Long duration gamma-ray bursts (GRBs) and broad-line, type Ic supernovae (SNe) are strongly connected. We aim at characterizing SN 2013dx, associated with GRB,130702A, through sensitive and extensive ground-based observational campaigns in the optica
l-IR band. We monitored the field of the Swift GRB 130702A (redshift z = 0.145) using the 8.2-m VLT, the 3.6-m TNG and the 0.6-m REM telescopes during the time interval between 4 and 40 days after the burst. Photometric and spectroscopic observations revealed the presence of the associated Type Ic SN 2013dx. Our multi-band photometry allowed the construction of a bolometric light curve.} The bolometric light curve of SN 2013dx resembles that of 2003dh (associated with GRB 030329), but is ~10% faster and ~25% dimmer. From this we infer a synthesized 56Ni mass of ~0.2 solar masses. The multi-epoch optical spectroscopy shows that the SN 2013dx behavior is best matched by SN 1998bw, among the other well-known low-redshift SNe associated with GRBs and XRFs, and by SN 2010ah, an energetic Type Ic SN not associated with any GRB. The photospheric velocity of the ejected material declines from ~2.7X10^4 km/s at 8 rest frame days from the explosion, to ~3.5X10^3 km/s at 40 days. These values are extremely close to those of SN1998bw and 2010ah. We deduce for SN 2013dx a kinetic energy of ~35X10^51 erg, and an ejected mass of ~7 solar masses. This suggests that the progenitor of SN2013dx had a mass of ~25 solar masses, i.e., 15-20% less massive than that of SN 1998bw. Finally, we performed a study of the SN 2013dx environment, through spectroscopy of the closeby galaxies. 9 out of the 14 inspected galaxies lie within 0.03 in redshift from z=0.145, indicating that the host of GRB 130702A/SN 2013dx belongs to a group of galaxies, an unprecedented finding for a GRB-associated SN and, to our knowledge, for long GRBs in general.
Long-duration gamma-ray bursts (GRBs) have been found to be associated with broad-lined type-Ic supernovae (SNe), but only a handful of cases have been studied in detail. Prompted by the discovery of the exceptionally bright, nearby GRB130427A (redsh
ift z=0.3399), we aim at characterising the properties of its associated SN2013cq. This is the first opportunity to test directly the progenitors of high-luminosity GRBs. We monitored the field of the Swift long duration GRB130427A using the 3.6-m TNG and the 8.2-m VLT during the time interval between 3.6 and 51.6 days after the burst. Photometric and spectroscopic observations revealed the presence of the type Ic SN2013cq. Spectroscopic analysis suggests that SN2013cq resembles two previous GRB-SNe, SN1998bw and SN2010bh associated with GRB980425 and XRF100316D, respectively. The bolometric light curve of SN2013cq, which is significantly affected by the host galaxy contribution, is systematically more luminous than that of SN2010bh ($sim$ 2 mag at peak), but is consistent with SN1998bw. The comparison with the light curve model of another GRB-connected SN2003dh, indicates that SN2013cq is consistent with the model when brightened by 20%. This suggests a synthesised radioactive $^{56}$Ni mass of $sim 0.4 M_odot$. GRB130427A/SN2013cq is the first case of low-z GRB-SN connection where the GRB energetics are extreme ($E_{rm gamma, iso} sim 10^{54}$ erg). We show that the maximum luminosities attained by SNe associated with GRBs span a very narrow range, but those associated with XRFs are significantly less luminous. On the other hand the isotropic energies of the accompanying GRBs span 6 orders of magnitude (10$^{48}$ erg $< E_{rm gamma, iso} <$ 10$^{54}$ erg), although this range is reduced when corrected for jet collimation. The GRB total radiated energy is in fact a small fraction of the SN energy budget.
LOFT is a satellite mission currently in Assessment Phase for the ESA M3 selection. The payload is composed of the Large Area Detector (LAD), with 2-50 keV energy band, a peak effective area of about 10 m2 and an energy resolution better than 260 eV,
and the Wide Field Monitor (WFM), a coded mask imager with a FOV of several steradians, an energy resolution of about 300 eV and a point source location accuracy of 1 arcmin in the 2-30 keV energy range. Based on preliminary considerations and estimates, we show how the scientific performances of the WFM are particularly suited to investigate some of the most relevant open issues in the study of GRBs: the physics of the prompt emission, the spectral absorption features by circum-burst material (and hence the nature of the progenitors), the population and properties of XRFs, and the detection and rate of high-z GRBs. Measurements of the early afterglow emission with the Large Area Detector (LAD) may also be possible depending on the mission slewing capabilities and TOO observations policy. More refined analysis and simulations are in progress.
One of the most intriguing features discovered by Swift is a plateau phase in the X-ray flux decay of about 70% of the afterglows of gamma-ray bursts (GRBs). The physical origin of this feature is still being debated. We constrain the proposed interp
retations, based on the intrinsic temporal properties of the plateau phase. We selected and analyzed all the Swift/XRT GRB afterglows at known redshift observed between March 2005 and June 2008 featuring a shallow decay phase in their X-ray lightcurves. For our sample of 21 GRBs we find an anticorrelation of the logarithm of the duration of the shallow phase with re dshift, with a Spearman rank-order correlation coefficient of r=-0.4 and a null hypothesis probability of 5%. When we correct the durations for cosmological dilation, the anticorrelation strenghtens, with r=-0.6 and a null hypothesis probability of 0.4%. Considering only those GRBs in our sample that have a well-measured burst peak energy (8 out of 21), we find an anticorrelation between the energy of the burst and the shallow phase duration, with r=-0.80 and a null hypothesis probability of 1.8%. If the burst energy anticorrelation with the shallow phase duration is real, then the dependence of the shallow phase on redshift could be the result of a selection effect, since on average high-redshift bursts with lower energies and longer plateaus would be missed. A burst energy anticorrelation with the shallow phase duration would be expected if the end of the plateau arises from a collimated outflow. Alternative scenarios are briefly discussed involving a possible cosmological evolution of the mechanism responsible for the X-ray shallow decay.
GRB080319B reached 5th optical magnitude during the burst prompt emission. Thanks to the VLT/UVES rapid response mode, we observed its afterglow just 8m:30s after the GRB onset when the magnitude was R ~ 12. This allowed us to obtain the best signal-
to-noise, high resolution spectrum of a GRB afterglow ever (S/N per resolution element ~ 50). The spectrum is rich of absorption features belonging to the main system at z=0.937, divided in at least six components spanning a total velocity range of 100 km/s. The VLT/UVES observations caught the absorbing gas in a highly excited state, producing the strongest Fe II fine structure lines ever observed in a GRB. A few hours later the optical depth of these lines was reduced by a factor of 4-20, and the optical/UV flux by a factor of ~ 60. This proves that the excitation of the observed fine structure lines is due to pumping by the GRB UV photons. A comparison of the observed ratio between the number of photons absorbed by the excited state and those in the Fe II ground state suggests that the six absorbers are ~ 2-6 kpc from the GRB site, with component I ~ 3 times closer to the GRB site than components III to VI. Component I is characterized also by the lack of Mg I absorption, unlike all other components. This may be due both to a closer distance and a lower density, suggesting a structured ISM in this galaxy complex.
