2,839 research outputs found
Iron line afterglows: how to produce them
We discuss how a powerful iron line emission can be produced if ~1-5 iron
rich solar masses are concentrated in the close vicinity of the burst.
Recombination, thermal and fluorescent reflection are discussed. We find that
recombination suffers the high Compton temperature of the plasma while the
other two scenarios are not mutually exclusive and could account for the
claimed iron line detected in two afterglows.Comment: 2 pages, A&AS in press, proceedings of the Workshop "Gamma Ray Bursts
in the Afterglow Era" held in Rome, November 199
Iron line in the afterglow: a key to unveil Gamma-Ray Burst progenitors
The discovery of a powerful and transient iron line feature in the X-ray
afterglow spectra of gamma-ray bursts would be a major breakthrough for
understanding the nature of their progenitors. Piro et al. (1999) and Yoshida
et al. (1999) report such a detection in the afterglow of GRB 970508 and GRB
970828, respectively. We discuss how such a strong line could be produced in
the various scenarios proposed for the event progenitor. We show that the
observed line intensity requires a large iron mass, concentrated in the
vicinity of the burst. The previous explosion of a supernova, predicted in the
Supranova scenario, is the most straightforward way to account for such a large
amount of matter. We discuss three different physical processes that could
account for the line: recombination, reflection and thermal emission. Among
these, reflection and thermal emission may explain the observed line features:
reflection should be important if the remnant is optically thick, while thermal
lines can be produced only in a thin plasma. The recombination process requires
extremely high densities to efficiently reprocess the burst photons, whereas
this process could work during the X-ray afterglow. Future key observations for
discriminating the actual radiating process are discussed.Comment: 5 pages, 1 figure, MNRAS letters in pres
Iron line afterglows: general constraints
The discovery of a powerful and transient iron line feature in the X-ray
afterglow spectra of gamma-ray bursts would be a major breakthrough for
understanding the nature of their progenitors, strongly suggesting the presence
of a large, iron rich, mass in the vicinity of the burst event.
Model-independent limits to the size and the mass of the the iron line emitting
region are derived and discussed. We also discuss how these results can be used
to constrain the amount of beaming or anisotropy of the burst emission.Comment: 2 pages, A&AS in press, proceedings of the Workshop "Gamma Ray Bursts
in the Afterglow Era" held in Rome, November 199
GRB Redshift determination in the X-ray band
If gamma-ray bursts originate in dense stellar forming regions, the
interstellar material can imprint detectable absorption features on the
observed X-ray spectrum. Such features can be detected by existing and planned
X-ray satellites, as long as the X-ray afterglow is observed after a few
minutes from the burst. Detection of these X-ray features will make possible
the determination of the redshift of gamma-ray bursts even when their optical
afterglows are severely dimmed by extinction.Comment: 2 pages, A&AS in press, proceedings of the Workshop "Gamma Ray Bursts
in the Afterglow Era" held in Rome, November 199
Dynorphin gene expression and release in the myocardial cell.
The expression of the prodynorphin gene was investigated in adult cultured rat ventricular cardiac myocytes by using a sensitive solution hybridization RNase protection assay for the quantitative analysis of prodynorphin mRNA. Myocyte culture in high KCl resulted, after 4 h, in a marked increase in cellular prodynorphin mRNA, while a KCl treatment for 6, 12, or 24 h progressively down-regulated the levels of prodynorphin mRNA below the control value. Immunoreactive dynorphin B, a biologically active end product of the precursor, was found to be present in the culture medium in significantly higher amounts than in the cardiac myocytes. The levels of this biologically active K opioid receptor agonist significantly increased after 4 h of KCl treatment and were markedly reduced following a 24-h exposure of the cardiac myocytes to KCl. These KCl-induced effects were all abolished by cell incubation in the presence of the calcium channel blocker verapamil. In single cardiac myocytes, acute stimulation of K opioid receptors with dynorphin B or with the selective agonist U-50,488H increased the level of cytosolic calcium. This effect was abolished by the specific K opioid receptor antagonist (Mr-1452) and was not affected by the removal of calcium from the bathing medium. These results suggest that an opioid gene may influence the myocardial function in an autocrine or paracrine fashion
Redshift determination in the X-ray band of gamma-ray bursts
If gamma-ray bursts originate in dense stellar forming regions, the
interstellar material can imprint detectable absorption features on the
observed X-ray spectrum. Such features can be detected by existing and planned
X-ray satellites, as long as the X-ray afterglow is observed after a few
minutes from the burst. If the column density of the interstellar material
exceeds ~10^{23} cm^{-2} there exists the possibility to detect the K_alpha
fluorescent iron line, which should be visible for more than one year, long
after the X-ray afterglow continuum has faded away. Detection of these X-ray
features will make possible the determination of the redshift of gamma-ray
bursts even when their optical afterglow is severely dimmed by extinction.Comment: 15 pages with 5 figures. Submitted to Ap
Discovery of periodicities in two highly variable intermediate polars towards the Galactic Center
We discovered Fe complex emission and pulsation in two highly
variable sources (4XMM J174917.7--283329, 4XMM J174954.6--294336). The
equivalent widths of 6.4 and 6.7 keV lines of 4XMM J174917.7--283329 are
and eV, respectively. The continuum is
fitted by a partially absorbed apec model with plasma temperature of
keV. The inferred mass of the white dwarf (WD) is
. We detected pulsations with a period of
s and a pulsed fraction of . The light curves of 4XMM
J174954.6--294336 display asymmetric eclipse and dipping behaviour. To date,
this is only the second intermediate polar (IP) that shows a total eclipse in
X-rays. The spectrum of the sources is characterized by a power-law model with
photon index . The equivalent widths of the 6.4 keV and 6.7
keV iron lines are and eV, respectively.
