21 research outputs found
Hard X-ray emission of the microquasar GRO J1655-40 during the rise of its 2005 outburst
We present the analysis of the high energy emission of the Galactic black
hole GRO J1655-40 at the beginning of its 2005 outburst. The data from 458 ks
of INTEGRAL observations, spread over 4 weeks, are analyzed, along with the
existing simultaneous RXTE and Swift data. The high energy data allow us to
detect the presence of a high energy cut-off and to study its evolution during
the outburst rise. This high energy feature is generally related to thermal
mechanisms in the framework of Comptonization models from which we can estimate
the plasma parameters. We found an electron temperature of about 30-40 keV and
an optical depth around 1.8-2.1. The high energy cut-off decreased along with
the radio flux, and disappeared as the jet turned off.Comment: 24 pages, 5 figures, accepted for publication in the Astrophsical
Journa
IGR J08408--4503: a new recurrent Supergiant Fast X-ray Transient
The supergiant fast X-ray transient IGR J08408-4503 was discovered by
INTEGRAL on May 15, 2006, during a bright flare. The source shows sporadic
recurrent short bright flares, reaching a peak luminosity of 10^36 erg s^-1
within less than one hour. The companion star is HD 74194, an Ob5Ib(f)
supergiant star located at 3 kpc in the Vela region. We report the light curves
and broad-band spectra (0.1-200 keV) of all the three flares of IGR J08408-4503
detected up to now based on INTEGRAL and Swift data. The flare spectra are well
described by a power-law model with a high energy cut-off at ~15 keV. The
absorption column density during the flares was found to be ~10^21 cm^-2,
indicating a very low matter density around the compact object. Using the
supergiant donor star parameters, the wind accretion conditions imply an
orbital period of the order of one year, a spin period of the order of hours
and a magnetic field of the order of 10^13 G.Comment: 5 pages, 2 figures, accepted for publication in Astrophysical Journal
Letter
The 2008 October Swift detection of X-ray bursts/outburst from the transient SGR-like AXP 1E 1547.0-5408
We report on the detailed study of the 2008 October outburst from the
anomalous X-ray pulsar (AXP) 1E 1547.0-5408 discovered through the Swift/Burst
Alert Telescope (BAT) detection of SGR-like short X-ray bursts on 2008 October
3. The Swift/X-ray Telescope (XRT) started observing the source after less than
100 s since the BAT trigger, when the flux (about 6E-11 erg/cm^2/s in the 2-10
keV range) was >50 times higher than its quiescent level. Swift monitored the
outbursting activity of 1E 1547.0-5408 on a daily basis for approximately three
weeks. This strategy allowed us to find a phase-coherent solution for the
source pulsations after the burst, which, besides period and period derivative,
requires a positive Period second derivative term (spin-down increase). The
time evolution of the pulse shape is complex and variable, with the pulsed
fraction increasing from 20% to 50% within the Swift observational window. The
XRT spectra can be fitted well by means of a single component, either a
power-law (PL) or a blackbody (BB). During the very initial phases of the
outburst the spectrum is hard, with a PL photon index about 2 (or kT about 1.4
keV) which steepens to about 4 (or kT about 0.8 keV) within one day from the
BAT trigger, though the two components are likely present simultaneously during
the first day spectra. An INTEGRAL observation carried out five days after the
trigger provided an upper limit of about 2E-11 erg/cm^2/s to the emission of 1E
1547.0-5408 in the 18-60 keV band.Comment: 10 pages, 2 tables, 6 figures; accepted for publication in MNRA
Detectability of gamma-ray emission from classical novae with Swift/BAT
Classical novae are expected to emit gamma rays during their explosions. The
most important contribution to the early gamma-ray emission comes from the
annihilation with electrons of the positrons generated by the decay of 13N and
18F. The photons are expected to be down-scattered to a few tens of keV, and
the emission is predicted to occur some days before the visual discovery and to
last ~2 days. Despite a number of attempts, no positive detections of such
emission have been made, due to lack of sensitivity and of sky coverage.
Because of its huge field of view, good sensitivity, and well-adapted energy
band, Swift/BAT offers a new opportunity for such searches. BAT data can be
retrospectively used to search for prompt gamma-ray emission from the direction
of novae after their optical discovery. We have estimated the expected success
rate for the detection with BAT of gamma rays from classical novae using a
Monte Carlo approach. Searches were performed for emission from novae occurring
since the launch of Swift. Using the actual observing program during the first
2.3 years of BAT operations as an example, and sensitivity achieved, we
estimate the expected rate of detection of classical novae with BAT as
~0.2-0.5/yr, implying that several should be seen within a 10 yr mission. The
search for emission in the directions of the 24 classical novae discovered
since the Swift launch yielded no positive results, but none of these was known
to be close enough for this to be a surprise. Detections of a recurrent nova
(RS Oph) and a nearby dwarf nova (V455 And) demonstrate the efficacy of the
technique. The absence of detections is consistent with the expectations from
the Monte Carlo simulations, but the long-term prospects are encouraging given
an anticipated Swift operating lifetime of ~10 years.Comment: 10 pages, 6 figures. Accepted for publication in A&
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind
RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts
have indicated that high velocity ejecta interact with a pre-existing red giant
wind, setting up shock systems analogous to those seen in Supernova Remnants.
