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

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

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

Elaborazione e visualizzazione di modelli di grandi dimensioni di terreni

2006-11-22Sardegna Ricerche, Edificio 2, Località Piscinamanna 09010 Pula (CA) - ItaliaGis Day 2006: dal Gis al Geo-We

The effect of X-ray dust-scattering on a bright burst from the magnetar 1E 1547.0-5408

A bright burst, followed by an X-ray tail lasting ~10 ks, was detected during an XMM-Newton observation of the magnetar 1E 1547.0-5408 carried out on 2009 February 3. The burst, also observed by SWIFT/BAT, had a spectrum well fit by the sum of two blackbodies with temperatures of ~4 keV and 10 keV and a fluence in the 0.3-150 keV energy range of ~1e-5 erg/cm2. The X-ray tail had a fluence of ~4e-8 erg/cm2. Thanks to the knowledge of the distances and relative optical depths of three dust clouds between us and 1E 1547.0-5408, we show that most of the X-rays in the tail can be explained by dust scattering of the burst emission, except for the first ~20-30 s. We point out that other X-ray tails observed after strong magnetar bursts may contain a non-negligible contribution due to dust scattering.Comment: 8 pages, 2 tables and 10 figures; accepted to publication in MNRA

Advances in massive model visualization in the CYBERSAR project

We provide a survey of the major results obtained within the CYBERSAR project in the area of massive data visualization. Despite the impressive improvements in graphics and computational hardware performance, interactive visualization of massive models still remains a challenging problem. To address this problem, we developed methods that exploit the programmability of latest generation graphics hardware, and combine coarse-grained multiresolution models, chunk-based data management with compression, incremental view-dependent level-of-detail selection, and visibility culling. The models that can be interactively rendered with our methods range from multi-gigabyte-sized datasets for general 3D meshes or scalar volumes, to terabyte-sized datasets in the restricted 2.5D case of digital terrain models. Such a performance enables novel ways of exploring massive datasets. In particular, we have demonstrated the capability of driving innovative light field displays able of giving multiple freely moving naked-eye viewers the illusion of seeing and manipulating massive 3D objects with continuous viewer-independent parallax.233-23

Discovery of hard phase lags in the pulsed emission of GRO J1744-28

We report on the discovery and energy dependence of hard phase lags in the 2.14 Hz pulsed profiles of GRO J1744-28. We used data from XMM-Newton and NuSTAR. We were able to well constrain the lag spectrum with respect to the softest (0.3--2.3 keV) band: the delay shows increasing lag values reaching a maximum delay of $\sim$ 12 ms, between 6 and 6.4 keV. After this maximum, the value of the hard lag drops to 7 ms, followed by a recovery to a plateau at 9 ms for energies above 8 keV. NuSTAR data confirm this trend up to 30 keV, but the measurements are statistically poorer, and therefore, less constraining. The lag-energy pattern up to the discontinuity is well described by a logarithmic function. Assuming this is due to a Compton reverberation mechanism, we derive a size for the Compton cloud $R_{\rm{cc}}$ $\sim$ 120 $R_{\rm g}$, consistent with previous estimates on the magnetospheric radius. In this scenario, the sharp discontinuity at $\sim$ 6.5 keV appears difficult to interpret and suggests the possible influence of the reflected component in this energy range. We therefore propose the possible coexistence of both Compton and disk reverberation to explain the scale of the lags and its energy dependence.Comment: Accepted for publication in MNRAS Letters on 2016 June 0

Discovery of spin-up in the X-ray pulsar companion of the hot subdwarf HD 49798

The hot subdwarf HD 49798 has an X-ray emitting compact companion with a spin-period of 13.2 s and a dynamically measured mass of 1.28+/-0.05 M_sun, consistent with either a neutron star or a white dwarf. Using all the available XMM-Newton and Swift observations of this source, we could perform a phase-connected timing analysis extending back to the ROSAT data obtained in 1992. We found that the pulsar is spinning up at a rate of (2.15+/-0.05)x10^{-15} s/s. This result is best interpreted in terms of a neutron star accreting from the wind of its subdwarf companion, although the remarkably steady period derivative over more than 20 years is unusual in wind-accreting neutron stars. The possibility that the compact object is a massive white dwarf accreting through a disk cannot be excluded, but it requires a larger distance and/or properties of the stellar wind of HD 49798 different from those derived from the modelling of its optical/UV spectra.Comment: Accepted for publication in MNRA

Study of the reflection spectrum of the accreting neutron star GX 3+1 using XMM-Newton and INTEGRAL

Broad emission features of abundant chemical elements, such as Iron, are commonly seen in the X-ray spectra of accreting compact objects and their studies can provide useful information about the geometry of the accretion processes. In this work, we focus our attention on GX 3+1, a bright, persistent accreting low mass X-ray binary, classified as an atoll source. Its spectrum is well described by an accretion disc plus a stable comptonizing, optically thick corona which dominates the X-ray emission in the 0.3-20 keV energy band. In addition, four broad emission lines are found and we associate them with reflection of hard photons from the inner regions of the accretion disc where doppler and relativistic effects are important. We used self-consistent reflection models to fit the spectra of the 2010 XMM-Newton observation and the stacking of the whole datasets of 2010 INTEGRAL observations. We conclude that the spectra are consistent with reflection produced at ~10 gravitational radii by an accretion disc with an ionization parameter of xi~600 erg cm/s and viewed under an inclination angle of the system of ~35{\deg}. Furthermore, we detected for the first time for GX 3+1, the presence of a powerlaw component dominant at energies higher than 20 keV, possibly associated with an optically thin component of non-thermal electrons.Comment: Accepted to appear on MNRAS, 9 pages, 5 figur