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Swift J1818.0-1607 is a radio-loud magnetar with a spin period of 1.36 s and a dipolar magnetic field strength of B~3E14 G, which is very young compared to the Galactic pulsar population. We report here on the long-term X-ray monitoring campaign of this young magnetar using XMM-Newton, NuSTAR, and Swift from the activation of its first outburst in March 2020 until October 2021, as well as INTEGRAL upper limits on its hard X-ray emission. The 1-10 keV magnetar spectrum is well modeled by an absorbed blackbody with a temperature of kT_BB~1.1 keV, and apparent reduction in the radius of the emitting region from ~0.6 to ~0.2 km. We also confirm the bright diffuse X-ray emission around the source extending between ~50'' and ~110''. A timing analysis revealed large torque variability, with an average spin-down rate nudot~-2.3E-11 Hz^2 that appears to decrease in magnitude over time. We also observed Swift J1818.0-1607 with the Karl G. Jansky Very Large Array (VLA) on 2021 March 22. We detected the radio counterpart to Swift J1818.0-1607 measuring a flux density of S_v = 4.38+/-0.05 mJy at 3 GHz, and a half ring-like structure of bright diffuse radio emission located at ~90'' to the west of the magnetar. We tentatively suggest that the diffuse X-ray emission is due to a dust scattering halo and that the radio structure may be associated with the supernova remnant of this young pulsar, based on its morphology., Comment: 19 pages, 8 figures, accepted for publication on ApJ

The surface temperature distributions of central compact objects (CCOs) are powerful probes of their crustal magnetic field strengths and geometries. Here we model the surface temperature distribution of RX J0822$-$4300, the CCO in the Puppis A supernova remnant (SNR), using $471$ ks of XMM-Newton data. We compute the energy-dependent pulse profiles in sixteen energy bands, fully including the general relativistic effects of gravitational redshift and light bending, to accurately model the two heated surface regions of different temperatures and areas, in addition to constraining the viewing geometry. This results in precise measurements of the two temperatures: $kT_{\rm warm} = (1+z) \times 0.222_{-0.019}^{+0.018}$ keV and $kT_{\rm hot} = (1+z) \times 0.411\pm0.011$ keV. For the first time, we are able to measure a deviation from a pure antipodal hot-spot geometry, with a minimum value of $1.\!^{\circ}1 \pm 0.\!^{\circ}2$, and an expectation value of $9.\!^{\circ}35 \pm 0.\!^{\circ}17$ among the most probable geometries. The discovery of this asymmetry, along with the factor of $\approx2$ temperature difference between the two emitting regions, may indicate that RX J0822$-$4300 was born with a strong, tangled crustal magnetic field., Comment: 19 pages, 10 figures, Submitted to ApJ

Active galactic nuclei (AGNs) can act as black hole assembly lines, funneling some of the stellar-mass black holes from the vicinity of the galactic center into the inner plane of the AGN disk where the black holes can merge through dynamical friction and gravitational wave emission. Here, we show that stars near the galactic center are also brought into the AGN disk, where they can be tidally disrupted by the stellar-mass black holes in the disk. Such micro-tidal disruption events (micro-TDEs) could be useful probe of stellar interaction with the AGN disk. We find that micro-TDEs in AGNs occur at a rate of $\sim170$ Gpc$^{-3}$yr$^{-1}$. Their cleanest observational probe may be the detection of tidal disruption in AGNs by heavy supermassive black holes ($M_{\bullet}\gtrsim10^{8}$ M$_{\odot}$) so that cannot tidally disrupt solar-type stars. We discuss two such TDE candidates observed to date (ASASSN-15lh and ZTF19aailpwl)., Comment: 7 pages, 1 figure

After 15 years, in late 2018, the magnetar XTE J1810-197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM-Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of two change in its spin-down rate from $\sim7.2\times10^{-12}$ s s$^{-1}$ to $\sim1.5\times10^{-11}$ s s$^{-1}$ after two months. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. Following the initial outburst, the spectrum of XTE J1810-197 is well-modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. We model the energy-dependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of XTE J1810-197 has likely evolved relative to that found for the 2003 event., Comment: 14 pages, 12 figures, 4 tables. Accepted for publications in MNRAS

In this paper, we consider triple systems composed of main-sequence (MS) stars, and their internal evolution due to stellar and binary evolution. Our focus is on triples that produce white dwarfs (WDs), where Roche lobe overflow of an evolving tertiary triggers accretion onto the inner binary via a circumbinary disk (CBD) driving it toward a mass ratio of unity. We present a combination of analytic- and population synthesis-based calculations performed using the \texttt{SeBa} code to constrain the expected frequency of such systems, given a realistic initial population of MS triples, and provide the predicted distributions of orbital periods. We identify the parameter space for triples that can accommodate a CBD, to inform future numerical simulations of suitable initial conditions. We find that $\lesssim$ 10\% of all MS triples should be able to accommodate a CBD around the inner binary, and compute lower limits for the production rates. This scenario broadly predicts mergers of near equal-mass binaries, producing blue stragglers (BSs), Type Ia supernovae, gamma ray bursts and gravitational wave-induced mergers, along with the presence of an outer WD tertiary companion. We compare our predicted distributions to a sample of field BS binaries, and argue that our proposed mechanism explains the observed range of orbital periods. Finally, the mechanism considered here could produce hypervelocity MS stars, WDs and even millisecond pulsars with masses close to the Chandrasekhar mass limit, and be used to constrain the maximum remnant masses at the time of any supernova explosion., Comment: 15 pages, 7 figures; accepted for publication in MNRAS