However, in the previous outburst in 1985, X-ray observations did not commence
until 55 days after the initial explosion. Here we report on Swift observations
covering the first month of the 2006 outburst with the Burst Alert (BAT) and
X-ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25
keV band from t=0 to days. XRT observationsfrom 0.3-10 keV, started at
3.17 days after outburst. The rapidly evolving XRT spectra clearly show the
presence of both line and continuum emission which can be fitted by thermal
emission from hot gas whose characteristic temperature, overlying absorbing
column, , and resulting unabsorbed total flux decline monotonically
after the first few days. Derived shock velocities are in good agreement with
those found from observations at other wavelengths. Similarly, is in
accord with that expected from the red giant wind ahead of the forward shock.
We confirm the basic models of the 1985 outburst and conclude that standard
Phase I remnant evolution terminated by days and the remnant then
rapidly evolved to display behaviour characteristic of Phase III. Around t=26
days however, a new, luminous and highly variable soft X-ray source began to
appear whose origin will be explored in a subsequent paper.Comment: 20 pages, 4 figures (2 updated), accepted by Ap
Multiwavelength observations of 3C 454.3. I. The AGILE 2007 November campaign on the "Crazy Diamond"
[Abridged] We report on a multiwavelength observation of the blazar 3C 454.3
(which we dubbed "crazy diamond") carried out on November 2007 by means of the
astrophysical satellites AGILE, INTEGRAL, Swift, the WEBT Consortium, and the
optical-NIR telescope REM. 3C 454.3 is detected at a level
during the 3-week observing period, with an average flux above 100 MeV of
\phcmsec. The gamma-ray
spectrum can be fit with a single power-law with photon index between 100 MeV and 1 GeV. We detect significant
day-by-day variability of the gamma-ray emission during our observations, and
we can exclude that the fluxes are constant at the 99.6% ()
level. The source was detected typically around 40 degrees off-axis, and it was
substantially off--axis in the field of view of the AGILE hard X-ray imager.
However, a 5-day long ToO observation by INTEGRAL detected 3C 454.3 at an
average flux of about \phcmsec with
an average photon index of between 20--200
keV. Swift also detected 3C 454.3 with a flux in the 0.3--10 keV energy band in
the range \phcmsec{} and a photon index in the
range . In the optical band, both WEBT and REM
show an extremely variable behavior in the band. A correlation analysis
based on the entire data set is consistent with no time-lags between the
gamma-ray and the optical flux variations. Our simultaneous multifrequency
observations strongly indicate that the dominant emission mechanism between 30
MeV and 30 GeV is dominated by inverse Compton scattering of relativistic
electrons in the jet on the external photons from the broad line region.Comment: Accepted for publication in ApJ. Abridged Abstract. 37 pages, 14
Figures, 3 Table
Novae Ejecta as Colliding Shells
Following on our initial absorption-line analysis of fifteen novae spectra we
present additional evidence for the existence of two distinct components of
novae ejecta having different origins. As argued in Paper I one component is
the rapidly expanding gas ejected from the outer layers of the white dwarf by
the outburst. The second component is pre-existing outer, more slowly expanding
circumbinary gas that represents ejecta from the secondary star or accretion
disk. We present measurements of the emission-line widths that show them to be
significantly narrower than the broad P Cygni profiles that immediately precede
them. The emission profiles of novae in the nebular phase are distinctly
rectangular, i.e., strongly suggestive of emission from a relatively thin,
roughly spherical shell. We thus interpret novae spectral evolution in terms of
the collision between the two components of ejecta, which converts the early
absorption spectrum to an emission-line spectrum within weeks of the outburst.
The narrow emission widths require the outer circumbinary gas to be much more
massive than the white dwarf ejecta, thereby slowing the latter's expansion
upon collision. The presence of a large reservoir of circumbinary gas at the
time of outburst is suggestive that novae outbursts may sometime be triggered
by collapse of gas onto the white dwarf, as occurs for dwarf novae, rather than
steady mass transfer through the inner Lagrangian point.Comment: 12 pages, 3 figures; Revised manuscript; Accepted for publication in
Astrophysics & Space Scienc
The 2008 May burst activation of SGR 1627-41
In May 2008 the soft gamma-ray repeater SGR 1627-41 resumed its bursting
activity after nearly a decade of quiescence. After detection of a bright
burst, Swift pointed its X-ray telescope in the direction of the source in less
than five hours and followed it for over five weeks. In this paper we present
an analysis of the data from these Swift observations and an XMM-Newton one
performed when SGR 1627-41 was still in a quiescent state. The analysis of the
bursts detected with Swift/BAT shows that their temporal and spectral
properties are similar to those found in previous observations of SGR 1627-41
and other soft gamma-ray repeaters. The maximum peak luminosity of the bursts
was about 2E+41 erg/s. Our data show that the outburst was accompanied by a
fast flux enhancement and by a hardening of the spectrum with respect to the
persistent emission.Comment: 5 pages, 4 figures, 2 tables; accepted for publication in MNRAS
Letter