In this paper, we compute rest-frame extinctions for the afterglows of a sample of Swift gamma-ray bursts (GRBs) complete in redshift. The selection criteria of the sample are based on observational ...high-energy parameters of the prompt emission and therefore our sample should not be biased against dusty sight-lines. It is therefore expected that our inferences hold for the general population of GRBs. Our main result is that the optical/near-infrared extinction of GRB afterglows in our sample does not follow a single distribution. 87 per cent of the events are absorbed by less than 2 mag, and 50 per cent suffer from less than 0.3-0.4 mag extinction. The remaining 13 per cent of the afterglows are highly absorbed. The true percentage of GRB afterglows showing high absorption could be even higher since a fair fraction of the events without reliable redshift measurement are probably part of this class. These events may be due to highly dusty molecular clouds/star-forming regions associated with the GRB progenitor or along the afterglow line of sight, and/or due to massive dusty host galaxies. No clear evolution in the dust extinction properties is evident within the redshift range of our sample, although the largest extinctions are at z ∼ 1.5-2, close to the expected peak of the star formation rate. Those events classified as dark are characterized, on average, by a higher extinction than typical events in the sample. A correlation between optical/near-infrared extinction and hydrogen-equivalent column density based on X-ray studies is shown, although the observed N
H appears to be well in excess compared to those observed in the Local Group. Dust extinction does not seem to correlate with GRB energetics or luminosity.
We present a carefully selected sub-sample of Swift long gamma-ray bursts (GRBs) that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions ...for ground-based follow-up searches, which are bright in the 15-150 keV Swift/BAT band, i.e., with 1-s peak photon fluxes in excess to 2.6 photons s super(-1) cm super(-2). The sample is composed of 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in an unbiased way. We find that strong evolution in luminosity ( delta sub()l = 2.3 + or - 0.6) or in density ( delta sub()d = 1.7 + or - 0.5) is required in order to account for the observations. The derived redshift distributions in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distributions. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.
We present the 1SXPS (Swift-XRT point source) catalog of 151, 524 X-ray point sources detected by the Swift-XRT in 8 yr of operation. The catalog covers 1905 deg super(2) distributed approximately ...uniformly on the sky. We analyze the data in two ways. First we consider all observations individually, for which we have a typical sensitivity of ~3 x 10 super(-13) erg cm super(-2) s super(-1) (0.3-10 keV). Then we co-add all data covering the same location on the sky: these images have a typical sensitivity of ~9 x 10 super(-14) erg cm super(-2) s super(-1) (0.3-10 keV). Our sky coverage is nearly 2.5 times that of 3XMM-DR4, although the catalog is a factor of ~1.5 less sensitive. The median position error is 5''.5 (90% confidence), including systematics. Our source detection method improves on that used in previous X-ray Telescope (XRT) catalogs and we report >68,000 new X-ray sources. The goals and observing strategy of the Swift satellite allow us to probe source variability on multiple timescales, and we find ~30,000 variable objects in our catalog. For every source we give positions, fluxes, time series (in four energy bands and two hardness ratios), estimates of the spectral properties, spectra and spectral fits for the brightest sources, and variability probabilities in multiple energy bands and timescales.
