We propose a scenario where blazars are classified into flat-spectrum radio quasars (FSRQs), BL Lacertae (BL Lac) objects, low-synchrotron, or high-synchrotron peaked objects according to a varying ...mix of the Doppler-boosted radiation from the jet, the emission from the accretion disc, the broad-line region, and the light from the host galaxy. In this framework, the peak energy of the synchrotron power (
) in blazars is independent of source type and radio luminosity. We test this new approach, which builds upon unified schemes, using extensive Monte Carlo simulations, and show that it can provide simple answers to a number of long-standing issues, including, amongst others, the different cosmological evolution of BL Lac objects selected in the radio and X-ray bands, the larger
values observed in BL Lac objects, the fact that high-synchrotron peaked blazars are always of BL Lac type, and the existence of FSRQ-BL Lac transition objects. Objects so far classified as BL Lac objects on the basis of their observed weak, or undetectable, emission lines are of two physically different classes: intrinsically weak lined objects, more common in X-ray-selected samples, and heavily diluted broad-lined sources, more frequent in radio-selected samples, which explains some of the confusion in the literature. We also show that strong selection effects are the main cause of the diversity observed in radio and X-ray samples, and that the correlation between luminosity and
, which led to the proposal of the 'blazar sequence', is also a selection effect arising from the comparison between shallow radio and X-ray surveys, and to the fact that high-
-high-radio-power objects have never been considered because their redshift is not measurable.
We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from similar0.03 to ...similar1.2 M, but mostly with 0.1 M<Msub *< 0.5 M. Our aim is twofold: firstly, to analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (Lsubacc), and in turn the accretion rate (Msubacc ), was derived by modelling the excess emission from the UV to the near-infrared as the continuum emission of a slab of hydrogen. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude in Msubacc, confirming previous suggestions that the geometry of the accretion flow controls the rate at which the disc material accretes onto the central star.
The nature of the prompt gamma-ray emission of gamma-ray bursts (GRBs) is still far from being completely elucidated. The measure of linear polarization is a powerful tool that can be used to put ...further constraints on the content and magnetization of the GRB relativistic outflows, as well as on the radiation processes at work. To date, only a handful of polarization measurements are available for the prompt emission of GRBs. Here we present the analysis of the prompt emission of GRB 140206A, obtained with INTEGRAL/IBIS, Swift/BAT, and Fermi/GBM. Using INTEGRAL/IBIS as a Compton polarimeter, we were able to constrain the linear polarization level of the second peak of this GRB as being larger than 28 per cent at 90 per cent c.l. We also present the GRB afterglow optical spectroscopy obtained at the Telescopio Nazionale Galileo, which allowed us to measure the distance of this GRB, z = 2.739. This distance value together with the polarization measure obtained with IBIS allowed us to derive the deepest and most reliable limit to date (ξ < 1 × 10−16) on the possibility of Lorentz invariance violation, measured through the vacuum birefringence effect on a cosmological source.
Disks around T Tauri stars are known to lose mass, as best shown by the profiles of the forbidden emission lines of low-ionization species. At least two separate kinematic components have been ...identified, one characterized by velocity shifts of tens to hundreds of km s super(-1) (HVC) and one with a much lower velocity of a few km s super(-1) (LVC). The HVC are convincingly associated to the emission of jets, but the origin of the LVC is still unknown. In this paper we analyze the forbidden line spectrum of a sample of 44 mostly low-mass young stars in Lupus and sigma Ori observed with the X-shooter ESO spectrometer. We detect forbidden line emission of OI, OII, S II, N I, and N II, and characterize the line profiles as LVC, blueshifted HVC, and redshifted HVC. We focus our study on the LVC. We show that there is a good correlation between line luminosity and both L sub(star) and the accretion luminosity (or the mass accretion rate) over a large interval of values (L sub(star) ~ 10 super(-2)-1 L sub(middot in circle); L sub(acc) ~ 10 super(-5)-10 super(-1) L sub(middot in circle); M sub(acc) ~ 10 super(-11)-10 super(-7) M sub(middot in circle)/yr). The lines show the presence of a slow wind (V sub(peak) < 20 km s super(-1)) that is dense (n sub(H) > 10 super(8) cm super(-3) ), warm (T ~ 5000-10 000 K), mostly neutral. We estimate the mass of the emitting gas and provide a value for the maximum volume it occupies. Both quantities increase steeply with the stellar mass, from ~10 super(-12) M sub(middot in circle) and ~0.01 AU super(3) for M sub(star) ~ 0.1 M sub(middot in circle), to ~3 x 10 super(-10) M sub(middot in circle) and ~1 AU super(3) for M sub(star) ~ 1 M sub(middot in circle), respectively. These results provide quite stringent constraints to wind models in low-mass young stars, that need to be explored further.
