Da Milano a Galliano: qualche nuova riflessione sulla committenza dell'arcivescovo Ariberto di Intimiano

Uomo di chiesa e di potere, dotato di un carattere energico e ambizioso, di un forte carisma e di un\u2019indomita coscienza del proprio ruolo istituzionale, Ariberto di Intimiano \ue8 stato uno dei pi\uf9 grandi arcivescovi di Milano (1018-1045), fiero erede di sant\u2019Ambrogio, il fondatore della chiesa milanese, dal cui esempio sembra aver tratto ispirazione per la propria azione politica e pastorale, come attestano le testimonianze fornite dalle fonti e gli indizi rintracciabili nelle numerose opere d\u2019arte da lui promosse e/o commissionate, a partire dalla ristrutturazione e dalla decorazione nel 1007 della chiesa plebana di San Vincenzo a Galliano, di cui era stato custode negli anni iniziali del suo cursus honorum. Proprio una attenta rilettura delle fonti documentarie, delle notizie fornite dalle cronache dell\u2019epoca, dei dati archeologici e delle evidenze pittoriche pertinenti a questo cantiere, poste in relazione con il corpus delle altre opere promosse dall\u2019arcivescovo e con i caratteri della sua azione e della sua personalit\ue0 messe a fuoco dagli studi pi\uf9 recenti, permette di avanzare un\u2019inedita interpretazione degli interventi eseguiti a Galliano per volere del prelato e di cogliervi l\u2019emblematico avvio di quel piano preordinato di assimilazione della propria figura a quella di sant\u2019Ambrogio su cui Ariberto incardin\uf2 tutta la sua successiva azione politica e pastorale

Il complesso del ponte di San Rocco a Vimercate. Identit\ue0 e storia

Con le sue due poderose porte medievali a torre passante poste a inquadrare i resti di un ponte in pietra d'et\ue0 romana, quello di San Rocco a Vimercate \ue8 l'unico ponte fortificato d'et\ue0 antica e medievale sopravvissuto in Lombardia. Frutto di una serie di campagne costruttive scalate sull'arco di oltre mille e cinquecento anni, si presenta come un complesso edilizio stratificato, segnato da numerosi interventi di trasformazione, manomissione e restauro, che vi hanno determinato profonde mutazioni degli assetti strutturali e funzionali, trasformandolo in una sorta di palinsesto architettonico di non aguile lettura. Sorto probabilmente nel IV secolo come ponte a cinque arcate al servizio della strada che dal vicus romano di Vimercate si indirizzava verso l'attraversamento dell'Adda, dopo una presumibile fase di decadenza altomedievale viene recuperato nell'XI secolo come elemento liminare della perimetrazione del borgo e fornito nel XII secolo di una porta a torre passante aperta alla gola, eretta sulla testata occidentale. Nella seconda met\ue0 del XIV secolo, nell'ambito forse dei vasti interventi di realizzazione di una efficiente scacchiera difensiva dello stato promossa dai Visconti, la porta gi\ue0 esistente venne soprelevata e fronteggiata, sulla testata orientale del ponte, da un altro accesso a torre passante, la cui fronte, venute meno le necessit\ue0 difensive del borgo con il passaggio dello stato di milano alla corona spagnola nel 1535, fu per\uf2 riedificata nel XVI secolo, in uno con lo smantellamento delle attrezzature difensive dell'edificio. Che sopravvisse cos\uec per un paio di secoli, finch\ue8, all'inizio dell'Ottocento, non venne ridotto in lunghezza con l'abbattimento della prima arcata est del vallo. Scampato alla distruzione cui sono incorse nella seconda met\ue0 del XIX secolo le altre due porte della cinta muraria vimercate, nel Novecento il complesso \ue8 stato sottoposto a una serie di restauri, l'ultimo dei quali condotto nel 2002. Risultato di una ricerca decennale che non ha trascurato nessun tipo di indagine e/o testimonianza (dall'analisi stratigrafica degli elevati alla lettura dei dati stilistici, dalla ricerca delle fonti documentarie e letterarie alla riscoperta e valorizzazione di quelle iconografiche, dallo studio del contesto ambientale alla ricostruzione delle vicende storiche e militari in cui l'organismo pot\ue8 essere coinvolto) la monografia affronta per la prima volta lo studio dell'edificio in tutti i suoi aspetti: da quelli architettonici e archeologici (tramite un'approfondita analisi stratigrafica degli elevati, analiticamente descritta nel testo e documentata da rilievi stratigrafici e matrici di harris appositamente apprestati), a quelli funzionali, a quelli simboli. Ne risulta una microstoria dalla quale il ponte di San Rocco emerge come una sorta di ideogramma del borgo di Vimercate, che in un certo qual modo riflette e riassume nel divenire delle sue forme, nelle sue complesse vicende architettoniche, nel mutare delle sue funzioni e nella sua fortuna visiva l'identit\ue0 e la storia dell'insediamento

