A new investigation of the possible X-ray counterparts of the magnetar candidate AX J1845-0258

AX J1845-0258 is a transient X-ray pulsar, with spin period of 6.97s, discovered with the ASCA satellite in 1993. Its soft spectrum and the possible association with a supernova remnant suggest that AX J1845-0258 might be a magnetar, but this has not been confirmed yet. A possible counterpart one order of magnitude fainter, AX J184453-025640, has been found in later X-ray observations, but no pulsations have been detected. In addition, some other X-ray sources are compatible with the pulsar location, which is in a crowded region of the Galactic plane. We have carried out a new investigation of all the X-ray sources in the ASCA error region of AX J1845-0258, using archival data obtained with Chandra in 2007 and 2010, and with XMM-Newton in 2010. We set an upper limit of 6% on the pulsed fraction of AX J184453-025640 and confirmed its rather hard spectrum (power law photon index of 1.2 +\- 0.3). In addition to the other two fainter sources already reported in the literature, we found other X-ray sources positionally consistent with AX J1845-0258. Although many of them are possibly foreground stars likely unrelated to the pulsar, at least another new source, CXOU J184457.5-025823, could be a plausible counterpart of AX J1845-0258. It has a flux of 6x10^{-14} erg cm^{-2} s^{-1} and a spectrum well fit by a power law with photon index ~1.3 and Nh ~ 10^{22} cm^{-2}.Comment: 6 pages, 6 figures, 3 tables; accepted for publication in MNRA

Ultra Luminous X-ray Sources: a deeper insight into their spectral evolution

We select a sample of nearby Ultraluminous X-ray sources with long XMM-Newton observations and analyse all the available XMM-Newton data using both X-ray spectral fitting techniques and hardness-intensity diagrams. The sample includes IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg II X-1, NGC 1313 X-1, NGC 1313 X-2 and NGC 253 X-1. We found that, although a common reference model can be used to describe the X-ray spectra, the sources show different spectral evolutions, phenomenologically described in terms of variations in the properties of a soft component and a high energy tail. Variations at low energies are accounted for (mostly) by changes in the normalization of the soft component and/or in the column density to the source, while variations in the high energy tail by changes in the parameters of an optically thick corona. This spectral variability is rather well characterized on a colour-colour and hardness-intensity diagram in terms of suitably defined hardness ratios. We suggest the existence of a variability pattern on the hardness-intensity diagram and we interpret it in terms of the switch between a near-Eddington and a super-Eddington accretion regime. The transition between the two regimes seems to be driven mostly by changes in the contribution of the soft component, which can be explained in terms of the increasing importance of wind emission. The analysis is complemented by an investigation of the short-term time variability of all the sources. In general, no clear correlation between the spectral and temporal properties is found.Comment: 15 pages, 7 figures, 5 tables. Accepted for publication in MNRA

A rule-governed gunman writ large? Il posto della coercizione ne "Il Concetto di Diritto"

Título del seminario: "50º aniversario de The Concept of Law"Premessa. Ci sono molte ragioni per dolersi della piega che ha preso il dibattito filosoficogiuridico post-hartiano, specie dopo la pubblicazione del Postscript. Vorrei qui focalizzarmi su una di esse, vale a dire sulla pressoché totale assenza, dall’agenda di questa discussione, del tema della coercizione. Quest’assenza non è casuale ma è il frutto di una precisa opzione metateorica. L’idea è che la coercizione non rappresenta un elemento necessario del diritto o, come anche si usa dire, non è parte della sua natura; essa va perciò affidata alla cura dei sociologi, dato che alla teoria compete dedicarsi ai soli aspetti necessari del diritto, precipuamente alla sua autorità o normatività che dir si vogli

«A Rule-Governed Gunman Writ Large?» The rol of coertion in The Concept of Law

Frente a las opiniones que consideran que la coerción ha ido perdiendo su lugar central en la teoría del Derecho y sitúan el comienzo de esta tendencia en El concepto de derecho, este artículo trata de reivindicar el tratamiento de la coerción por parte de Hart en tres momentos de su obra: 1) el análisis de las reglas y de las obligaciones sociales; 2) la crítica a Austin y Kelsen y la elaboración de la teoría de los legal systems; y 3) el tratamiento del contenido mínimo del derecho natural y del derecho internacional. La conclusión de la autora es que «El concepto de derecho no marca en absoluto una ruptura de la tradición iuspositivista en tema de coerción sino que se coloca en línea de continuidad con ella».Objecting to those who claim that coercion has been losing its central role in Legal Theory and considered that the point of departure of that new tendency is to be found in The concept of Law, this paper vindicates Harts way to address the topic on coercion in three moments of his work: 1) His analyses of rules and social obligation; 2) His objection against Austin and Kelsen and his elaboration of a theory of legal systems; 3) His way to address the minimum content of Natural Law and International Law. The conclusion of this paper is to consider The concept of Law fully align with the iuspositivist tradition on this issue rejecting it to be any departure or turning point

