1,095 research outputs found
The spectacular X-ray echo of a magnetar burst
The Anomalous X-ray Pulsar (AXP) 1E 1547.0-5408 reactivated in 2009 January
with the emission of dozens of short bursts. Follow-up observations with
Swift/XRT and XMM-Newton showed the presence of multiple expanding rings around
the position of the AXP. These rings are due to scattering, by different layers
of interstellar dust, of a very high fluence burst emitted by 1E 1547.0-5408 on
2009 January 22. Thanks to the exceptional brightness of the X-ray rings, we
could carry out a detailed study of their spatial and spectral time evolution
until 2009 February 4. This analysis gives the possibility to estimate the
distance of 1E 1547.0-5408. We also derived constraints on the properties of
the dust and of the burst responsible for this rare phenomenon.Comment: Proceedings of the conference X-Ray Astronomy 2009, Present Status,
multiwavelength approach and future perspectives, September 7 - 11, 2009,
Bologna, Ital
Three new X-ray emitting sdO stars discovered with Chandra
X-ray observations of sdO stars are a useful tool to investigate their
properties, but so far only two sdO stars were detected at X-rays. We observed
a complete flux-limited sample of 19 sdO stars with the Chandra HRC-I camera to
measure the count rate of the detected sources or to set a tight upper limit on
it for the undetected sources. We obtained a robust detection of BD+37 1977 and
Feige 34 and a marginal detection of BD+28 4211. The estimated luminosity of
BD+37 1977 is above 10^31 erg/s, which is high enough to suggest the possible
presence of an accreting compact companion. This possibility is unlikely for
all the other targets (both detected and undetected), since in their case L_X <
10^30 erg/s. On the other hand, for all 19 targets the estimated value of L_X
(or its upper limit) implies an X-ray/bolometric flux ratio that agrees with
log(L_X/L_bol) = -6.7 +/- 0.5, which is the range of values typical of
main-sequence and giant O stars. Therefore, for Feige 34 and BD+28 4211 the
observed X-ray flux is most probably due to intrinsic emission. The same is
possibile for the 16 undetected stars.Comment: 6 pages. Accepted for publication by Astronomy and Astrophysic
Exploring the role of X-ray reprocessing and irradiation in the anomalous bright optical outbursts of A0538-66
In 1981, the Be/X-ray binary (Be/XRB) A0538-66 showed outbursts characterized
by high peak luminosities in the X-ray and optical bands. The optical outbursts
were qualitatively explained as X-ray reprocessing in a gas cloud surrounding
the binary system. Since then, further important information about A0538-66
have been obtained, and sophisticated photoionization codes have been developed
to calculate the radiation emerging from a gas nebula illuminated by a central
X-ray source. In the light of the new information and tools available, we
studied again the enhanced optical emission displayed by A0538-66 to understand
the mechanisms responsible for these unique events among the class of Be/XRBs.
We performed about 10^5 simulations of a gas envelope photoionized by an X-ray
source. We assumed for the shape of the gas cloud either a sphere or a
circumstellar disc observed edge-on. We studied the effects of varying the main
properties of the envelope and the influence of different input X-ray spectra
on the optical/UV emission emerging from the photoionized cloud. We compared
the computed spectra with the IUE spectrum and photometric UBV measurements
obtained during the outburst of 29 April 1981. We also explored the role played
by the X-ray heating of the surface of the donor star irradiated by the X-ray
emission of the neutron star (NS). We found that reprocessing in a spherical
cloud with a shallow radial density distribution can reproduce the optical/UV
emission. To our knowledge, this configuration has never been observed either
in A0538-66 during other epochs or in other Be/XRBs. We found, contrary to the
case of most other Be/XRBs, that the optical/UV radiation produced by the X-ray
heating of the surface of the donor star irradiated by the NS is
non-negligible, due to the particular orbital parameters of this system that
bring the NS very close to its companion.Comment: Accepted for publication in Astronomy & Astrophysics. Abstract
abridged to meet arXiv requirement
Follow-up observations of X-ray emitting hot subdwarf star: the He-rich sdO BD +37{\deg} 1977
We report on the results of the first XMM-Newton satellite observation of the
luminous and helium-rich O-type subdwarf BD +37{\deg} 1977 carried out in April
2014. X-ray emission is detected with a flux of about 4*10^(-14) erg/cm2/s
(0.2-1.5 keV), corresponding to a f_X/f_bol ratio about 10^(-7); the source
spectrum is very soft, and is well fit by the sum of two plasma components at
different temperatures. Both characteristics are in agreement with what is
observed in the main-sequence early-type stars, where the observed X-ray
emission is due to turbulence and shocks in the stellar wind. A smaller but
still significant stellar wind has been observed also in BD +37{\deg} 1977;
therefore, we suggest that also in this case the detected X-ray flux has the
same origin.Comment: 6 pages. Accepted for publication by Astronomy and Astrophysic
Spectral properties of the soft excess pulsar RX J0059.2-7138 during its 2013 outburst
We report on an X-ray observation of the Be X-ray Binary Pulsar RX
J0059.2-7138, performed by XMM-Newton in March 2014. The 19 ks long observation
was carried out about three months after the discovery of the latest outburst
from this Small Magellanic Cloud transient, when the source luminosity was Lx ~
10 erg/s. A spin period of P=2.762383(5) s was derived, corresponding to
an average spin-up of
s from the only previous period measurement, obtained more than 20
years earlier. The time-averaged continuum spectrum (0.2-12 keV) consisted of a
hard power-law (photon index ~0.44) with an exponential cut-off at a
phase-dependent energy (20-50 keV) plus a significant soft excess below about
0.5 keV. In addition, several features were observed in the spectrum: an
emission line at 6.6 keV from highly ionized iron, a broad feature at 0.9-1 keV
likely due to a blend of Fe L-shell lines, and narrow emission and absorption
lines consistent with transitions in highly ionized oxygen, nitrogen and iron
visible in the high resolution RGS data (0.4-2.1 keV). Given the different
ionization stages of the narrow line components, indicative of photoionization
from the luminous X-ray pulsar, we argue that the soft excess in RX
J0059.2-7138 is produced by reprocessing of the pulsar emission in the inner
regions of the accretion disc.Comment: Accepted for publication in Mon. Not. R. Astron. Soc. 9 pages, 5
figure
The Broad-Band Spectrum and Infrared Variability of the Magnetar AXP 1E1048.1-5937
We present photometry of the Anomalous X-ray pulsar 1E1048.1-5937 in the
infrared and optical, taken at Magellan and the VLT. The object is detected in
the I, J and Ks bands under excellent conditions. We find that the source has
varied greatly in its infrared brightness and present these new magnitudes. No
correlation is found between the infrared flux and spin-down rate, but the
infrared flux and X-ray flux may be anti-correlated. Assuming nominal reddening
values, the resultant spectral energy distribution is found to be inconsistent
with the only other AXP SED available (for 4U0142+61). We consider the effect
of the uncertainty in the reddening to the source on its SED. We find that
although both the X-ray and infrared fluxes have varied greatly for this
source, the most recent flux ratio is remarkably consistent with what is is
found for other AXPs. Finally, we discuss the implications of our findings in
the context of the magnetar model.Comment: 21 pages, 5 eps figures. Submitted to Ap
- …