173 research outputs found
Chandra and XMM-Newton observations of the exceptional pulsar PSR B0628-28
PSR B0628-28 is a radio pulsar which was first detected in the X-ray band by
ROSAT and then later observed with Chandra and XMM-Newton. The Chandra
observation yielded an X-ray luminosity two orders of magnitude higher than
what is expected for spin-powered pulsars, also there were no pulsations
detected. The XMM-Newton observation, however, reveals pulsations at the
expected radio period, P=1.244 s. The simultaneously analyzed spectra also
gives a luminosity (in cgs) Log Lx=30.34, which is ~350 times greater than what
would be expected from the correlation between Lx-Edot.Comment: 13 pages, 4 figures, to be published in ApJ
The Reanalysis of the ROSAT Data of GQ Mus (1983) Using White Dwarf Atmosphere Emission Models
The analyses of X-ray emission from classical novae during the outburst stage
have shown that the soft X-ray emission below 1 keV, which is thought to
originate from the photosphere of the white dwarf, is inconsistent with the
simple blackbody model of emission. Thus, Position Sensitive
Proportional Counter (PSPC) archival data of the classical novae GQ Mus 1983
(GQ Mus) have been reanalyzed in order to understand the spectral development
in the X-ray wavelengths during the outburst stage. The X-ray spectra are
fitted with the hot white dwarf atmosphere emission models developed for the
remnants of classical novae near the Eddington luminosity. The post-outburst
X-ray spectra of the remnant white dwarf is examined in the context of
evolution on the Hertzsprung-Russell diagram using C-O enhanced atmosphere
models. The data obtained in 1991 August (during the ROSAT All Sky Survey)
indicate that the effective temperature is kT_e<54 eV (<6.2x10^5 K). The 1992
February data show that the white dwarf had reached an effective temperature in
the range 38.3-43.3 eV (4.4-5.1x10^5 K) with an unabsorbed X-ray flux (i.e.,
bolometric flux) between 2.5x10^-9 and 2.3x10^-10 erg s^-1 cm^-2. We
show that the H burning at the surface of the WD had most likely ceased at the
time of the X-ray observations. Only the 1991 August data show evidence for
ongoing H burning.Comment: 17 pages and 3 figures. Accepted to be published in MNRA
The X-ray emission from Nova V382 Velorum: I. The hard component observed with BeppoSAX
We present BeppoSAX observations of Nova Velorum 1999 (V382 Vel), done in a
broad X-ray band covering 0.1-300 keV only 15 days after the discovery and
again after 6 months. The nova was detected at day 15 with the BeppoSAX
instruments in the energy range 1.8-10 keV and we attribute the emission to
shocks in the ejecta. The plasma temperature was kT~6 keV and the unabsorbed
flux was F(x)~4.3 x 10(-11) erg/cm**2/s. The nebular material was affected by
high intrinsic absorption of the ejecta. 6 months after after the outburst, the
intrinsic absorption did not play a role, the nova had turned into a bright
supersoft source, and the hot nebular component previously detected had cooled
to a plasma temperature kT<=1 keV. No emission was detected in either
observation above 20 keV.Comment: 1 tex file, 2 figures as .ps, and 1 .sty file of MNRA
Multicolor Photometry of the Vela Pulsar
Multicolor photometry of the Vela pulsar (PSR B0833-45), updated by recent
HST/WFPC2 observations obtained in the 555W, 675W and 814W filters, is
presented. The available data provide the best characterization so far of the
pulsar spectral shape, which is dominated by a flat power law continuum with
spectral index \alpha = -0.2 +/- 0.2, consistent with the extrapolation in the
optical domain of the power law component of the X-ray spectrum detected by
Chandra. In addition, a marginally significant dip (~ 3 sigma) seems to be
present at about 6500 AA. Spectroscopic observations with the VLT, now in
progress, will undoubtly provide a much better assessment of the reality and
characteristics of this feature.Comment: 4 pages, LaTex, 2 Postscript figures. Astronomy & Astrophysics,
accepte
The ASCA Spectrum of the Vela Pulsar Jet
ROSAT observations of the Vela pulsar and its surroundings revealed a
collimated X-ray feature almost 45' in length (Markwardt & Ogelman 1995),
interpreted as the signature ``cocoon'' of a one-sided jet from the Vela
pulsar. We report on a new ASCA observation of the Vela pulsar jet at its head,
the point where the jet is believed to interact with the supernova remnant. The
head is clearly detected, and its X-ray spectrum is remarkably similar to the
surrounding supernova remnant spectrum, extending to X-ray energies of at least
7 keV. A ROSAT+ASCA spectrum can be fit by two-component emission models but
not standard one-component models. The lower energy component is thermal and
has a temperature of 0.29+/-0.03 keV (1 sigma); the higher energy component can
