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The GRB 190829A has been widely studied due to its nature and the high energy emission presented. Due to the detection of a very-high-energy component by the High Energy Stereoscopic System and the event's atypically middling luminosity, it has been categorized in a select, limited group of bursts bordering classic GRBs and nearby sub-energetic events. Given the range of models utilized to adequately characterize the afterglow of this burst, it has proven challenging to identify the most probable explanation. Nevertheless, the detection of polarization data provided by the MASTER collaboration has added a new aspect to GRB 190829A that permits us to attempt to explore this degeneracy. In this paper, we present a polarization model coupled with a synchrotron forward-shock model -- a component in all models used to describe GRB 190829A's afterglow -- in order to fit the polarization's temporal evolution with the existing upper limits ($\Pi < 6\%$). We find that the polarization generated from an on-axis emission is favored for strongly anisotropic magnetic field ratios, while an off-axis scenario cannot be fully ruled out when a more isotropic framework is taken into account., Comment: 16 pages, 7 figures, 1 table

As we further our studies on Gamma-ray bursts (GRBs), both on theoretical models and observational tools, more and more options begin to open for exploration of its physical properties. As transient events primarily dominated by synchrotron radiation, it is expected that the synchrotron photons emitted by GRBs should present some degree of polarization throughout the evolution of the burst. Whereas observing this polarization can still be challenging due to the constraints on observational tools, especially for short GRBs, it is paramount that the groundwork is laid for the day we have abundant data. In this work, we present a polarization model linked with an off-axis spreading top-hat jet synchrotron scenario in a stratified environment with a density profile $n(r)\propto r^ {-k}$. We present this model's expected temporal polarization evolution for a realistic set of afterglow parameters constrained within the values observed in the GRB literature for four degrees of stratification $k=0,1,1.5 {\rm \, and\,} 2$ and two magnetic field configurations with high extreme anisotropy. We apply this model and predict polarization from a set of GRBs exhibiting off-axis afterglow emission. In particular, for GRB 170817A, we use the available polarimetric upper limits to rule out the possibility of a extremely anisotropic configuration for the magnetic field., Comment: 17 pages, 3 tables, 6 figures. Accepted for publication in the Astrophysical Journal (ApJ)

Relativistic jets propagating through an ambient medium must produce some observational effects along their side boundaries because of interactions across the large velocity gradient. One possible effect of such an interaction would be a sheared magnetic field structure at the jet boundaries, leading to a characteristic radio polarization pattern. As proposed by Ostrowski, another effect can come from the generation of a high energy cosmic ray component at the boundary, producing dynamic effects on the medium surrounding the jet and forming a cocoon dominated by cosmic rays with a decreased thermal gas emissivity. We selected this process for our first attempt to look for the effects of this type of interaction. We analyzed the Chandra X-ray data for the radio galaxy M87 in order to verify if the expected regions of diminished emissivity may be present near the spectacular X-ray jet in this source. The detailed analysis of the data, merged from 42 separate observations, shows signatures of lower emissivity surrounding the jet. In particular we detect an intensity dip along the part of the jet, which would be approximately 150 pc x 2 kpc in size, if situated along the jet which is inclined toward us. Due to a highly non-uniform X-ray background in the central region we are not able to claim the discovery of a cosmic ray cocoon around the M87 jet: we only have demonstrated that the data show morphological structures which could be accounted for if a cosmic ray cocoon exists., Comment: 8 pages, 8 pictures accepted for publication in MNRAS

We present the Hubble diagram (HD) of 66 Gamma Ray Bursts (GRBs) derived using only data from their X - ray afterglow lightcurve. To this end, we use the recently updated L_X - T_a correlation between the break time T_a and the X - ray luminosity L_X measured at T_a calibrated from a sample of Swift GRBs with lightcurves well fitted by the Willingale et al. (2007) model. We then investigate the use of this HD to constrain cosmological parameters when used alone or in combination with other data showing that the use of GRBs leads to constraints in agreement with previous results in literature. We finally argue that a larger sample of high luminosity GRBs can provide a valuable information in the search for the correct cosmological model., Comment: 6 pages, 2 figures, 2 tables, submitted

THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRBs and X-ray transients detection over a broad FOV (more than 1sr) with 0.5-1 arcmin localization, an energy band extending from several MeVs down to 0.3 keV and high sensitivity to transient sources in the soft X-ray domain, as well as on-board prompt (few minutes) follow-up with a 0.7 m class IR telescope with both imaging and spectroscopic capabilities. THESEUS will be perfectly suited for addressing the main open issues in cosmology such as, e.g., star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift ∼10, signatures of Pop III stars, sources and physics of re-ionization, and the faint end of the galaxy luminosity function. In addition, it will provide unprecedented capability to monitor the X-ray variable sky, thus detecting, localizing, and identifying the electromagnetic counterparts to sources of gravitational radiation, which may be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/ aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUS will also provide powerful synergies with the next generation of multi-wavelength observatories (e.g., LSST, ELT, SKA, CTA, ATHENA).

Thanks to their enormous energy release, Gamma Rays Bursts (GRBs) have recently attracted a lot of interest to probe the Hubble diagram (HD) deep into the matter dominated era and hence complement Type Ia Supernovae (SNeIa). We consider here three different calibration methods based on the use of a fiducial LCDM model, on cosmographic parameters and on the local regression on SNeIa to calibrate the scaling relations proposed as an equivalent to the Phillips law to standardize GRBs finding any significant dependence. We then investigate the evolution of these parameters with the redshift to obtain any statistical improvement. Under this assumption, we then consider possible systematics effects on the HDs introduced by the calibration method, the averaging procedure and the homogeneity of the sample arguing against any significant bias., 5 pages, 1 figure to appear in the Proceedings of Frascati Workshop 2011 (Vulcano, Italy, 23th - 28th May)

Emerging high redshift cosmological probes, in particular quasars (QSOs), show a preference for larger matter densities, $\Omega_{m} \approx 1$, within the flat $\Lambda$CDM framework. Here, using the Risaliti-Lusso relation for standardizable QSOs, we demonstrate that the QSOs recover the \textit{same} Planck-$\Lambda$CDM Universe as Type Ia supernovae (SN), $\Omega_m \approx 0.3$ at lower redshifts $ 0 < z \lesssim 0.7$, before transitioning to an Einstein-de Sitter Universe ($\Omega_m =1$) at higher redshifts $z \gtrsim 1$. We illustrate the same trend, namely increasing $\Omega_{m}$ and decreasing $H_0$ with redshift, in SN but poor statistics prevent a definitive statement. We explain physically why the trend is expected in the flat $\Lambda$CDM cosmology, illustrate the intrinsic bias and non-Gaussian tails with mock Pantheon data, and identify a similar trend in BAO below $z=1$. Our results highlight an intrinsic bias in the flat $\Lambda$CDM Universe, whereby $\Omega_m$ increases, $H_0$ decreases and $S_8$ increases with effective redshift, thus providing a new perspective on $\Lambda$CDM tensions; even in a Planck-$\Lambda$CDM Universe the current tensions might have been expected., Comment: 6 pages, 7 figures, comments welcome; v2 matches published version

The Large Area Telescope (LAT) on board the \Fermi Gamma-ray Space Telescope (\Fermi) shows long-lasting high-energy emission in many gamma-ray bursts (GRBs), similar to X-ray afterglows observed by the Neil Gehrels Swift Observatory \citep[\textit{Swift};][]{gehrels2004}. Some LAT light curves (LCs) show a late-time flattening reminiscent of X-ray plateaus. We explore the presence of plateaus in LAT temporally extended emission analyzing GRBs from the second \lat GRB Catalog \citep[2FLGC;][]{Ajello2019apj} from 2008 to May 2016 with known redshifts, and check whether they follow closure relations corresponding to 4 distinct astrophysical environments predicted by the external forward shock (ES) model. We find that three LCs can be fit by the same phenomenological model used to fit X-ray plateaus \citep{Willingale2007} and show tentative evidence for the existence of plateaus in their high-energy extended emission. The most favorable scenario is a slow cooling regime, whereas the preferred density profile for each GRBs varies from a constant density ISM to a $r^{-2}$ wind environment. We also compare the end time of the plateaus in $\gamma$-rays and X-rays using a statistical comparison with 222 \textit{Swift} GRBs with plateaus and known redshifts from January 2005 to August 2019. Within this comparison, the case of GRB 090510 shows an indication of chromaticity at the end time of the plateau. Finally, we update the 3-D fundamental plane relation among the rest frame end time of the plateau, its correspondent luminosity, and the peak prompt luminosity for 222 GRBs observed by \textit{Swift}. We find that these three LAT GRBs follow this relation., Comment: 28 pages, 10 pictures, 3 Tables to appear on ApJ Supplement