The magnetar Swift ,J1818.0-1607 was discovered in March 2020 when Swift detected a 9 ms hard X-ray burst and a long-lived outburst. Prompt X-ray observations revealed a spin period of 1.36 s, soon confirmed by the discovery of radio pulsations. We report here on the analysis of the Swift burst and follow-up X-ray and radio observations. The burst average luminosity was $L_{\rm burst} \sim2\times 10^{39}$ erg/s (at 4.8 kpc). Simultaneous observations with XMM-Newton and NuSTAR three days after the burst provided a source spectrum well fit by an absorbed blackbody ($N_{\rm H} = (1.13\pm0.03) \times 10^{23}$ cm$^{-2}$ and $kT = 1.16\pm0.03$ keV) plus a power-law ($\Gamma=0.0\pm1.3$) in the 1-20 keV band, with a luminosity of $\sim$$8\times10^{34}$ erg/s, dominated by the blackbody emission. From our timing analysis, we derive a dipolar magnetic field $B \sim 7\times10^{14}$ G, spin-down luminosity $\dot{E}_{\rm rot} \sim 1.4\times10^{36}$ erg/s and characteristic age of 240 yr, the shortest currently known. Archival observations led to an upper limit on the quiescent luminosity $<$$5.5\times10^{33}$ erg/s, lower than the value expected from magnetar cooling models at the source characteristic age. A 1 hr radio observation with the Sardinia Radio Telescope taken about 1 week after the X-ray burst detected a number of strong and short radio pulses at 1.5 GHz, in addition to regular pulsed emission; they were emitted at an average rate 0.9 min$^{-1}$ and accounted for $\sim$50% of the total pulsed radio fluence. We conclude that Swift ,J1818.0-1607 is a peculiar magnetar belonging to the small, diverse group of young neutron stars with properties straddling those of rotationally and magnetically powered pulsars. Future observations will make a better estimation of the age possible by measuring the spin-down rate in quiescence., Comment: 10 pages, 4 figures, 1 table; revised version accepted for publication in The Astrophysical Journal Letters

The magnetar SGR J1745-2900 discovered at parsecs distance from the Milky Way central black hole, Sagittarius A*, represents the closest pulsar to a supermassive black hole ever detected. Furthermore, its intriguing radio emission has been used to study the environment of the black hole, as well as to derive a precise position and proper motion for this object. The discovery of SGR J1745-2900 has opened interesting debates about the number, age and nature of pulsars expected in the Galactic center region. In this work, we present extensive X-ray monitoring of the outburst of SGR J1745-2900 using the Chandra X-ray Observatory, the only instrument with the spatial resolution to distinguish the magnetar from the supermassive black hole (2.4" angular distance). It was monitored from its outburst onset in April 2013 until August 2019, collecting more than fifty Chandra observations for a total of more than 2.3 Ms of data. Soon after the outburst onset, the magnetar emission settled onto a purely thermal emission state that cooled from a temperature of about 0.9 to 0.6 keV over 6 years. The pulsar timing properties showed at least two changes in the period derivative, increasing by a factor of about 4 during the outburst decay. We find that the long-term properties of this outburst challenge current models for the magnetar outbursts., Comment: 11 pages, 6 figures. Accepted by ApJ

As of today, we have directly detected exactly one source in both gravitational waves (GWs) and electromagnetic (EM) radiation, the binary neutron star merger GW170817, its associated gamma-ray burst GRB170817A, and the subsequent kilonova SSS17a/AT 2017gfo. Within ten years, we will detect hundreds of events, including new classes of events such as neutron-star-black-hole mergers, core-collapse supernovae, and almost certainly something completely unexpected. As we build this sample, we will explore exotic astrophysical topics ranging from nucleosynthesis, stellar evolution, general relativity, high-energy astrophysics, nuclear matter, to cosmology. The discovery potential is extraordinary, and investments in this area will yield major scientific breakthroughs. Here we outline some of the most exciting scientific questions that can be answered by combining GW and EM observations., Comment: submitted to Astro2020

After two major outbursts in 2006 and 2011, on 2017 May 16 the magnetar CXOU J164710.2-455216, hosted within the massive star cluster Westerlund I, emitted a short (20 ms) burst, which marked the onset of a new active phase. We started a long-term monitoring campaign with Swift (45 observations), Chandra (5 observations) and NuSTAR (4 observations) from the activation until 2018 April. During the campaign, Swift BAT registered the occurrence of multiple bursts, accompanied by two other enhancements of the X-ray persistent flux. The long time span covered by our observations allowed us to study the spectral as well as the timing evolution of the source. After 11 months since the 2017 May outburst onset, the observed flux was about 15 times higher than its historical minimum level and a factor of 3 higher than the level reached after the 2006 outburst. This suggests that the crust has not fully relaxed to the quiescent level, or that the source quiescent level has changed following the multiple outburst activities in the past 10 years or so. This is another case of multiple outbursts from the same source on a yearly time scale, a somehow recently discovered behaviour in magnetars., Comment: Accepted for publication in MNRAS, 14 pages, 7 figures

We report on a new NuSTAR observation and on the ongoing Swift XRT monitoring campaign of the peculiar source 1E 161348-5055, located at the centre of the supernova remnant RCW 103, which is recovering from its last outburst in June 2016. The X-ray spectrum at the epoch of the NuSTAR observation can be described by either two absorbed blackbodies ($kT_{BB_1}$ ~ 0.5 keV, $kT_{BB_2}$ ~ 1.2 keV) or an absorbed blackbody plus a power law ($kT_{BB_1}$ ~ 0.6 keV, $\Gamma$ ~ 3.9). The observed flux was ~ 9 $\times$ 10$^{-12}$ erg s$^{-1}$ cm$^{-2}$, ~ 3 times lower than what observed at the outburst onset, but about one order of magnitude higher than the historical quiescent level. A periodic modulation was detected at the known 6.67 hr periodicity. The spectral decomposition and evolution along the outburst decay are consistent with 1E 161348-5055 being a magnetar, the slowest ever detected., Comment: Accepted for publication in MNRAS

The mass of the stellar precursor of supernova (SN) 1987A and the burst of neutrinos observed at the moment of the explosion are consistent with the core-collapse formation of a neutron star. However, no compelling evidence for the presence in SN 1987A of a compact object of any kind has been found yet in any band of the electromagnetic spectrum, prompting questions on whether the neutron star survived and, if it did, on its properties. Starting from the analysis of recent Chandra observations, here we appraise the current observational situation. We derived limits on the X-ray luminosity of a compact object with a nonthermal, Crab-pulsar-like spectrum of the order of $\approx$$(1$-$5)\times10^{35}$ erg s$^{-1}$, corresponding to limits on the rotational energy loss of a possible X-ray pulsar in SN 1987A of $\approx$$(0.5$-$1.5)\times10^{38}$ erg s$^{-1}$. However, a much brighter X-ray source cannot be excluded if, as is likely, it is enshrouded in a cloud of absorbing matter with metallicity similar to that expected in the outer layers of a massive star towards the end of its life. We found that other limits obtained from various arguments and observations in other energy ranges either are unbinding or allow a similar maximum luminosity of the order of $\approx$$10^{35}$ erg s$^{-1}$. We conclude that while a pulsar alike the one in the Crab Nebula in both luminosity and spectrum is hardly compatible with the observations, there is ample space for an `ordinary' X-ray-emitting young neutron star, born with normal initial spin period, temperature and magnetic field, to be hiding inside the evolving remnant of SN 1987A., Comment: Accepted for publication in the Astrophysical Journal; 10 pages, 3 color figures, 2 tables