be fit by either a thermal component of temperature ~4 keV or a power law with
photon index ~2.0. Compared to the ROSAT-only results, the mechanical
properties of the jet and its cocoon do not change much. If the observed
spectrum is that of a hot jet cocoon, then the speed of the jet is at least 800
km s^-1, depending on the angle of inclination. The mechanical power driving
the jet is >10^36 erg s^-1, and the mass flow rate at the head is > 10^-6 M_sun
yr^-1. We conclude that the jet must be entraining material all along its
length in order to generate such a large mass flow rate. We also explore the
possibility that the cocoon emission is synchrotron radiation instead of
thermal.Comment: 12 pages, LaTeX in AAS v4.0 preprint style, two PS figures, accepted
for publication in the ApJ Letter
Timing, glitches and braking index of PSR B0540-69
We report a pulse-time history of PSR B054069 based on the analysis of an
extended Data set including ASCA, BeppoSAX and RXTE observations spanning a
time interval of about 8 years. This interval includes also the epoch of the
glitch episode reported by Zhang et al. (2001). Our analysis shows the presence
of a relevant timing noise and does not give a clear evidence of the glitch
occurrence. We performed an accurate evaluation of the main timing parameters,
, and and derived a mean braking index of
quite different from the lower value found by Zhang et al.
(2001), but in rather good agreement with other several values reported in the
literature.Comment: 9 pages 5 figures, accepted by A&A, main journa
X-ray observations of classical novae. Theoretical implications
Detection of X-rays from classical novae, both in outburst and post-outburst,
provides unique and crucial information about the explosion mechanism. Soft
X-rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear
burning is still on and expanding envelope is transparent enough, whereas
harder X-rays give information about the ejecta and/or the accretion flow in
the reborn cataclysmic variable. The duration of the supersoft X-ray emission
phase is related to the turn-off of the classical nova, i.e., of the H-burning
on top of the white dwarf core. A review of X-ray observations is presented,
with a special emphasis on the implications for the duration of post-outburst
steady H-burning and its theoretical explanation. The particular case of
recurrent novae (both the "standard" objects and the recently discovered ones)
is also reviewed, in terms of theoretical feasibility of short recurrence
periods, as well as regarding implications for scenarios of type Ia supernovae.Comment: 6 pages, review paper accepted for publication in Astronomische
Nachrichte
Cosmic Gamma-Ray Background from Star-Forming Galaxies
The origin of the extragalactic gamma-ray background is a pressing
cosmological mystery. The Fermi Gamma-Ray Space Telescope has recently measured
the intensity and spectrum of this background; both are substantially different
from previous measurements. We present a novel calculation of the gamma-ray
background from normal star-forming galaxies. Contrary to long-standing
expectations, we find that numerous but individually faint normal galaxies may
comprise the bulk of the Fermi signal, rather than rare but intrinsically
bright active galaxies. This result has wide-ranging implications, including:
the possibility to probe the cosmic star formation history with gamma rays; the
ability to infer the cosmological evolution of cosmic rays and galactic
magnetic fields; and an increased likelihood of identifying subdominant
components from rare sources (e.g., dark matter clumps) through their large
anisotropy.Comment: 5 pages, 1 figure. Matches version published in ApJL. Sharper focus
on normal (vs starburst) galaxies. Basic conclusions unchange
BeppoSAX observations of the three Gamma-ray pulsars PSR B0656+14, PSR B1055-52 and PSR B1706-44
We report the results of the observations of the three gamma-ray pulsars PSR
B0656+14, PSR B1055-52 and PSR B1706-44 performed with BeppoSAX. We detected a
pulsed emission only for PSR B1055-52: in the range 0.1-6.5 keV the pulse
profile is sinusoidal and the statistical significance is 4.5 sigma. The pulsed
fraction was estimated 0.64+/-0.17. This pulsation was detected also at
energies greater than 2.5 keV suggesting either a non-thermal origin or a quite
high temperature region on the neutron star surface. Spectral analysis showed
that only the X-ray spectrum of PSR B1706-44 can be fitted by a single
power-law component, while that of PSR B1055-52 requires also a blackbody
component (kT = 0.075 keV) and that of PSR B0656+14 two blackbody components
(kT_1 = 0.059, kT_2 = 0.12 keV).Comment: 8 pages, 5 figures. Accepted for publication in A&
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