Sensitivity analysis of cohesive zone model parameters to simulate hydrogen embrittlement effect

For many steels and alloys used in engineering field, the presence of atomic hydrogen in working environment can produce a deleterious effect. In fact, when this small element penetrates into the material lattice induces a drastically decrease of the mechanical properties. This process is known as hydrogen embrittlement. This complex phenomenon involves chemical and physical factors that are strictly dependent on the microstructure of the material. Some examples are hydrogen diffusivity, solubility of hydrogen into the material and concentration related not only to the interstitial lattice sites (NILS) but also to the traps sites that is the most difficult part to quantify. The present work starts from the development of 2D finite elements cohesive zone model reproducing a toughness test of a high-strength low carbon steel, AISI 4130 operating in hydrogen-contaminated environment. With three consequent steps of simulations, the model implements diffusion and crack propagation analyses using cohesive elements. The embrittlement effect of hydrogen is considered by decreasing the cohesive law (TSL), which expresses the constitutive response of the material to the fracture behavior, based on the total hydrogen concentration. It includes NILS and traps sites. Aim of the work is a sensitivity analysis of the parameters included into the model. In particular, the influence of the hydrogen diffusion coefficient as well as the initial concentration set to calculate the total hydrogen concentration at the crack tip are taken into account. Both a comparison of the values used in the model with literature data and a critical discussion of the results obtained by the sensitivity analysis will be presented

Effective absorbing column density in the gamma-ray burst afterglow X-ray spectra

We investigate the scaling relation between the observed amount of absorption in the X-ray spectra of Gamma Ray Burst (GRB) afterglows and the absorber redshift. Through dedicated numerical simulations of an ideal instrument, we establish that this dependence has a power law shape with index 2.4. However, for real instruments, this value depends on their low energy cut-off, spectral resolution and on the detector spectral response in general. We thus provide appropriate scaling laws for specific instruments. Finally, we discuss the possibility to measure the absorber redshift from X-ray data alone. We find that 10^5-10^6 counts in the 0.3-10 keV band are needed to constrain the redshift with 10% accuracy. As a test case we discuss the XMM-Newton observation of GRB 090618 at z=0.54. We are able to recover the correct redshift of this burst with the expected accuracy.Comment: MNRAS accepted. 6 figures. 3 table

A complete sample of bright Swift Long Gamma Ray Bursts: testing the spectral-energy correlations

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 Ep of the prompt emission, the isotropic energetics Eiso and the isotropic luminosity Liso. This GRB sample is characterized by a high level of completeness in redshift (90%). 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 Ep-Eiso correlation. Despite this case, we find strong Ep-Eiso and Ep-Liso correlations for the bursts of the complete sample. Their slopes, normalisations and dispersions are consistent with those found with the whole sample of bursts with measured redshift and Ep. 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 Ep-Eiso and Ep-Liso correlations.Comment: MNRAS in press, 9 pages, 4 figures, 2 tables. This version matches the published version in MNRA