A peculiar infrared ring-like structure was discovered by {\em Spitzer} around the strongly magnetised neutron star SGR 1900$+$14. This infrared structure was suggested to be due to a dust-free cavity, produced by the SGR Giant Flare occurred in 1998, and kept illuminated by surrounding stars. Using a 3D dust radiative transfer code, we aimed at reproducing the emission morphology and the integrated emission flux of this structure assuming different spatial distributions and densities for the dust, and different positions for the illuminating stars. We found that a dust-free ellipsoidal cavity can reproduce the shape, flux, and spectrum of the ring-like infrared emission, provided that the illuminating stars are inside the cavity and that the interstellar medium has high gas density ($n_H\sim$1000 cm$^{-3}$). We further constrain the emitting region to have a sharp inner boundary and to be significantly extended in the radial direction, possibly even just a cavity in a smooth molecular cloud. We discuss possible scenarios for the formation of the dustless cavity and the particular geometry that allows it to be IR-bright., Comment: 13 pages, 8 figures, accepted for publication in The Astrophysical Journal

We study the contribution of binary black hole (BH-BH) mergers from the first, metal-free stars in the Universe (Pop III) to gravitational wave detection rates. Our study combines initial conditions for the formation of Pop III stars based on N-body simulations of binary formation (including rates, binary fraction, initial mass function, orbital separation and eccentricity distributions) with an updated model of stellar evolution specific for Pop III stars. We find that the merger rate of these Pop III BH-BH systems is relatively small (< 0.1 Gpc^-3 yr^-1) at low redshifts (z<2), where it can be compared with the LIGO empirical estimate of 9-240 Gpc^-3 yr^-1 (Abbott et al. 2016). The predicted rates are even smaller for Pop III double neutron star and black hole neutron star mergers. Our rates are compatible with those of Hartwig et al. (2016), but significantly smaller than those found in previous work (Bond & Carr 1984; Belczynski et al. 2004; Kinugawa et al. 2014, 2016). We explain the reasons for this discrepancy by means of detailed model comparisons and point out that (i) identification of Pop III BH-BH mergers may not be possible by advanced LIGO, and (ii) the level of stochastic gravitational wave background from Pop III mergers may be lower than recently estimated (Kowalska et al. 2012; Inayoshi et al. 2016; Dvorkin et al. 2016). We further estimate gravitational wave detection rates for third-generation interferometric detectors. Our calculations are relevant for low to moderately rotating Pop III stars. We can now exclude significant (> 1 per cent) contribution of these stars to low-redshift BH-BH mergers. However, it remains to be tested whether (and at what level) rapidly spinning Pop III stars (homogeneous evolution) can contribute to BH-BH mergers in the local Universe., Comment: submitted to MNRAS, comments welcome

We present the detection of persistent soft X-ray radiation with L_x ~ 10^41-10^42 erg/s at the location of the extremely luminous, double-humped transient ASASSN-15lh as revealed by Chandra and Swift. We interpret this finding in the context of observations from our multiwavelength campaign, which revealed the presence of weak narrow nebular emission features from the host-galaxy nucleus and clear differences with respect to superluminous supernova optical spectra. Significant UV flux variability on short time-scales detected at the time of the re-brightening disfavors the shock interaction scenario as the source of energy powering the long-lived UV emission, while deep radio limits exclude the presence of relativistic jets propagating into a low-density environment. We propose a model where the extreme luminosity and double-peaked temporal structure of ASASSN-15lh is powered by a central source of ionizing radiation that produces a sudden change of the ejecta opacity at later times. As a result, UV radiation can more easily escape, producing the second bump in the light-curve. We discuss different interpretations for the intrinsic nature of the ionizing source. We conclude that, IF the X-ray source is physically associated with the optical-UV transient, ASASSN-15lh most likely represents the tidal disruption of a main-sequence star by the most massive spinning black hole detected to date. In this case, ASASSN-15lh and similar events discovered in the future would constitute the most direct probes of very massive, dormant, spinning, supermassive black holes in galaxies. Future monitoring of the X-rays may allow us to distinguish between the supernova and TDE hypothesis., Comment: Submitted to ApJ

Population synthesis studies constitute a powerful method to reconstruct the birth distribution of periods and magnetic fields of the pulsar population. When this method is applied to populations in different wavelengths, it can break the degeneracy in the inferred properties of initial distributions that arises from single-band studies. In this context, we extend previous works to include $X$-ray thermal emitting pulsars within the same evolutionary model as radio-pulsars. We find that the cumulative distribution of the number of X-ray pulsars can be well reproduced by several models that, simultaneously, reproduce the characteristics of the radio-pulsar distribution. However, even considering the most favourable magneto-thermal evolution models with fast field decay, log-normal distributions of the initial magnetic field over-predict the number of visible sources with periods longer than 12 s. We then show that the problem can be solved with different distributions of magnetic field, such as a truncated log-normal distribution, or a binormal distribution with two distinct populations. We use the observational lack of isolated NSs with spin periods P>12 s to establish an upper limit to the fraction of magnetars born with B > 10^{15} G (less than 1\%). As future detections keep increasing the magnetar and high-B pulsar statistics, our approach can be used to establish a severe constraint on the maximum magnetic field at birth of NSs., Comment: 12 pages, 11 figures, 5 tables

In 2013 April a new magnetar, SGR 1745-2900, was discovered as it entered an outburst, at only 2.4 arcsec angular distance from the supermassive black hole at the Centre of the Milky Way, Sagittarius A*. SGR 1745-2900 has a surface dipolar magnetic field of ~ 2x10^{14} G, and it is the neutron star closest to a black hole ever observed. The new source was detected both in the radio and X-ray bands, with a peak X-ray luminosity L_X ~ 5x10^{35} erg s^{-1}. Here we report on the long-term Chandra (25 observations) and XMM-Newton (8 observations) X-ray monitoring campaign of SGR 1745-2900, from the onset of the outburst in April 2013 until September 2014. This unprecedented dataset allows us to refine the timing properties of the source, as well as to study the outburst spectral evolution as a function of time and rotational phase. Our timing analysis confirms the increase in the spin period derivative by a factor of ~2 around June 2013, and reveals that a further increase occurred between 2013 Oct 30 and 2014 Feb 21. We find that the period derivative changed from 6.6x10^{-12} s s^{-1} to 3.3x10^{-11} s s^{-1} in 1.5 yr. On the other hand, this magnetar shows a slow flux decay compared to other magnetars and a rather inefficient surface cooling. In particular, starquake-induced crustal cooling models alone have difficulty in explaining the high luminosity of the source for the first ~200 days of its outburst, and additional heating of the star surface from currents flowing in a twisted magnetic bundle is probably playing an important role in the outburst evolution., Comment: 16 pages, 12 figures; accepted for publication on MNRAS

