181 research outputs found
Searching for annihilation radiation from SN 1006 with SPI on INTEGRAL
Historical Type Ia supernovae are a leading candidate for the source of
positrons observed through their diffuse annihilation emission in the Galaxy.
However, search for annihilation emission from individual Type Ia supernovae
has not been possible before the improved sensitivity of \integral. The total
511 keV annihilation flux from individual SNe Ia, as well as their contribution
to the overall diffuse emission, depends critically on the escape fraction of
positrons produced in Co decays. Late optical light curves suggest that
this fraction may be as high as 5%. We searched for positron annihilation
radiation from the historical Type Ia supernova SN 1006 using the SPI
instrument on \integral. We did not detect significant 511 keV line emission,
with a 3 flux upper limit of 0.59 x 10 ergs cm^-2 s^-1 for \wsim
1 Msec exposure time, assuming a FWHM of 2.5 keV. This upper limit corresponds
to a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and
rules out the possibility that Type Ia supernovae produce all of the positrons
in the Galaxy (~ 12% escape fraction), if the mean positron lifetime is less
than 10 years. Future observations with \integral will provide stronger
limits on the escape fraction of positrons, the mean positron lifetime, and the
contribution of Type Ia supernovae to the overall positron content of the
Galaxy.Comment: 3 pages, 2 figures, accepted for publication in ApJ
Detection of Low-Hard State Spectral and Timing Signatures from the Black Hole X-Ray Transient XTE J1650-500 at Low X-Ray Luminosities
Using the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer, we
have studied the black hole candidate (BHC) X-ray transient XTE J1650-500 near
the end of its 2001-2002 outburst after its transition to the low-hard state at
X-ray luminosities down to L = 1.5E34 erg/s (1-9 keV, assuming a source
distance of 4 kpc). Our results include a characterization of the spectral and
timing properties. At the lowest sampled luminosity, we used an 18 ks Chandra
observation to measure the power spectrum at low frequencies. For the 3 epochs
at which we obtained Chandra/RXTE observations, the 0.5-20 keV energy spectrum
is consistent with a spectral model consisting of a power-law with interstellar
absorption. We detect evolution in the power-law photon index from 1.66 +/-
0.05 to 1.93 +/- 0.13 (90% confidence errors), indicating that the source
softens at low luminosities. The power spectra are characterized by strong
(20-35% fractional rms) band-limited noise, which we model as a zero-centered
Lorentzian. Including results from an RXTE study of XTE J1650-500 near the
transition to the low-hard state by Kalemci et al. (2003), the half-width of
the zero-centered Lorentzian (roughly where the band-limited noise cuts off)
drops from 4 Hz at L = 7E36 erg/s (1-9 keV, absorbed) to 0.067 +/- 0.007 Hz at
L = 9E34 erg/s to 0.0035 +/- 0.0010 Hz at the lowest luminosity. While the
spectral and timing parameters evolve with luminosity, it is notable that the
general shapes of the energy and power spectra remain the same, indicating that
the source stays in the low-hard state. This implies that the X-ray emitting
region of the system likely keeps the same overall structure, while the
luminosity changes by a factor of 470. We discuss how these results may
constrain theoretical black hole accretion models.Comment: 11 pages, accepted by ApJ after minor revision
The appearance of a compact jet in the soft-intermediate state of 4U 1543-47
Recent advancements in the understanding of jet-disc coupling in black hole
candidate X-ray binaries (BHXBs) have provided close links between radio jet
emission and X-ray spectral and variability behaviour. In 'soft' X-ray states
the jets are suppressed, but the current picture lacks an understanding of the
X-ray features associated with the quenching or recovering of these jets. Here
we show that a brief, ~4 day infrared (IR) brightening during a predominantly
soft X-ray state of the BHXB 4U 1543-47 is contemporaneous with a strong X-ray
Type B quasi-periodic oscillation (QPO), a slight spectral hardening and an
increase in the rms variability, indicating an excursion to the
soft-intermediate state (SIMS). This IR 'flare' has a spectral index consistent
with optically thin synchrotron emission and most likely originates from the
steady, compact jet. This core jet emitting in the IR is usually only
associated with the hard state, and its appearance during the SIMS places the
'jet line' between the SIMS and the soft state in the hardness-intensity
diagram for this source. IR emission is produced in a small region of the jets
close to where they are launched (~ 0.1 light-seconds), and the timescale of
the IR flare in 4U 1543-47 is far too long to be caused by a single, discrete
ejection. We also present a summary of the evolution of the jet and X-ray
spectral/variability properties throughout the whole outburst, constraining the
jet contribution to the X-ray flux during the decay.Comment: Accepted to MNRAS. 11 pages, 6 figure
X-Ray Observations of SN 1006 with Integral
The remnant of the supernova of 1006 AD, the remnant first showing evidence
for the presence of X-ray synchrotron emission from shock-accelerated
electrons, was observed for ~1000 ksec with INTEGRAL for the study of electron
acceleration to very high energies. The aim of the observation was to
characterize the synchrotron emission, and attempt to detect non-thermal
bremsstrahlung, using the combination of IBIS and JEM-X spatial and spectral
coverage. The source was detected with JEM-X between 2.4 and 8.4 keV bands, and
not detected with either ISGRI or SPI above 20 keV. The ISGRI upper limit is
about a factor of four above current model predictions, but confirms the
presence of steepening in the power-law extrapolated from lower energies (< 4
keV).Comment: 5 pages, 3 figures, 1 table, accepted for publication in the
Astrophysical Journa
The infrared/X-ray correlation of GX 339-4: Probing hard X-ray emission in accreting black holes
GX 339-4 has been one of the key sources for unravelling the accretion
ejection coupling in accreting stellar mass black holes. After a long period of
quiescence between 1999 and 2002, GX 339-4 underwent a series of 4 outbursts
that have been intensively observed by many ground based observatories [radio,
infrared(IR), optical] and satellites (X-rays). Here, we present results of
these broad-band observational campaigns, focusing on the optical-IR
(OIR)/X-ray flux correlations over the four outbursts. We found tight OIR/X-ray
correlations over four decades with the presence of a break in the IR/X-ray
correlation in the hard state. This correlation is the same for all four
outbursts. This can be interpreted in a consistent way by considering a
synchrotron self-Compton origin of the X-rays in which the break frequency
varies between the optically thick and thin regime of the jet spectrum. We also
highlight the similarities and differences between optical/X-ray and IR/X-ray
correlations which suggest a jet origin of the near-IR emission in the hard
state while the optical is more likely dominated by the blackbody emission of
the accretion disc in both hard and soft state. However we find a non
negligible contribution of 40 per cent of the jet emission in the V-band during
the hard state.
