70 research outputs found
Time Dependent Modeling of the Markarian 501 X-ray and TeV Gamma-Ray Data Taken During March and April, 1997
If the high-energy emission from TeV blazars is produced by the Synchrotron
Self-Compton (SSC) mechanism, then simultaneous X-ray and Gamma-ray
observations of these objects are a powerful probe of the electron (and/or
positron) populations responsible for this emission. Understanding the emitting
particle distributions and their evolution in turn allow us to probe physical
conditions in the inner blazar jet and test, for example, various acceleration
scenarios. By constraining the SSC emission model parameters, such observations
also allow us to predict the intrinsic (unabsorbed) Gamma-ray spectra of these
sources, a major uncertainty in current attempts to use the observed Gamma-ray
spectra to constrain the intensity of the extragalactic background at
optical/infrared wavelengths. As a next step in testing the SSC model and as a
demonstration of the potential power of coordinated X-ray and Gamma-ray
observations, we attempt to model in detail the X-ray and Gamma-ray light
curves of the TeV Blazar Mrk 501 during its April-May 1997 outburst using a
time dependent SSC emission model. Extensive, quasi-simultaneous X-ray and
gamma-ray coverage exists for this period. We discuss and explore
quantitatively several of the flare scenarios presented in the literature. We
show that simple two-component models (with a soft, steady X-ray component plus
a variable SSC component) involving substantial pre-acceleration of electrons
to Lorentz factors on the order of 1E+5 describe the data train surprisingly
well. All considered models imply an emission region that is strongly out of
equipartition and low radiative efficiencies (ratio between kinetic jet
luminosity and comoving radiative luminosity) of 1 per-mill and less.Comment: 16 pages, Refereed Manuscript. Minor changes to previous versio
Jet-Intracluster Medium Interactions of the Head Tail Radio Galaxy 3C 129
The 50 ksec XMM observations of the galaxy cluster 3C 129 were taken as scheduled, and the data are of good quality. We analyzed the data in the following way. After standard cleaning, we flat-fielded the XMM surface brightness maps. Combining the data from the EPIC MOS and PN Camera CCDs, we performed a cross-correlation analysis of the X-ray surface brightness distribution with the 1.4 GHz VLA radio map. We found evidence for cavities in the X-ray emitting Intra-Cluster Medium (ICM) associated with the radio tail of the head-tail radio galaxy 3C 129. This discovery is very interesting as it excludes the presence of a large fraction of thermal plasma in the radio tail. Together with the observation of an apparent pressure mismatch between the radio plasma and the ICM, and an upper limit on the magnetic field inside the radio tail (from the radio spectral indices map) the observation implies that the tail pressure is dominated either by low-energy electrons/positrons, or, by relativistic protons. Furthermore, we studied the energy spectrum of an X-ray "hot-spot" associated with the head of the radio galaxy 3C 129. It seems likely that the X-ray hot-spot originates from shocked gas in front of the radio galaxy. , The analysis turned out to be much more difficult than anticipated. The main reason is the lack of a comprehensive, publicly available background model that is key for the analysis of extended sources. Small groups like our do not have the man-power to come up with a background model themselves. We used the model from Read & Ponman (A&A 409, 395, 2003). However, the background subtracted X-ray surface brightness maps show a bright ring in the outer 20% of the camera. We tried to get rid of this ring and contacted the XMM helpdesk and Read & Ponman, the authors of the background paper. However, up to this day, we did not entirely succeed to remove the brightness enhancement at the outer parts of the camera. Unfortunately, our results are somewhat sensitive to the uncertainty, as the radio galaxy 3C 129 is very large and occupies a rather large fraction of the XMM field of view. We are now working on a paper describing the results. The paper will include a detailed discussion of the uncertainties associated with the non- perfect background subtraction
A gamma-ray burst remnant in our Galaxy: HESS J1303-631
We present the results of our investigation of the multiwavelength data on
HESS J1303-631, an unidentified TeV source serendipitously discovered in the
Galactic plane by the HESS collaboration. These results strongly suggest the
identification of this particular source as the remnant of a Gamma-Ray Burst
(GRB) that happened some few tens of thousands years ago in our Galaxy at a
distance on the order of 10 kpc from us. We show, through detailed calculations
of particle diffusion, interaction and radiation processes of relativistic
