1,179 research outputs found
Modelling the variable broad-band optical/UV/X-ray spectrum of PG1211+143: Implications for the ionized outflow
We present the results from a detailed analysis of the 2007 Swift monitoring
campaign of the quasar PG1211+143. We constructed broad-band, optical/UV/X-ray
spectral energy distributions over three X-ray flux intervals. We fitted them
with a model which accounts for the disc and the X-ray coronal emission and the
warm absorber (well established in this source). The three flux spectra are
well fitted by the model we considered. The disc inner temperature remains
constant at ~2 eV, while X-rays are variable both in spectral slope and
normalization. The absorber covers almost 90% of the central source. It is
outflowing with a velocity less than 2.3*10^4 km/s (3sigma upper limit), and
has a column density of ~10^23.2. Its ionization parameter varies by a factor
of 1.6, and it is in photo-ionizing equilibrium with the ionizing flux. It is
located at a distance of less than 0.35 pc from the central source and its
relative thickness, DR/R is less than 0.1. The absorber' s ionization parameter
variations can explain the larger than average amplitude of the X-ray
variations. The absence of optical/UV variations (consistent with the high
black hole mass estimate) argues against the presence of inward propagating
disc fluctuations and strong X-ray illumination of the disc (in agreement with
the low ratio of X-ray over the bolometric luminosity of ~20-35). We estimate
an upper limit for the mass outflow of ~5 solar masses per year (~2.3 times the
Eddington mass accretion rate). If the outflow rate is indeed that high, then
it must be a short-lived episode in the quasar's life time. Finally, we
estimate an upper limit for the kinetic power of the outflow of ~1.4*10^43
ergs/s. This outflow cannot deploy significant mechanical energy to the
surrounding ISM of the quasar's host galaxy, but is sufficient to heat the ISM
to 10^7 K and to produce a fast decline to the star formation rate of the
galaxy.Comment: Accepted for publication by A&
Unveiling GRB hard X-ray afterglow emission with Simbol-X
Despite the enormous progress occurred in the last 10 years, the Gamma-Ray
Bursts (GRB) phenomenon is still far to be fully understood. One of the most
important open issues that have still to be settled is the afterglow emission
above 10 keV, which is almost completely unexplored. This is due to the lack of
sensitive enough detectors operating in this energy band. The only detection,
by the BeppoSAX/PDS instrument (15-200 keV), of hard X-ray emission from a GRB
(the very bright GRB 990123), combined with optical and radio observations,
seriously challenged the standard scenario in which the dominant mechanism is
synchrotron radiation produced in the shock of a ultra-relativistic fireball
with the ISM, showing the need of a substantial revision of present models. In
this respect, thanks to its unprecedented sensitivity in the 10-80 keV energy
band, Simbol-X, through follow-up observations of bright GRBs detected and
localized by GRB dedicated experiments that will fly in the >2010 time frame,
will provide an important breakthrough in the GRB field.Comment: 4 pages, 2 figures. Paper presented at "Simbol-X: the hard X-ray
universe in focus", held in Bologna, Italy, on 14-16 May 2007. To be
published in Memorie della Societa' Astronomica Italian
The X-ray afterglow of GRB980519
Over a total of 20 gamma-ray bursts localized with arcmin accuracies,
GRB980519 represents the 13th detected by the BeppoSAX Wide Field Cameras. An
X-ray TOO observation performed by the BeppoSAX Narrow Field Instruments,
starting about 9.5 hours after the high energy event, revealed X-ray afterglow
emission in the 0.1--10 keV energy range. The flux decay was particularly fast
with a power-law index of ~ 1.8. This is the fastest decay so far measured.
Signs of bursting activity are evident. The power-law spectral index of 2.8
(+0.6, -0.5) is quite soft but not unique among GRB afterglows.Comment: 2 pages; 3 EPS figures; requires aa.cls, psfig.sty; accepted for
publication on A&AS (Proc. of ``Gamma-Ray Bursts in the Afterglow Era'', Rome
3-6 Nov. 1998.
Spectral catalogue of bright gamma-ray bursts detected with the BeppoSAX/GRBM
The emission process responsible for the so-called "prompt" emission of
gamma-ray bursts is still unknown. A number of empirical models fitting the
typical spectrum still lack a satisfactory interpretation. A few GRB spectral
catalogues derived from past and present experiments are known in the
literature and allow to tackle the issue of spectral properties of gamma-ray
bursts on a statistical ground. We extracted and studied the time-integrated
photon spectra of the 200 brightest GRBs observed with the Gamma-Ray Burst
Monitor which flew aboard the BeppoSAX mission (1996-2002) to provide an
independent statistical characterisation of GRB spectra. The spectra were fit
with three models: a simple power-law, a cut-off power law or a Band function.
The typical photon spectrum of a bright GRB consists of a low-energy index
around 1.0 and a peak energy of the nuFnu spectrum E_p~240 keV in agreement
with previous results on a sample of bright CGRO/BATSE bursts. Spectra of ~35%
of GRBs can be fit with a power-law with a photon index around 2, indicative of
peak energies either close to or outside the GRBM energy boundaries. We confirm
the correlation between E_p and fluence, with a logarithmic dispersion of 0.13
around the power-law with index 0.21+-0.06. The low-energy and peak energy
distributions are not yet explained in the current literature. The capability
of measuring time-resolved spectra over a broadband energy range, ensuring
precise measurements of parameters such as E_p, will be crucial for future
experiments (abridged).Comment: 28 pages, 20 figures, 3 tables, accepted to A&
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