GRB orphan afterglows in present and future radio transient surveys

Orphan Afterglows (OA) are slow transients produced by Gamma Ray Bursts seen off-axis that become visible on timescales of days/years at optical/NIR and radio frequencies, when the prompt emission at high energies (X and gamma rays) has already ceased. Given the typically estimated jet opening angle of GRBs theta_jet ~ 3 deg, for each burst pointing to the Earth there should be a factor ~ 700 more GRBs pointing in other directions. Despite this, no secure OAs have been detected so far. Through a population synthesis code we study the emission properties of the population of OA at radio frequencies. OAs reach their emission peak on year-timescales and they last for a comparable amount of time. The typical peak fluxes (which depend on the observing frequency) are of few micro Jy in the radio band with only a few OA reaching the mJy level. These values are consistent with the upper limits on the radio flux of SN Ib/c observed at late times. We find that the OA radio number count distribution has a typical slope -1.7 at high fluxes and a flatter (-0.4) slope at low fluxes with a break at a frequency-dependent flux. Our predictions of the OA rates are consistent with the (upper) limits of recent radio surveys and archive searches for radio transients. Future radio surveys like VAST/ASKAP at 1.4 GHz should detect ~ 3x10^-3 OA deg^-2 yr-1, MeerKAT and EVLA at 8.4 GHz should see ~ 3x10^-1 OA deg-2 yr-1. The SKA, reaching the micro Jy flux limit, could see up to ~ 0.2-1.5 OA deg^-2 yr^-1. These rates also depend on the duration of the OA above a certain flux limit and we discuss this effect with respect to the survey cadence.Comment: (10 pages, 5 figures, 1 table) Accepted for publication by PAS

Optical and X-ray Rest-frame Light Curves of the BAT6 sample

We present the rest-frame light curves in the optical and X-ray bands of an unbiased and complete sample of Swift long Gamma-Ray Bursts (GRBs), namely the BAT6 sample. The unbiased BAT6 sample (consisting of 58 events) has the highest level of completeness in redshift ($\sim$ 95%), allowing us to compute the rest-frame X-ray and optical light curves for 55 and 47 objects, respectively. We compute the X-ray and optical luminosities accounting for any possible source of absorption (Galactic and intrinsic) that could affect the observed fluxes in these two bands. We compare the behaviour observed in the X-ray and in the optical bands to assess the relative contribution of the emission during the prompt and afterglow phases. We unarguably demonstrate that the GRBs rest-frame optical luminosity distribution is not bimodal, being rather clustered around the mean value Log(L$_{R}$) = 29.9 $\pm$ 0.8 when estimated at a rest frame time of 12 hr. This is in contrast with what found in previous works and confirms that the GRB population has an intrinsic unimodal luminosity distribution. For more than 70% of the events the rest-frame light curves in the X-ray and optical bands have a different evolution, indicating distinct emitting regions and/or mechanisms. The X-ray light curves normalised to the GRB isotropic energy (E$_{\rm iso}$), provide evidence for X-ray emission still powered by the prompt emission until late times ($\sim$ hours after the burst event). On the other hand, the same test performed for the E$_{\rm iso}$-normalised optical light curves shows that the optical emission is a better proxy of the afterglow emission from early to late times.Comment: Accepted for publication in A&A: 10 pages, 5 figures, 2 table

Unveiling the population of orphan Gamma Ray Bursts

Gamma Ray Bursts are detectable in the gamma-ray band if their jets are oriented towards the observer. However, for each GRB with a typical theta_jet, there should be ~2/theta_jet^2 bursts whose emission cone is oriented elsewhere in space. These off-axis bursts can be eventually detected when, due to the deceleration of their relativistic jets, the beaming angle becomes comparable to the viewing angle. Orphan Afterglows (OA) should outnumber the current population of bursts detected in the gamma-ray band even if they have not been conclusively observed so far at any frequency. We compute the expected flux of the population of orphan afterglows in the mm, optical and X-ray bands through a population synthesis code of GRBs and the standard afterglow emission model. We estimate the detection rate of OA by on-going and forthcoming surveys. The average duration of OA as transients above a given limiting flux is derived and described with analytical expressions: in general OA should appear as daily transients in optical surveys and as monthly/yearly transients in the mm/radio band. We find that ~ 2 OA yr^-1 could already be detected by Gaia and up to 20 OA yr^-1 could be observed by the ZTF survey. A larger number of 50 OA yr^-1 should be detected by LSST in the optical band. For the X-ray band, ~ 26 OA yr^-1 could be detected by the eROSITA. For the large population of OA detectable by LSST, the X-ray and optical follow up of the light curve (for the brightest cases) and/or the extensive follow up of their emission in the mm and radio band could be the key to disentangle their GRB nature from other extragalactic transients of comparable flux density.Comment: 9 pages, 4 figures, 2 tables. Accepted for publication by Astronomy and Astrophysic