particles in the interstellar medium, that it is possible for a GRB remnant
(GRBR) to be a strong TeV emitter with no observable synchrotron emission. We
predict spectral and spatial signatures that would unambiguously distinguish
GRBRs from ordinary supernova remnants, including: (1) large energy budgets
inferred from their TeV emission, but at the same time, (2) suppressed fluxes
in the radio through GeV wavebands; (3) extended center-filled emission with an
energy-dependent spatial profile; and (4) a possible elongation in the
direction of the past pair of GRB jets. While GRBRs can best be detected by
ground-based gamma-ray detectors, the future GLAST mission will play a crucial
role in confirming the predicted low level of GeV emission.Comment: Replaced by the version accepted in ApJ Letters (to appear in
April/May 2006); 4 pages, 3 figure
X-ray polarimetry with the Polarization Spectroscopic Telescope Array (PolSTAR)
This paper describes the Polarization Spectroscopic Telescope Array (PolSTAR), a mission proposed to NASAâs 2014 Small Explorer (SMEX) announcement of opportunity. PolSTAR measures the linear polarization of 3â50 keV (requirement; goal: 2.5â70 keV) X-rays probing the behavior of matter, radiation and the very fabric of spacetime under the extreme conditions close to the event horizons of black holes, as well as in and around magnetars and neutron stars. The PolSTAR design is based on the technology developed for the Nuclear Spectroscopic Telescope Array (NuSTAR) mission launched in June 2012. In particular, it uses the same X-ray optics, extendable telescope boom, optical bench, and CdZnTe detectors as NuSTAR. The mission has the sensitivity to measure âŒ1% linear polarization fractions for X-ray sources with fluxes down to âŒ5 mCrab. This paper describes the PolSTAR design as well as the science drivers and the potential science return
Observations of 4U 1626-67 with the Imaging X-ray Polarimetry Explorer
We present measurements of the polarization of X-rays in the 2-8 keV band
from the pulsar in the ultracompact low mass X-ray binary 4U1626-67 using data
from the Imaging X-ray Polarimetry Explorer (IXPE). The 7.66 s pulsations were
clearly detected throughout the IXPE observations as well as in the NICER soft
X-ray observations, which we use as the basis for our timing analysis and to
constrain the spectral shape over 0.4-10 keV energy band. Chandra HETGS
high-resolution X-ray spectra were also obtained near the times of the IXPE
observations for firm spectral modeling. We find an upper limit on the
pulse-averaged linear polarization of <4% (at 95% confidence). Similarly, there
was no significant detection of polarized flux in pulse phase intervals when
subdividing the bandpass by energy. However, spectropolarimetric modeling over
the full bandpass in pulse phase intervals provide a marginal detection of
polarization of the power-law spectral component at the 4.8 +/- 2.3% level (90%
confidence). We discuss the implications concerning the accretion geometry onto
the pulsar, favoring two-component models of the pulsed emission.Comment: 19 pages, 7 figures, 7 tables; accepted for publication in the
Astrophysical Journa
X-ray polarimetry reveals the magnetic field topology on sub-parsec scales in Tycho's supernova remnant
Supernova remnants are commonly considered to produce most of the Galactic
cosmic rays via diffusive shock acceleration. However, many questions about the
physical conditions at shock fronts, such as the magnetic-field morphology
close to the particle acceleration sites, remain open. Here we report the
detection of a localized polarization signal from some synchrotron X-ray
emitting regions of Tycho's supernova remnant made by the Imaging X-ray
Polarimetry Explorer. The derived polarization degree of the X-ray synchrotron
emission is 9+/-2% averaged over the whole remnant, and 12+/-2% at the rim,
higher than the 7-8% polarization value observed in the radio band. In the west
region the polarization degree is 23+/-4%. The X-ray polarization degree in
Tycho is higher than for Cassiopeia A, suggesting a more ordered magnetic-field
or a larger maximum turbulence scale. The measured tangential polarization
direction corresponds to a radial magnetic field, and is consistent with that
observed in the radio band. These results are compatible with the expectation
of turbulence produced by an anisotropic cascade of a radial magnetic-field
near the shock, where we derive a magnetic-field amplification factor of
3.4+/-0.3. The fact that this value is significantly smaller than those
expected from acceleration models is indicative of highly anisotropic
magnetic-field turbulence, or that the emitting electrons either favor regions
of lower turbulence, or accumulate close to where the magnetic-field
orientation is preferentially radially oriented due to hydrodynamical
instabilities.Comment: 31 pages, 7 figures, 3 tables. Accepted for publication in ApJ.
