aged, Nearby Pulsars?
Geminga and B0656+14 are the closest pulsars with characteristic ages in the
ran ge of 100 kyr to 1 Myr. They both have spindown powers of the order 3e34
erg/s at present. The winds of these pulsars had most probably powered pulsar
wind nebulae (PWNe) that broke up less than about 100 kyr after the birth of
the pulsars. Assuming that leptonic particles accelerated by the pulsars were
confined in th e PWNe and were released into the interstellar medium (ISM) on
breakup of the PW Ne, we show that, depending on the pulsar parameters, both
pulsars make a non-ne gligible contribution to the local cosmic ray (CR)
positron spectrum, and they m ay be the main contributors above several GeV.
The relatively small angular dist ance between Geminga and B0656+14 thus
implies an anisotropy in the local CR po sitron flux at these energies. We
calculate the contribution of these pulsars to the locally observed CR electr
on and positron spectra depending on the pulsar birth period and the magnitude
o f the local CR diffusion coefficient. We further give an estimate of the
expecte d anisotropy in the local CR positron flux. Our calculations show that
within the framework of our model, the local CR posit ron spectrum imposes
constraints on pulsar parameters for Geminga and B0656+14, notably the pulsar
period at birth, and also the local interstellar diffusion co efficient for CR
leptons.Comment: accepted for publication in ApJ
Spatially resolved XMM-Newton analysis and a model of the nonthermal emission of MSH 15-52
We present an X-ray analysis and a model of the nonthermal emission of the
pulsar wind nebula (PWN) MSH15-52. We analyzed XMM-Newton data to obtain the
spatially resolved spectral parameters around the pulsar PSRB1509-58. A
steepening of the fitted power-law spectra and decrease in the surface
brightness is observed with increasing distance from the pulsar. In the second
part of this paper, we introduce a model for the nonthermal emission, based on
assuming the ideal magnetohydrodynamic limit. This model is used to constrain
the parameters of the termination shock and the bulk velocity of the leptons in
the PWN. Our model is able to reproduce the spatial variation of the X-ray
spectra. The parameter ranges that we found agree well with the parameter
estimates found by other authors with different approaches. In the last part of
this paper, we calculate the inverse Compton emission from our model and
compare it to the emission detected with the H.E.S.S. telescope system. Our
model is able to reproduce the flux level observed with H.E.S.S., but not the
spectral shape of the observed TeV {\gamma}-ray emission.Comment: Accepted for publication in A&A, 9 pages, 15 figure
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
MACHO 311.37557.169: A VY Scl star
Optical surveys, such as the MACHO project, often uncover variable stars
whose classification requires followup observations by other instruments. We
performed X-ray spectroscopy and photometry of the unusual variable star MACHO
311.37557.169 with \XMM\ in April 2018, supplemented by archival X-ray and
optical spectrographic data. The star has a bolometric X-ray luminosity of
about erg s cm and a heavily absorbed
two-temperature plasma spectrum. The shape of its light curve, its overall
brightness, its X-ray spectrum, and the emission lines in its optical spectrum
suggest that it is most likely a VY~Scl cataclysmic variable.Comment: 7 pages, 5 figure
- …