The center of our Galaxy hosts a supermassive black hole, Sagittarius (Sgr) A*. Young, massive stars within 0.5 pc of SgrA* are evidence of an episode of intense star formation near the black hole a few Myr ago, which might have left behind a young neutron star traveling deep into SgrA*'s gravitational potential. On 2013 April 25, a short X-ray burst was observed from the direction of the Galactic center. Thanks to a series of observations with the Chandra and the Swift satellites, we pinpoint the associated magnetar at an angular distance of 2.4+/-0.3 arcsec from SgrA*, and refine the source spin period and its derivative (P=3.7635537(2) s and \dot{P} = 6.61(4)x10^{-12} s/s), confirmed by quasi simultaneous radio observations performed with the Green Bank (GBT) and Parkes antennas, which also constrain a Dispersion Measure of DM=1750+/-50 pc cm^{-3}, the highest ever observed for a radio pulsar. We have found that this X-ray source is a young magnetar at ~0.07-2 pc from SgrA*. Simulations of its possible motion around SgrA* show that it is likely (~90% probability) in a bound orbit around the black hole. The radiation front produced by the past activity from the magnetar passing through the molecular clouds surrounding the Galactic center region, might be responsible for a large fraction of the light echoes observed in the Fe fluorescence features., Comment: ApJ Letters in press

Gamma Ray Bursts (GRBs) and galaxies at high redshift represent complementary probes of the star formation history of the Universe. In fact, both the GRB rate and the galaxy luminosity density are connected to the underlying star formation. Here, we combine a star formation model for the evolution of the galaxy luminosity function from z=0 to z=10 with a metallicity-dependent efficiency for GRB formation to simultaneously predict the comoving GRB rate. Our model sheds light on the physical origin of the empirical relation often assumed between GRB rate and luminosity density-derived star formation rate: Rgrb(z) = \epsilon(z)*SFR_{obs}(z), with \epsilon(z) (1+z)^{1.2}. At z<4, \epsilon(z) is dominated by the effects of metallicity evolution in the GRB efficiency. Our best-fitting model only requires a moderate preference for low-metallicity, that is a GRB rate per unit stellar mass about four times higher for log(Z/Zsun)<-3 compared to log(Z/Zsun)>0. Models with total suppression of GRB formation at log(Z/Zsun)>0 are disfavored. At z>4, most of the star formation happens in low-metallicity hosts with nearly saturated efficiency of GRB production per unit stellar mass. However at the same epoch, galaxy surveys miss an increasing fraction of the predicted luminosity density because of flux limits, driving an accelerated evolution of \epsilon(z) compared to the empirical power-law fit from lower z. Our findings are consistent with the non-detections of GRB hosts in ultradeep imaging at z>5, and point toward current galaxy surveys at z>8 only observing the top 15-20 % of the total luminosity density., Comment: Minor changes, ApJL accepted (7 pages, 4 figures)

We report on the long term X-ray monitoring of the outburst decay of the low magnetic field magnetar SGR 0418+5729, using all the available X-ray data obtained with RXTE, SWIFT, Chandra, and XMM-Newton observations, from the discovery of the source in June 2009, up to August 2012. The timing analysis allowed us to obtain the first measurement of the period derivative of SGR 0418+5729: \dot{P}=4(1)x10^{-15} s/s, significant at ~3.5 sigma confidence level. This leads to a surface dipolar magnetic field of B_dip ~6x 10^{12} G. This measurement confirms SGR 0418+5729 as the lowest magnetic field magnetar. Following the flux and spectral evolution from the beginning of the outburst up to ~1200 days, we observe a gradual cooling of the tiny hot spot responsible for the X-ray emission, from a temperature of ~0.9 to 0.3 keV. Simultaneously, the X-ray flux decreased by about 3 orders of magnitude: from about 1.4x10^{-11} to 1.2x10^{-14} erg/s/cm^2 . Deep radio, millimeter, optical and gamma-ray observations did not detect the source counterpart, implying stringent limits on its multi-band emission, as well as constraints on the presence of a fossil disk. By modeling the magneto-thermal secular evolution of SGR 0418+5729, we infer a realistic age of ~550 kyr, and a dipolar magnetic field at birth of ~10^{14} G. The outburst characteristics suggest the presence of a thin twisted bundle with a small heated spot at its base. The bundle untwisted in the first few months following the outburst, while the hot spot decreases in temperature and size. We estimate the outburst rate of low magnetic field magnetars to be about one per year per galaxy, and we briefly discuss the consequences of such result in several other astrophysical contexts., Comment: ApJ in press (15 pages, 11 figures)

We present a long-term study of the 2011 outburst of the magnetar Swift J1834.9-0846 carried out using new Chandra observations, as well as all the available Swift, RXTE, and XMM-Newton data. The last observation was performed on 2011 November 12, about 100 days after the onset of the bursting activity that had led to the discovery of the source on 2011 August 07. This long time span enabled us to refine the rotational ephemeris and observe a downturn in the decay of the X-ray flux. Assuming a broken power law for the long-term light curve, the break was at ~46 d after the outburst onset, when the decay index changed from alpha ~ 0.4 to ~4.5. The flux decreased by a factor ~2 in the first ~50 d and then by a factor ~40 until November 2011 (overall, by a factor ~70 in ~100 d). At the same time, the spectrum, which was well described by an absorbed blackbody all along the outburst, softened, the temperature dropping from ~1 to ~0.6 keV. Diffuse X-ray emission extending up to 20" from the source was clearly detected in all Chandra observations. Its spatial and spectral properties, as well as its time evolution, are consistent with a dust-scattering halo due to a single cloud located at a distance of $\approx$200 pc from Swift J1834.9-0846, which should be in turn located at a distance of ~5 kpc. Considering the time delay of the scattered photons, the same dust cloud might also be responsible for the more extended emission detected in XMM-Newton data taken in September 2011. We searched for the radio signature of Swift J1834.9-0846 at radio frequencies using the Green Bank Radio Telescope and in archival data collected at Parkes from 1998 to 2003. No evidence for radio emission was found, down to a flux density of 0.05 mJy (at 2 GHz) during the outburst and ~0.2-0.3 mJy (at 1.4 GHz) in the older data., Comment: 11 pages, 9 figures and 4 tables, accepted for publication in MNRAS