Context.
GRB 220627A is a rare burst with two distinct
γ
-ray emission episodes separated by almost 1000 s that triggered the
Fermi
Gamma-ray Burst Monitor twice. High-energy GeV emission was ...detected by the
Fermi
Large Area Telescope coincident with the first emission episode but not the second. The discovery of the optical afterglow with MeerLICHT led to MUSE observations which secured the burst redshift to
z
= 3.08, making this the most distant ultra-long gamma-ray burst (GRB) detected to date.
Aims.
The progenitors of some ultra-long GRBs have been suggested in the literature to be different to those of normal long GRBs. Our aim is to determine whether the afterglow and host properties of GRB 220627A agree with this interpretation.
Methods.
We performed empirical and theoretical modelling of the afterglow data within the external forward shock framework, and determined the metallicity of the GRB environment through modelling the absorption lines in the MUSE spectrum.
Results.
Our optical data show evidence for a jet break in the light curve at ∼1.2 days, while our theoretical modelling shows a preference for a homogeneous circumburst medium. Our forward shock parameters are typical for the wider GRB population, and we find that the environment of the burst is characterised by a sub-solar metallicity.
Conclusions.
Our observations and modelling of GRB 220627A do not suggest that a different progenitor compared to the progenitor of normal long GRBs is required. We find that more observations of ultra-long GRBs are needed to determine if they form a separate population with distinct prompt and afterglow features, and possibly distinct progenitors.
The lack of knowledge of photospheric parameters and the level of chromospheric activity in young low-mass pre-main sequence stars introduces uncertainties when measuring mass accretion rates in ...accreting (Class II) young stellar objects. A detailed investigation of the effect of chromospheilc emission on the estimates of mass accretion rate in young low-mass stars is still missing. This can be undertaken using samples of young diskless (Class III) K and M-type stars. Our goal is to measure the chromospheric activity of Class III pre main sequence stars to determine its effect on the estimates of the accretion luminosity (Lsubacc) and mass accretion rate (Msubacc) in young stellar objects with disks. Values of Lsubacc < or=10sup -3 obtained in accreting low-mass pre main sequence stars through line luminosity should be treated with caution because the line emission may be dominated by the contribution of chromospheric activity.
We use a nearly complete sample of gamma-ray bursts (GRBs) detected by the Swift satellite to study the correlations between the spectral peak energy E
peak of the prompt emission, the isotropic ...energetics E
iso and the isotropic luminosity L
iso. This GRB sample is characterized by a high level of completeness in redshift (90 per cent). This allows us to probe in an unbiased way the issue related to the physical origin of these correlations against selection effects. We find that one burst, GRB 061021, is an outlier to the E
peak-E
iso correlation. Despite this case, we find strong E
peak-E
iso and E
peak-L
iso correlations for the bursts of the complete sample. Their slopes, normalizations and dispersions are consistent with those found with the whole sample of bursts with measured redshift and E
peak. This means that the biases present in the total sample commonly used to study these correlations do not affect their properties. Finally, we also find no evolution with redshift of the E
peak-E
iso and E
peak-L
iso correlations.
The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical-near-infrared source ...powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named 'macronovae' or 'kilonovae', are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.
We present a carefully selected sample of short gamma-ray bursts (SGRBs) observed by the Swift satellite up to 2013 June. Inspired by the criteria we used to build a similar sample of bright long ...GRBs (the BAT6 sample), we selected SGRBs with favourable observing conditions for the redshift determination on ground, ending up with a sample of 36 events, almost half of which with a redshift measure. The redshift completeness increases up to about 70 per cent (with an average redshift value of z = 0.85) by restricting to those events that are bright in the 15–150 keV Swift Burst Alert Telescope energy band. Such flux-limited sample minimizes any redshift-related selection effects, and can provide a robust base for the study of the energetics, redshift distribution and environment of the Swift bright population of SGRBs. For all the events of the sample, we derived the prompt and afterglow emission in both the observer and (when possible) rest frame and tested the consistency with the correlations valid for long GRBs. The redshift and intrinsic X-ray absorbing column density distributions we obtain are consistent with the scenario of SGRBs originated by the coalescence of compact objects in primordial binaries, with a possible minor contribution (∼10–25 per cent) of binaries formed by dynamical capture (or experiencing large natal kicks). This sample is expected to significantly increase with further years of Swift activity.
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