Context. Gamma-ray bursts (GRBs) occurring in the local Universe constitute an interesting sub-class of the GRB family, since their luminosity is on average lower than that of their cosmological ...analogs. Attempts to understand in a global way this peculiar behaviour is still not possible, since the sample of low redshift GRBs is small, and the properties of individual objects are too different from each other. In addition, their closeness (and consequently high fluxes) make these sources ideal targets for extensive follow-up even with small telescopes, considering also that these GRBs are conclusively associated with supernova (SN) explosions. Aims. We aim to contribute to the study of local bursts by reporting the case of GRB 171205A. This source was discovered by Swift Burst Alert Telescope (BAT) on 2017, December 5 and soon associated with a low redshift host galaxy (z = 0.037), and an emerging SN (SN 2017iuk). Methods. We analyzed the full Swift dataset, comprising the UV-Optical Telescope (UVOT), X-ray Telescope (XRT) and BAT data. In addition, we employed the Konus-Wind high energy data as a valuable extension at γ-ray energies. Results. The photometric SN signature is clearly visible in the UVOT u, b and ν filters. The maximum emission is reached at ∼13 (rest frame) days, and the whole bump resembles that of SN 2006aj, but lower in magnitude and with a shift in time of +2 d. A prebump in the ν-band is also clearly visible, and this is the first time that such a feature is not observed achromatically in GRB–SNe. Its physical origin cannot be easily explained. The X-ray spectrum shows an intrinsic Hydrogen column density NH,int = 7.4+4.1−3.6 × 1020 N H , int = 7 . 4 − 3.6 + 4.1 × 10 20 $ N_{\mathrm{H,int}} = 7.4^{+4.1}_{-3.6}\times 10^{20} $ cm−2, which is at the low end of the N H, int, even considering just low redshift GRBs. The spectrum also features a thermal component, which is quite common in GRBs associated with SNe, but whose origin is still a matter of debate. Finally, the isotropic energy in the γ-ray band, Eiso = 2.18+0.63−5.0 × 1049 E iso = 2 . 18 − 0.50 + 0.63 × 10 49 $ E_{\mathrm{iso}} = 2.18^{+0.63}_{-0.50} \times 10^{49} $ erg, is lower than those of cosmological GRBs. Combining this value with the peak energy in the same band, Ep = 125+141−37 E p = 125 − 37 + 141 $ E_{\mathrm{p}}=125^{+141}_{-37} $ keV, implies that GRB 171205A is an outlier of the Amati relation, as are some other low redshift GRBs, and its emission mechanism should be different from that of canonical, farther away GRBs.
The jet opening angle θjet and the bulk Lorentz factor Γ0 are crucial parameters for the computation of the energetics of gamma-ray bursts (GRBs). From the ∼30 GRBs with measured θjet or Γ0 it is ...known that (i) the real energetic E
γ, obtained by correcting the isotropic equivalent energy E
iso for the collimation factor ∼ θ2
jet, is clustered around 1050-1051 erg and it is correlated with the peak energy E
p of the prompt emission and (ii) the comoving frame E′p and E′γ are clustered around typical values. Current estimates of Γ0 and θjet are based on incomplete data samples and their observed distributions could be subject to biases. Through a population synthesis code we investigate whether different assumed intrinsic distributions of Γ0 and θjet can reproduce a set of observational constraints. Assuming that all bursts have the same E′p and E′γ in the comoving frame, we find that Γ0 and θjet cannot be distributed as single power laws. The best agreement between our simulation and the available data is obtained assuming (a) log-normal distributions for θjet and Γ0 and (b) an intrinsic relation between the peak values of their distributions, i.e. θjet
2.5Γ0 = const. On average, larger values of Γ0 (i.e. the 'faster' bursts) correspond to smaller values of θjet (i.e. the 'narrower'). We predict that ∼6 per cent of the bursts that point to us should not show any jet break in their afterglow light curve since they have sin θjet < 1/Γ0. Finally, we estimate that the local rate of GRBs is ∼0.3 per cent of all local Type Ib/c supernova (SNIb/c) and ∼4.3 per cent of local hypernovae, i.e. SNIb/c with broad lines.
The red blazar PMN J2345−1555 becomes blue Ghisellini, G; Tavecchio, F; Foschini, L ...
Monthly notices of the Royal Astronomical Society. Letters,
05/2013, Letnik:
432, Številka:
1
Journal Article
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The flat spectrum radio quasar PMN J2345−1555 is a bright γ-ray source, that recently underwent a flaring episode in the infrared (IR), ultraviolet (UV) and γ-ray bands. The flux changed ...quasi-simultaneously at different frequencies, suggesting that it was produced by a single population of emitting particles, hence by a single and well-localized region of the jet. While the overall spectral energy distribution (SED) before the flare was typical of powerful blazars (namely two broad humps peaking in the far-IR and below 100 MeV bands, respectively), during the flare the peaks moved to the optical-UV and to energies larger than 1 GeV, to resemble low power BL Lac objects, even if the observed bolometric luminosity increased by more than one order of magnitude. We interpret this behaviour as due to a change of the location of the emission region in the jet, from within the broad-line region, to just outside. The corresponding decrease of the radiation energy density as seen in the comoving frame of the jet allowed the relativistic electrons to be accelerated to higher energies, and thus produce a 'bluer' SED.