We finally concentrate on a soft-to-hard state transition during the decay of
the 2004 outburst by comparing the radio, IR, optical and hard X-rays light
curves. It appears that unusual delays between the peak of emission in the
different energy domains may provide some important constraints on jet
formation scenario.Comment: Accepted for publication in MNRAS, 12 pages, 8 figure
Tracing the jet contribution to the mid-IR over the 2005 outburst of GRO J1655-40 via broadband spectral modeling
We present new results from a multi-wavelength (radio/infrared/optical/X-ray)
study of the black hole X-ray binary GRO J1655-40 during its 2005 outburst. We
detected, for the first time, mid-infrared emission at 24 um from the compact
jet of a black hole X-ray binary during its hard state, when the source shows
emission from a radio compact jet as well as a strong non-thermal hard X-ray
component. These detections strongly constrain the optically thick part of the
synchrotron spectrum of the compact jet, which is consistent with being flat
over four orders of magnitude in frequency. Moreover, using this unprecedented
coverage, and especially thanks to the new Spitzer observations, we can test
broadband disk and jet models during the hard state. Two of the hard state
broadband spectra are reasonably well fitted using a jet model with parameters
overall similar to those previously found for Cyg X-1 and GX 339-4. Differences
are also present; most notably, the jet power in GRO J1655-40 appears to be a
factor of at least ~3-5 higher (depending on the distance) than that of Cyg X-1
and GX 339-4 at comparable disk luminosities. Furthermore, a few discrepancies
between the model and the data, previously not found for the other two black
hole systems for which there was no mid-IR/IR and optical coverage, are
evident, and will help to constrain and refine theoretical models.Comment: accepted for publication in Ap
Search for polarization from the prompt gamma-ray emission of GRB 041219a with SPI on INTEGRAL
Measuring the polarization of the prompt gamma-ray emission from GRBs can significantly improve our understanding of both the GRB emission mechanisms, as well as the underlying engine driving the explosion. We searched for polarization in the prompt gamma-ray emission of GRB 041219a with the SPI instrument on INTEGRAL. Using multiple-detector coincidence events in the 100--350 keV energy band, our analysis yields a polarization fraction from this GRB of 99 +- 33 %. Statistically, we cannot claim a polarization detection from this source. Moreover, different event selection criteria lead to even less significant polarization fractions, e.g. lower polarization fractions are obtained when higher energies are included in the analysis. We cannot strongly rule out the possibility that the measured modulation is dominated by instrumental systematics. Therefore, SPI observations of GRB 041219a do not significantly constrain GRB models. However, this measurement demonstrates the capability of SPI to measure polarization, and the techniques developed for this analysis
State transitions in the 2001/2002 outburst of XTE J1650-500
We present a study of the X-ray transient and black hole candidate XTE
J1650-500 during its 2001/2002 outburst. The source made two state transitions
between the hard and soft states, at luminosity levels that differed by a
factor of ~5-10. The first transition, between hard and soft, lasted for ~30
days and showed two parts; one part in which the spectral properties evolve
smoothly away from the hard state and another that we identify as the 'steep
power law state'. The two parts showed different behavior of the Fe K emission
line and QPO frequencies. The second transition, from soft to hard, lasted only
\~15 days and showed no evidence of the presence of the 'steep power law
state'. Comparing observations from the early rise and the decay of the
outburst, we conclude that the source can be in the hard state in a range of
more 10^4 in luminosity. We briefly discuss the state transitions in the
framework of a two-flow model.Comment: 4 pages, 5 figures. To appear in Proc. of the II BeppoSAX Meeting:
"The Restless High-Energy Universe" (Amsterdam, May 5-8, 2003), E.P.J. van
den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers Ed
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