On the security of the Blockchain Bix Protocol and Certificates

The BIX protocol is a blockchain-based protocol that allows distribution of certificates linking a subject with his public key, hence providing a service similar to that of a PKI but without the need of a CA. In this paper we analyze the security of the BIX protocol in a formal way, in four steps. First, we identify formal security assumptions which are well-suited to this protocol. Second, we present some attack scenarios against the BIX protocol. Third, we provide a formal security proof that some of these attacks are not feasible under our previously established assumptions. Finally, we show how another attack may be carried on.Comment: 16 pages, 1 figur

Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1

NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We analysed a sample of 20 Swift observations, taken between 2013 April and August, and two Chandra observations taken in 2001 September and 2004 June. We found only marginal hints of a limited number of dips in the light curve, previously reported to occur in this source, although the uncertainties due to the low counting statistics of the data are large. The Chandra and Swift spectra showed clearly spectral variability which resembles those observed in other ULXs. We can account for this spectral variability in terms of changes in both the normalization and intrinsic column density of a two-components model consisting of a blackbody (for the soft component) and a multicolour accretion disc (for the hard component). We discuss the possibility that strong outflows ejected by the disc are in part responsible for such spectral changes.Comment: 9 pages, 6 figure; accepted to be published on MNRA

X-ray spectral states and metallicity of Ultra Luminous X-ray sources: a deeper insight into their spectral properties

Ultraluminous X-ray sources (ULXs) are a class of extragalactic, off nuclear and point-like sources with isotropic X-ray luminosities higher than 1e39 erg/s. They are supposed to be accreting Black Hole binaries systems but the accretion mechanisms at the basis of their extremely high X-ray luminosity are still matter of debate. We carried out a detailed spectral analysis of all the available XMM-Newton observations of two ULXs in NGC 1313, adopting a common model based on a multicolor disc plus a comptonizing component. We were able to describe the spectral evolution of the two sources within such a common framework. Furthermore, we investigated the chemical abundances of their local environments making use of both EPIC and RGS data. The results appear to indicate sub-solar metallicity for both sources. The possible existence of two spectral states in NGC 1313 X-1 and X-2 suggested to look for similar behaviours also in other ULXs. We then studied a larger sample of sources, including IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg II X-1, NGC 55 ULX1 and NGC 253 X-1. These sources were selected because they have a luminosity higher than 2e39 erg/s, are nearby, have one long observation and at least three other observations. The high quality observations provide at least 10000 counts in the EPIC instruments allowing us to constrain the curvature at high energy and to perform an analysis of the abundances of the material along the line of sight. We found that, in most of the spectra of the sources of our sample, the high energy component has a low temperature and is optically thick. However, because of the poor quality of some observations, the spectral fits are sometimes affected by a degeneracy between the spectral parameters and the roll-over of the spectrum at high energy is not easy to detect. For these reasons, similarly to what has been done for low counting statistics spectra of Galactic X-ray binaries (XRBs), we adopted the method of the hardness ratios that has also the advantage to allow us to study the spectral variability in a way completely independent of the spectral models. This analysis suggests the existence of possible characteristic evolutionary patterns on the color-color and intensity-color diagrams linking at least two different spectral states. This behaviour can be explained in terms of a non-standard accretion disc in which the increment of the accretion rate produces outflows that become more and more important at the highest luminosities. We tested the scenario of the ejection of a wind jointly analyzing the spectral and timing properties of the source NGC 55 ULX1 which shows a puzzling flux variability. In fact, fast drops in the flux are observed on time scales of minutes to hours that may be produced by optically thick blobs of matter that from time to time encounter our line of sight. We compared its variability properties with those of a Galactic accreting systems, EXO 0748-676, which is powered by a neutron star and is a known dipping source. We characterised the nature of the variability observed in the power density spectrum and, in particular, we checked the presence of a linear relation between the Root Mean Square (RMS) variability and the flux in several energy bands. We found that, in EXO 0748-676, the predominance of an (ionised) absorber strongly affects the RMS-flux relation which may anticorrelate when the absorption lines are unsaturated. On the other hand, no further variability is introduced when they are saturated and the variability is dominated by the accretion flow. In this case the source shows a positive correlation between RMS and flux. Since we found an anticorrelation in NGC ULX1, we suggest that at the highest flux levels, massive and unsaturated turbulent outflows are ejected. Finally, persistent ULXs, as those discussed above, do not allow us an easy comparison with the behaviour of Galactic XRBs. Transient ULXs are much more promising in this respect as they span different accretion regimes. Till now, only a handful of transient ULXs has been discovered and the link between them and the persistent sources is still unclear. We monitored the evolution of a new ULX (XMMU J004243.6+41251) discovered in January, 2012 in M31 by XMM-Newton. Its outburst showed that, at maximum luminosity, it entered in the ULX regime. It was then extensively followed by Swift during the flux decay. The source has experienced a fast rise in flux after discovery during which the XMM-Newton spectra changed from a powerlaw-like to a disk-like shape in the Swift spectra, suggesting a transition between the canonical low/hard and high/soft states. Its luminosity remained fairly constant for at least 40 days and then it faded below 1e38 erg/s. During the decay the disc emission softened and the temperature decreased from ~0.9 keV to ~0.5 keV. An optical follow-up and the UVOT images failed to provide evidence of a counterpart down to 22 mag in the optical band and to 23-24 mag in the near Ultraviolet. We compared the properties of XMMU J004243.6+412519 with those of other known ULXs and Galactic black hole transients, finding more similarities with the latter