We report on the long term X-ray monitoring with Swift, RXTE, Suzaku, Chandra and XMM-Newton of the outburst of the newly discovered magnetar Swift J1822.3-1606 (SGR 1822-1606), from the first observations soon after the detection of the short X-ray bursts which led to its discovery, through the first stages of its outburst decay (covering the time-span from July 2011, until end of April 2012). We also report on archival ROSAT observations which witnessed the source during its likely quiescent state, and on upper limits on Swift J1822.3-1606's radio-pulsed and optical emission during outburst, with the Green Bank Telescope (GBT) and the Gran Telescopio Canarias (GTC), respectively. Our X-ray timing analysis finds the source rotating with a period of P=8.43772016(2) s and a period derivative \dot{P}=8.3(2)x10^{-14} s s^{-1} , which entails an inferred dipolar surface magnetic field of B~2.7x10^{13} G at the equator. This measurement makes Swift J1822.3-1606 the second lowest magnetic field magnetar (after SGR 0418+5729; Rea et al. 2010). Following the flux and spectral evolution from the beginning of the outburst, we find that the flux decreased by about an order of magnitude, with a subtle softening of the spectrum, both typical of the outburst decay of magnetars. By modeling the secular thermal evolution of Swift J1822.3-1606, we find that the observed timing properties of the source, as well as its quiescent X-ray luminosity, can be reproduced if it was born with a poloidal and crustal toroidal fields of B_{p}~1.5x10^{14} G and B_{tor}~7x10^{14} G, respectively, and if its current age is ~550 kyr., Comment: 14 pages, 9 figures; new observations added; ApJ in press

We show that a pair of thermal, antipodal hot-spots on the neutron star surface is able to fully account for the pulsar's double blackbody spectrum and energy-dependent pulse profile, including the observed 180 degree phase reversal at approximately 1.2 keV. By comparing the observed pulse modulation and phase to the model predictions, we strongly constrain the hot-spot pole (xi) and the line-of-sight (psi) angles with respect to the spin axis. For a nominal radius of R = 12 km and distance D = 2.2 kpc, we find (xi,psi) = (86d,6d), with 1-sigma error ellipse of (2d,1d); this solution is degenerate in the two angles. The best-fit spectral model for this geometry requires that the temperatures of the two emission spots differ by a factor of 2 and their areas by a factor of ~ 20. Including a cosine-beamed pattern for the emitted intensity modifies the result, decreasing the angles to (84d,3d); however this model is not statistically distinguishable from the isotropic emission case. We also present a new upper limit on the period derivative of Pdot < 3.5E-16 (2-sigma), which limits the global dipole magnetic field to B_s < 2.0E11 G, confirming PSR J0821-4300 as an "anti-magnetar." We discuss the results in the context of observations and theories of nonuniform surface temperature on isolated NSs of both weak and strong magnetic field. To explain the nonuniform temperature of PSR J0821-4300 may require a crustal field that is much stronger than the external, global dipole field., Comment: 9 pages, 6 figure, Latex, emulateapj style. To appear in the Astrophysical Journal

We report on the detailed study of the 2008 October outburst from the anomalous X-ray pulsar (AXP) 1E 1547.0-5408 discovered through the Swift/Burst Alert Telescope (BAT) detection of SGR-like short X-ray bursts on 2008 October 3. The Swift/X-ray Telescope (XRT) started observing the source after less than 100 s since the BAT trigger, when the flux (about 6E-11 erg/cm^2/s in the 2-10 keV range) was >50 times higher than its quiescent level. Swift monitored the outbursting activity of 1E 1547.0-5408 on a daily basis for approximately three weeks. This strategy allowed us to find a phase-coherent solution for the source pulsations after the burst, which, besides period and period derivative, requires a positive Period second derivative term (spin-down increase). The time evolution of the pulse shape is complex and variable, with the pulsed fraction increasing from 20% to 50% within the Swift observational window. The XRT spectra can be fitted well by means of a single component, either a power-law (PL) or a blackbody (BB). During the very initial phases of the outburst the spectrum is hard, with a PL photon index about 2 (or kT about 1.4 keV) which steepens to about 4 (or kT about 0.8 keV) within one day from the BAT trigger, though the two components are likely present simultaneously during the first day spectra. An INTEGRAL observation carried out five days after the trigger provided an upper limit of about 2E-11 erg/cm^2/s to the emission of 1E 1547.0-5408 in the 18-60 keV band., Comment: 10 pages, 2 tables, 6 figures; accepted for publication in MNRAS

We report on submillimetre bolometer observations of the isolated neutron star RX J1856.5--3754 using the LABOCA bolometer array on the Atacama Pathfinder Experiment (APEX) Telescope. No cold dust continuum emission peak at the position of RX J1856.5--3754 was detected. The 3 sigma flux density upper limit of 5 mJy translates into a cold dust mass limit of a few earth masses. We use the new submillimetre limit, together with a previously obtained H-band limit, to constrain the presence of a gaseous, circumpulsar disc. Adopting a simple irradiated-disc model, we obtain a mass accretion limit of dM/dt less than 10^{14} g/s, and a maximum outer disc radius of around 10^{14} cm. By examining the projected proper motion of RX J1856.5--3754, we speculate about a possible encounter of the neutron star with a dense fragment of the CrA molecular cloud a few thousand years ago., Comment: 6 pages, 3 figures, 1 table; accepted by MNRAS

Aims. With an observed and rest-frame duration of < 2s and < 0.5s, respectively, GRB090426 could be classified as a short GRB. The prompt detection, both from space and ground-based telescopes, of a bright optical counterpart to this GRB offered a unique opportunity to complete a detailed study. Methods. Based on an extensive ground-based observational campaign, we obtained the spectrum of the optical afterglow of GRB090426, measuring its redshift and obtaining information about the medium in which the event took place. We completed follow-up observation of the afterglow optical light curve down to the brightness level of the host galaxy that we firmly identified and studied. We also retrieved and analyzed all the available high-energy data of this event, and compared the results with our findings in the optical. This represents one of the most detailed studies of a short-duration event presented so far. Results. The time properties qualify GRB090426 as a short burst. In this case, its redshift of z = 2.61 would be the highest yet found for a GRB of this class. On the other hand, the spectral and energy properties are more similar to those of long bursts. LBT late-time deep imaging identifies a star-forming galaxy at a redshift consistent with that of the GRB. The afterglow lies within the light of its host and shows evidence of local absorption., Comment: Accepted for publication on A&A as a Letter to the Editor. 8 pages, 7 figures