The continuum is described by emission from optically thin plasma with a
temperature of keV. The inferred mass of the WD is
. We discovered coherent pulsations from the
source with a period of s. The pulsed fraction is . The
measured spin period, hard photon index, and equivalent width of the
fluorescent Fe line in both sources are consistent with the values
found in IP. While 4XMM J174954.6--294336 was already previously classified as
an IP, we also suggest 4XMM J174917.7--283329 as a new IP. The X-ray eclipses
in 4XMM J174954.6--294336 are most likely caused by a low-mass companion star
obscuring the central X-ray source. The asymmetry in the eclipse is likely
caused by a thick bulge that intercepts the line of sight during the ingress
phase but not during the egress phase located behind the WD along the line of
sight.Comment: 9 pages, six figures, accepted for publication in A&
Evidence that AMP-activated protein kinase can negatively modulate Ornithine decarboxylase activity in cardiac myoblasts
AbstractThe responses of AMP-activated protein kinase (AMPK) and Ornithine decarboxylase (ODC) to isoproterenol have been examined in H9c2 cardiomyoblasts, AMPK represents the link between cell growth and energy availability whereas ODC, the key enzyme in polyamine biosynthesis, is essential for all growth processes and it is thought to have a role in the development of cardiac hypertrophy. Isoproterenol rapidly induced ODC activity in H9c2 cardiomyoblasts by promoting the synthesis of the enzyme protein and this effect was counteracted by inhibitors of the PI3K/Akt pathway. The increase in enzyme activity became significant between 15 and 30min after the treatment. At the same time, isoproterenol stimulated the phosphorylation of AMPKα catalytic subunits (Thr172), that was associated to an increase in acetyl coenzyme A carboxylase (Ser72) phosphorylation. Downregulation of both α1 and α2 isoforms of the AMPK catalytic subunit by siRNA to knockdown AMPK enzymatic activity, led to superinduction of ODC in isoproterenol-treated cardiomyoblasts. Downregulation of AMPKα increased ODC activity even in cells treated with other adrenergic agonists and in control cells. Analogue results were obtained in SH-SY5Y neuroblastoma cells transfected with a shRNA construct against AMPKα. In conclusion, isoproterenol quickly activates in H9c2 cardiomyoblasts two events that seem to contrast one another. The first one, an increase in ODC activity, is linked to cell growth, whereas the second, AMPK activation, is a homeostatic mechanism that negatively modulates the first. The modulation of ODC activity by AMPK represents a mechanism that may contribute to control cell growth processes
Matrix Modulation of the Bioactivation of Estragole by Constituents of Different Alkenylbenzene-containing Herbs and Spices and Physiologically Based Biokinetic Modeling of Possible In Vivo Effects
The alkenylbenzene estragole is a constituent of several herbs and spices. It induces hepatomas in rodents at high doses following bioactivation by cytochrome P450s and sulfotransferases (SULTs) giving rise to the ultimate carcinogenic metabolite 1'-sulfooxyestragole which forms DNA adducts. Methanolic extracts from different alkenylbenzene-containing herbs and spices were able to inhibit SULT activity. Flavonoids including quercetin, kaempferol, myricetin, apigenin, and nevadensin were the major constituents responsible for this inhibition with Ki values in the nano to micromolar range. In human HepG2 cells exposed to the proximate carcinogen 1ʹ-hydroxyestragole, the various flavonoids were able to inhibit estragole DNA adduct formation and shift metabolism in favor of glucuronidation which is a detoxification pathway for 1ʹ-hydroxyestragole. In a next step, the kinetics for SULT inhibition were incorporated in physiologically based biokinetic (PBBK) models for estragole in rat and human to predict the effect of co-exposure to estragole and (mixtures of) the different flavonoids on the bioactivation in vivo. The PBBK-model-based predictions indicate that the reduction of estragole bioactivation in rat and human by co-administration of the flavonoids is dependent on whether the intracellular liver concentrations of the flavonoids can reach their Ki values. It is expected that this is most easily achieved for nevadensin which has a Ki value in the nanomolar range and is, due to its methyl ation, more metabolically stable than the other flavonoid
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