We have conducted a multiwavelength survey of 42 radio loud narrow-1ine Seyfert 1 galaxies (RLNLS1s), selected by searching among all the known sources of this type and omitting those with steep ...radio spectra. We analyse data from radio frequencies to X-rays, and supplement these with information available from online catalogues and the literature in order to cover the full electromagnetic spectrum. This is the largest known multiwavelength survey for this type of source. We detected 90% of the sources in X-rays and found 17% at γ rays. Extreme variability at high energies was also found, down to timescales as short as hours. In some sources, dramatic spectral and flux changes suggest interplay between a relativistic jet and the accretion disk. The estimated masses of the central black holes are in the range ~106−8 M⊙, lower than those of blazars, while the accretion luminosities span a range from ~0.01 to ~0.49 times the Eddington limit, with an outlier at 0.003, similar to those of quasars. The distribution of the calculated jet power spans a range from ~1042.6 to ~1045.6 erg s-1, generally lower than quasars and BL Lac objects, but partially overlapping with the latter. Once normalised by the mass of the central black holes, the jet power of the three types of active galactic nuclei are consistent with each other, indicating that the jets are similar and the observational differences are due to scaling factors. Despite the observational differences, the central engine of RLNLS1s is apparently quite similar to that of blazars. The historical difficulties in finding radio-loud narrow-line Seyfert 1 galaxies might be due to their low power and to intermittent jetactivity.
We have selected SDSS J222032.50+002537.5 and SDSS J142048.01+120545.9 as best blazar candidates out of a complete sample of extremely radio-loud quasars at z > 4, with highly massive black holes. We ...observed them and a third serendipitous candidate with similar features (PMN J2134-0419) in the X-rays with the Swift/XRT telescope, to confirm their blazar nature. We observed strong and hard X-ray fluxes (i.e. alpha X greater than or equal to 0.6, where F(v) proportional to v super( alpha x) in the 0.3-10 keV observed energy range, similar to 1-40 keV rest frame) in all three cases. This allowed us to classify our candidates as real blazars, being characterized by large Lorentz factors ( similar to 13) and very small viewing angles ( similar to 3 degree ). All three sources have black hole masses exceeding 10 super(9) M. and their classification provides intriguing constraints on supermassive black hole formation and evolution models. We confirm our earlier suggestion that there are different formation epochs of extremely massive black holes hosted in jetted (z similar to 4) and non-jetted systems (z similar to 2.5).
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 delay in arrival times between high and low energy photons from cosmic sources can be used to test the violation of the Lorentz invariance (LIV), predicted by some quantum gravity theories, and ...to constrain its characteristic energy scale EQG that is of the order of the Planck energy. Gamma-ray bursts (GRBs) and blazars are ideal for this purpose thanks to their broad spectral energy distribution and cosmological distances: at first order approximation, the constraints on EQG are proportional to the photon energy separation and the distance of the source. However, the LIV tiny contribution to the total time delay can be dominated by intrinsic delays related to the physics of the sources: long GRBs typically show a delay between high and low energy photons related to their spectral evolution (spectral lag). Short GRBs have null intrinsic spectral lags and are therefore an ideal tool to measure any LIV effect. We considered a sample of 15 short GRBs with known redshift observed by Swift and we estimate a limit on EQG ≳ 1.5 × 1016 GeV. Our estimate represents an improvement with respect to the limit obtained with a larger (double) sample of long GRBs and is more robust than the estimates on single events because it accounts for the intrinsic delay in a statistical sense.
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