We study the Gamma Ray Burst (GRB) environment and intervening absorbers by analyzing the optical absorption features produced by gas surrounding the GRB or along its line of sight. We analyzed high resolution spectroscopic observations (R=40000, S/N=3 - 6) of the optical afterglow of GRB080330, taken with UVES at the VLT ~ 1.5 hours after the GRB trigger. The spectrum illustrates the complexity of the ISM of the GRB host galaxy at z = 1.51 which has at least four components in the main absorption system. We detect strong FeII, SiII, and NiII excited absorption lines associated with the bluemost component only. In addition to the host galaxy, at least two more absorbers lying along the line of sight to the afterglow have been detected in the redshift range 0.8 < z < 1.1, each exhibiting MgII absorption. For the bluemost component in the host galaxy, we derive information about its distance from the site of the GRB explosion. We do so by assuming that the excited absorption lines are produced by indirect UV pumping, and compare the data with a time dependent photo-excitation code. The distance of this component is found to be 280+40-50 pc, which is lower than found for other GRBs (1 - 6 kpc). We identify two additional MgII absorbers, one of them with a rest frame equivalent width larger than 1A. The distance between the GRB and the absorber measured in this paper confirms that the power of the GRB radiation can influence the conditions of the interstellar medium up to a distance of at least several hundred pc. For the intervening absorbers, we confirm the trend that on average one strong intervening system is found per afterglow, as has been noted in studies exhibiting an excess of strong MgII absorbers along GRB sightlines compared to quasars., Comment: 8 Pages, 7 ps figures, A&A in press

Short GRBs are commonly thought to originate from the merging of double compact object binaries but direct evidence for this scenario is still missing. Optical observations of short GRBs allow us to measure redshifts, firmly identify host galaxies, characterize their properties, and accurately localize GRBs within them. Multiwavelength observations of GRB afterglows provide useful information on the emission mechanisms at work. These are all key issues that allow one to discriminate among different models of these elusive events. We carried out photometric observations of the short/hard GRB 051227, GRB 061006, and GRB 071227 with the ESO-VLT starting from several hours after the explosion down to the host galaxy level several days later. For GRB 061006 and GRB 071227 we also obtained spectroscopic observations of the host galaxy. We compared the results obtained from our optical observations with the available X-ray data of these bursts. For all the three above bursts, we discovered optical afterglows and firmly identified their host galaxies. About half a day after the burst, the optical afterglows of GRB 051227 and GRB 061006 present a decay significatly steeper than in the X-rays. In the case of GRB 051227, the optical decay is so steep that it likely indicates different emission mechanisms in the two wavelengths ranges. The three hosts are blue, star forming galaxies at moderate redshifts and with metallicities comparable to the Solar one. The projected offsets of the optical afterglows from their host galaxies centers span a wide range, but all afterglows lie within the light of their hosts and present evidence for local absorption in their X-ray spectra. We discuss our findings in light of the current models of short GRB progenitors., Comment: Accepted for publication by A&A. 11 pages, 9 figures; v2: minor changes and new version of Fig. 3

Despite about a decade of observations, very little is known about the optical and infrared (IR) emission properties of the Soft Gamma-ray Repeaters (SGRs) and of the Anomalous X-ray Pulsars (AXPs), the magnetar candidates, and about the physical processes which drive their emission at these wavelengths. This is mainly due to the limited number of identifications achieved so far, five in total, and to the sparse spectral coverage obtained from multi-band optical/IR photometry. Aim of this work is to search for a likely candidate counterpart to the recently discovered transient radio AXP 1E 1547.0-5408. We performed the first deep near-IR (NIR) observations (Ks band) of 1E 1547.0-5408 with the VLT on three nights (July 8th, 12th, and August 17th), after the X-ray source rebrightening and during the subsequent decay reported around June 2007. We detected four objects within, or close to, the 3sigma radio position of 1E 1547.0-5408. The faintest of them (object 1) has a brightness K = 20.27 +/- 0.05, which would yield an unabsorbed X-ray to NIR flux ratio FX/FKs ~ 800 for 1E 1547.0-5408, i.e. on average lower than those derived for other magnetars. The non-detection of object 1 on the nights of July 8th and August 17th only allowed us to set an upper limit of Delta Ks ~ 0.2 on its NIR variability, which prevented us to search for correlations with the radio or X-ray flux. We detected no other object at the radio position down to a limit of Ks ~ 21.7 (at 5sigma), computed in our deepest VLT image (July 12th). From our observations we can not confidently propose a NIR counterpart to 1E 1547.0-5408. More NIR observations of object 1, e.g. to determine its colors and to monitor variability, would be conclusive to determine whether or not it can be considered a plausible candidate., Comment: 6 pages, 3 postscript figures, submitted to Astronomy & Astrophysics, referee's comments included

XTE J1810-197 is the first transient Anomalous X-ray Pulsar ever discovered. Its highly variable X-ray flux allowed us to study the timing and spectral emission properties of a magnetar candidate over a flux range of about two orders of magnitude. We analyzed nine XMM-Newton observations of XTE J1810-197 collected over a four years baseline (September 2003 - September 2007). EPIC PN and MOS data were reduced and used for detailed timing and spectral analysis. Pulse phase spectroscopic studies were also carried out for observations with sufficiently high signal to noise. We find that: (i) a three blackbodies model reproduces the spectral properties of the source over the entire outburst statistically better than the two blackbodies model previously used in the literature, (ii) the coldest blackbody is consistent with the thermal emission from the whole surface, and has temperature and radius similar to those inferred from ROSAT observations before the outburst onset, (iii) there is a spectral feature around 1.1 keV during six consecutive observations (since March 2005); if due to proton resonant cyclotron scattering, it would imply a magnetic field of around 2E14 G. This is in a very good agreement with the value from the spin period measurements., Comment: 16 pages, 11 figures, accepted for publication on A&A on 10/1/2009