A large fraction of Gamma-Ray Bursts (GRBs) lightcurves (LCs) shows X-ray plateaus. We analyze all GRBs with known redshifts presenting plateaus observed by \emph{The Neil Gehrels Swift Observatory} from its launch until August 2019. The fundamental plane relation between the rest-frame time and X-ray luminosity at the end of the plateau emission and the peak prompt luminosity holds for all the GRB classes when selection biases and cosmological evolutions are applied. We have discovered two important findings: 1) a new class of Long GRBs with good data coverage: the Platinum Sample; 2) the Platinum, the SNe-LGRB and the KN-SGRB samples, the second sample composed of GRBs associated spectroscopically with the SNe Ib,c, the third sample composed by 8 GRBs associated with Kilonovae or where there could have been such an association, yield the smallest intrinsic scatter, $\sigma_{platinum,GRB-SNe}=0.22 \pm 0.10$ and $\sigma_{KN-SGRB}=0.24 \pm 0.12$. The highest correlation coefficients yield for the SN-LGRB-ABC sample, which are GRBs spectroscopically associated with SNe Ib/c or with a clear optical bump in the LC resembling the SNe Ib/c, ($R^{2}_{SN-LGRB-ABC}=0.95$), for the SN-LGRBs ($R^{2}_{SN-LGRB}=0.91$) and the KN-SGRBs ($R^{2}_{KN-SGRB}=0.90$) when the redshift evolution is considered. These category planes are reliable candidates to be used as cosmological tools. Furthermore, the distance from the Gold fundamental plane is a crucial discriminant among classes. In fact, we find that the distributions of the distances of the SNe-LGRB, SNe-LGRB-ABC, KN-SGRBs and SGRBs samples from the Gold fundamental plane are statistically different from the distribution of the Gold GRBs' distances from the Gold fundamental plane with and without considering evolution cases., Comment: ApJ accepted, to be published 25th november, 19 Pages, 9 Figures, 6 Tables. Press Release has been issued by https://www.riken.jp/press/2020/20201119_3/index.html and http://www.oa.uj.edu.pl/AOM/aom20nov.en.html

Gamma-ray bursts (GRBs) visible up to very high redshift have become attractive targets as potential new distance indicators. It is still not clear whether the relations proposed so far originate from an unknown GRB physics or result from selection effects. We investigate this issue in the case of the L{sub X} -T{sub a}{sup ∗} (hereafter LT) correlation between the X-ray luminosity L{sub X} (T{sub a} ) at the end of the plateau phase, T{sub a} , and the rest-frame time T{sub a}{sup ∗}. We devise a general method to build mock data sets starting from a GRB world model and taking into account selection effects on both time and luminosity. This method shows how not knowing the efficiency function could influence the evaluation of the intrinsic slope of any correlation and the GRB density rate. We investigate biases (small offsets in slope or normalization) that would occur in the LT relation as a result of truncations, possibly present in the intrinsic distributions of L{sub X} and T{sub a}{sup ∗}. We compare these results with the ones in Dainotti et al. showing that in both cases the intrinsic slope of the LT correlation is ≈ – 1.0. This method is general andmore » therefore relevant for investigating whether or not any other GRB correlation is generated by the biases themselves. Moreover, because the farthest GRBs and star-forming galaxies probe the reionization epoch, we evaluate the redshift-dependent ratio Ψ(z) = (1 + z){sup α} of the GRB rate to the star formation rate. We found a modest evolution –0.2 ≤ α ≤ 0.5 consistent with a Swift GRB afterglow plateau in the redshift range 0.99 < z < 9.4.« less