Revised versio
IXPE and XMM-Newton observations of the Soft Gamma Repeater SGR 1806-20
Recent observations with the Imaging X-ray Polarimetry Explorer (IXPE) of two
anomalous X-ray pulsars provided evidence that X-ray emission from magnetar
sources is strongly polarized. Here we report on the joint IXPE and XMM-Newton
observations of the soft {\gamma}-repeater SGR 1806-20. The spectral and timing
properties of SGR 1806-20 derived from XMM-Newton data are in broad agreement
with previous measurements; however, we found the source at an all-time-low
persistent flux level. No significant polarization was measured apart from the
4-5 keV energy range, where a probable detection with PD=31.6\pm 10.5% and
PA=-17.6\pm 15 deg was obtained. The resulting polarization signal, together
with the upper limits we derive at lower and higher energies 2-4 and 5-8 keV,
respectively) is compatible with a picture in which thermal radiation from the
condensed star surface is reprocessed by resonant Compton scattering in the
magnetosphere, similar to what proposed for the bright magnetar 4U 0142+61.Comment: 11 pages, 3 figures, accepted for publication in Ap
IXPE Observations of the Quintessential Wind-accreting X-Ray Pulsar Vela X-1
The radiation from accreting X-ray pulsars was expected to be highly polarized, with some estimates for the polarization degree of up to 80%. However, phase-resolved and energy-resolved polarimetry of X-ray pulsars is required in order to test different models and to shed light on the emission processes and the geometry of the emission region. Here we present the first results of the observations of the accreting X-ray pulsar Vela X-1 performed with the Imaging X-ray Polarimetry Explorer. Vela X-1 is considered to be the archetypal example of a wind-accreting, high-mass X-ray binary system, consisting of a highly magnetized neutron star accreting matter from its supergiant stellar companion. The spectropolarimetric analysis of the phase-averaged data for Vela X-1 reveals a polarization degree (PD) of 2.3% ± 0.4% at the polarization angle (PA) of â47.°3 ± 5.°4. A low PD is consistent with the results obtained for other X-ray pulsars and is likely related to the inverse temperature structure of the neutron star atmosphere. The energy-resolved analysis shows the PD above 5 keV reaching 6%â10% and a âŒ90° difference in the PA compared to the data in the 2â3 keV range. The phase-resolved spectropolarimetric analysis finds a PD in the range 0%â9% with the PA varying between â80° and 40°
Magnetic structures and turbulence in SN 1006 revealed with imaging X-ray polarimetry
Young supernova remnants (SNRs) strongly modify surrounding magnetic fields,
which in turn play an essential role in accelerating cosmic rays (CRs). X-ray
polarization measurements probe magnetic field morphology and turbulence at the
immediate acceleration site. We report the X-ray polarization distribution in
the northeastern shell of SN1006 from a 1 Ms observation with the Imaging X-ray
Polarimetry Explorer (IXPE). We found an average polarization degree of
and an average polarization angle of
(measured on the plane of the sky from north to east). The X-ray polarization
angle distribution reveals that the magnetic fields immediately behind the
shock in the northeastern shell of SN 1006 are nearly parallel to the shock
normal or radially distributed, similar to that in the radio observations, and
consistent with the quasi-parallel CR acceleration scenario. The X-ray emission
is marginally more polarized than that in the radio band. The X-ray
polarization degree of SN 1006 is much larger than that in Cas A and Tycho,
together with the relatively tenuous and smooth ambient medium of the remnant,
favoring that CR-induced instabilities set the turbulence in SN 1006 and CR
acceleration is environment-dependent.Comment: 15 pages, 4 Figures, 2 Tables; accepted for publication in The
Astrophysical Journa
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