The aim of this paper is to investigate the environment of gamma ray bursts (GRBs) and the interstellar matter of their host galaxies. We use to this purpose high resolution spectroscopic observations of the afterglow of GRB050922C, obtained with UVES/VLT 3.5 hours after the GRB event. We found that, as for most high resolution spectra of GRBs, the spectrum of the afterglow of GRB050922C is complex. At least seven components contribute to the main absorption system at z=2.1992. The detection of lines of neutral elements like MgI and the detection of fine-structure levels of the ions FeII, SiII and CII allows us to separate components in the GRB ISM along the line of sight. Moreover, in addition to the main system, we have analyzed the five intervening systems between z = 2.077 and z = 1.5664 identified along the GRB line of sight. GRB afterglow spectra are very complex, but full of information. This can be used to disentangle the contribution of the different parts of the GRB host galaxy and to study their properties. Our metallicity estimates agree with the scenario of GRBs exploding in low metallicity galaxies, Comment: 11 pages, 12 figures, accepted by A&A

We present the results from an ESO/VLT campaign aimed at studying the afterglow properties of the short/hard gamma ray burst GRB 070707. Observations were carried out at ten different epochs from ~0.5 to ~80 days after the event. The optical flux decayed steeply with a power-law decay index greater than 3, later levelling off at R~27.3 mag; this is likely the emission level of the host galaxy, the faintest yet detected for a short GRB. Spectroscopic observations did not reveal any line features/edges that could unambiguously pinpoint the GRB redshift, but set a limit z < 3.6. In the range of allowed redshifts, the host has a low luminosity, comparable to that of long-duration GRBs. The existence of such faint host galaxies suggests caution when associating short GRBs with bright, offset galaxies, where the true host might just be too dim for detection. The steepness of the decay of the optical afterglow of GRB 070707 challenges external shock models for the optical afterglow of short/hard GRBs. We argue that this behaviour might results from prolonged activity of the central engine or require alternative scenarios., Comment: 6 pages, 5 figures, accepted by A&A

It is now recognized that long-duration Gamma-Ray Bursts (GRBs) are linked to the collapse of massive stars, based on the association between (low-redshift) GRBs and (type Ic) core-collapse supernovae (SNe). The census of massive stars and GRBs reveals, however, that not all massive stars do produce a GRB. Only ~1% of core collapse SNe are able to produce a highly relativistic collimated outflow, and hence a GRB. The extra crucial parameter has long been suspected to be metallicity and/or rotation. We find observational evidence strongly supporting that both ingredients are necessary in order to make a GRB out of a core-collapsing star. A detailed study of the absorption pattern in the X-ray spectrum of GRB060218 reveals evidence of material highly enriched in low atomic number metals ejected before the SN/GRB explosion. We find that, within the current scenarios of stellar evolution, only a progenitor star characterized by a fast stellar rotation and sub-solar initial metallicity could produce such a metal enrichment in its close surrounding., Comment: ApJL in press 4 pages 2 figures

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., Comment: 7 pages, 6 .ps figures, ApJL, submitted version #2. Substantial modification with respect to version #1

X-ray observations unveiled the existence of enigmatic point-like sources at the centre of young supernova remnants (SNRs). These sources, dubbed Central Compact Objects (CCOs), are thought to be neutron stars formed by the supernova explosion. However, their multi-wavelength phenomenology is surprisingly different from that of most young neutron stars.The aim of this work is to understand the nature of the CCO 1WGA J1713.4-3949 in the G347.3-0.5 SNR, through deep optical and IR observations, the first ever performed for this source.By exploiting its derived Chandra X-ray position we carried out optical (BVI) observations with the NTT and Adaptive Optics IR (JHKs) observations with the VLT. We detected two faint (I~23.5, I~24.3.) patchy objects in the NTT images, close to the Chandra error circle. They were clearly resolved in our VLT images which unveiled a total of six candidate counterparts (17.8

. We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1810-197, 1RXS J1708-4009, 1E 1841-045 and the soft gamma-ray repeater SGR 1900+14, taken with the ESO-VLT, the Gemini, and the CFHT telescopes. . This work is aimed at identifying and/or confirming the IR counterparts of these magnetars, as well as at measuring their possible IR variability. . In order to perform photometry of objects as faint as Ks~20, we have used data taken with the largest telescopes, equipped with the most advanced IR detectors and in most of the cases with Adaptive Optics devices. The latter are critical to achieve the sharp spatial accuracy required to pinpoint faint objects in crowded fields. . We confirm with high confidence the identification of the IR counterpart to XTE J1810-197, and its IR variability. For 1E 1841-045 and SGR 1900+14 we propose two candidate IR counterparts based on the detection of IR variability. For 1RXS J1708-4009 we show that none of the potential counterparts within the source X-ray error circle can be yet convincingly associated with this AXP. . The IR variability of the AXP XTE J1810-197 does not follow the same monotonic decrease of its post-outburst X-ray emission. Instead, the IR variability appears more similar to the one observed in radio band, although simultaneous IR and radio observations are crucial to draw any conclusion in this respect. For 1E 1841-045 and SGR 1900+14, follow-up observations are needed to confirm our proposed candidates with higher confidence., Comment: 9 pages, 10 figures; accepted by A&A (high resolution images at http://staff.science.uva.nl/~nrea/NIR_AXP_testa2007.pdf)

4U 2129+47 was discovered in the early 80's and classified as an accretion disk corona source due to its broad and partial X-ray eclipses. The 5.24 hr binary orbital period was inferred from the X-ray and optical light curve modulation, implying a late K or M spectral type companion star. The source entered a low state in 1983, during which the optical modulation disappeared and an F8 IV star was revealed, suggesting that 4U 2129+47 might be part of a triple system. The nature of 4U 2129+47 has since been investigated, but no definitive conclusion has been reached. Here, we present timing and spectral analyses of two XMM-Newton observations of this source, carried out in May and June, 2005. We find evidence for a delay between two mid-eclipse epochs measured ~22 days apart, and we show that this delay can be naturally explained as being due to the orbital motion of the binary 4U 2129+47 around the center of mass of a triple system. This result thus provides further support in favor of the triple nature of 4U 2129+47., Comment: 6 pages, 5 figures, accepted by A&A

We report on two new XMM-Newton observations of the Anomalous X-ray Pulsar (AXP) 4U 0142+614 performed in March and July 2004, collecting the most accurate spectrum for this source to date. Furthermore, we analyse two short archival observations performed in February 2002 and January 2003 (the latter already reported by Gohler et al. 2005) in order to study the long term behaviour of this AXP. 4U 0142+614 appears to be relatively steady in flux between 2002 and 2004, and the phase-averaged spectrum does not show any significant variability between the four epochs. We derive the deepest upper limits to date on the presence of lines in the 4U 0142+614 spectrum as a function of energy: equivalent width in the 1-3 keV energy range < 4 eV and < 8 eV for narrow and broad lines, respectively. A remarkable energy dependence in both the pulse profile and the pulsed fraction is detected, and consequently pulse-phase spectroscopy shows spectral variability as a function of phase. By making use of XMM-Newton and INTEGRAL data, we successfully model the 1-250 keV spectrum of 4U 0142+614 with three models presented in Rea et al. (2007a), namely the canonical absorbed blackbody plus two power-laws, a resonant cyclotron scattering model plus one power-law and two log-parabolic functions., Comment: 9 pages, 5 figures; accepted for publication on MNRAS (typos corrected and few references added)

X-ray observations have unveiled the existence of a family of radio-quiet Isolated Neutron Stars whose X-ray emission is purely thermal, hence dubbed X-ray Dim Isolated Neutron Stars (XDINSs). While optical observations have allowed to relate the thermal emission to the neutron star cooling and to build the neutron star surface thermal map, IR observations are critical to pinpoint a spectral turnover produced by a so far unseen magnetospheric component, or by the presence of a fallback disk. The detection of such a turnover can provide further evidence of a link between this class of isolated neutron stars and the magnetars, which show a distinctive spectral flattening in the IR. Here we present the deepest IR observations ever of five XDINSs, which we use to constrain a spectral turnover in the IR and the presence of a fallback disk. The data are obtained using the ISAAC instrument at the VLT. For none of our targets it was possible to identify the IR counterpart down to limiting magnitudes H = 21.5 - 22.9. Although these limits are the deepest ever obtained for neutron stars of this class, they are not deep enough to rule out the existence and the nature of a possible spectral flattening in the IR. We also derive, by using disk models, the upper limits on the mass inflow rate in a fallback disk. We find the existence of a putative fallback disk consistent (although not confirmed) with our observations., Comment: 6 pages, 2 figures, accepted by A&A on 26-06-2007

Recent radio observations have unveiled the existence of a number of radio pulsars with spin-down derived magnetic fields in the magnetar range. However, their observational properties appears to be more similar to classical radio pulsars than to magnetars. To shed light on this puzzle we first have to determine whether the spin-down derived magnetic field values for these radio pulsars are indeed representative of the actual neutron star magnetic field or if they are polluted, e.g. by the effects of a torque from a fallback disk. To investigate this possibility, we have performed deep IR observations of one of these high magnetic field radio pulsars (PSR J1119-6127) with the ESO VLT to search for IR emission which can be associated with a disk. No IR emission is detected from the pulsar position down to J=24, H=23, Ks=22. By comparing our flux upper limits with the predictions of fallback disk models, we have found that we can only exclude the presence of a disk with accretion rate dot M >3x10^16 g/s. This lower limit cannot rule out the presence of a substantial disk torque on the pulsar, which would then lead to overestimate the value of the magnetic field inferred from P and dot P., Comment: 8 pages, 4 figures, A&A, in press

On September 21, 2006, an intense (~10^39 erg/s) and short (20 ms) burst was detected by Swift BAT at a position consistent with that of the candidate Anomalous X-ray Pulsar, CXOU J164710.2-455216, discovered by Chandra in 2005. Swift follow-up observations began about 13 hours after the event and found the source at a 1-10keV flux level of about 4.5 x 10^-11 erg/s/cm^2, i.e. ~300 times brighter than measured 5 days earlier by XMM. We report the results obtained from Swift BAT observations of the burst and subsequent Swift XRT observations carried out during the first four months after the burst. These data are complemented with those from two XMM observations (carried out just before and after the BAT event) and four archival Chandra observations carried out between 2005 and 2007. We find a phase coherent solution for the source pulsations after the burst. The evolution of the pulse phase comprises an exponential component decaying with timescale of 1.4d which we interpret as the recovery stage following a large glitch (Delta nu / nu about 6 x 10^-5). We also detect a quadratic component corresponding to a spin-down rate of Pdot ~ 9 x 10^-13, implying a magnetic field strength of 10^14 Gauss. During the first Swift XRT observation taken 0.6 days after the burst, the spectrum showed a kT = 0.65keV blackbody (R_BB = 1.5km) plus a Gamma=2.3 power-law accounting for about 60% of the 1-10 keV observed flux. Analysis of Chandra archival data, taken during 2005 when the source was in quiescence, reveal that the modulation in quiescence is 100% pulsed at energies above 4 keV and consistent with the (unusually small-sized) blackbody component being occulted by the neutron star as it rotates., Comment: Accepted for publication on ApJ Main Journal; 24 pages, 8 figures

We compute the probability that intermediate mass black holes (IMBHs) capture companions due to dynamical interactions and become accreting sources, and explore the possibility that the accreting IMBHs would appear as ultra-luminous X-ray sources (ULXs). We focus on IMBHs originating from low-metallicity Population III stars. Two channels of IMBH formation are considered: from primordial halos in the framework of hierarchical clustering, and from non-mixed, zero-metallicity primeval gas in galactic discs. IMBHs can form binary systems due to tidal captures of single stars and exchange interactions with existing binary systems in galactic discs. We find that neither formation mechanism of the accreting IMBH binary is able to provide enough sources to explain the observed population of ULXs. Even at sub-ULX luminosity, the total number of accreting IMBHs with $L> 10^{36}$ erg s$^{-1}$ with dynamically captured companions is found to be $< 0.01$ per galaxy., Comment: 21 pages, 5 figures. Accepted to MNRAS

Aims. We derive simple estimates of the maximum efficiency with which matter can be ejected by the propeller mechanism in disk-fed, rotating magnetic neutron stars. Some binary evolution scenarios envisage that this mechanism is responsible for expelling to infinity the mass inflowing at a low rate from the companion star, therefore limiting the total amount of mass that can be accreted by the neutron star. Methods. We demonstrate that, for typical neutron star parameters, a maximum of \eta_{pro} < 5.7 (P_{-3})^{1/3} times more matter than accreted can be expelled through the propeller mechanism at the expenses of the neutron star rotational energy (P_{-3} is the NS spin period in unit of 10E-3 s). Approaching this value, however, would require a great deal of fine tuning in the system parameters and the properties of the interaction of matter and magnetic field at the magnetospheric boundary. Results. We conclude that some other mechanism must be invoked in order to prevent that too much mass accretes onto the neutron stars of some low mass X-ray binaries., Comment: 5 pages, 2 figures, accepted for publication in A&A

GRB050904 is the gamma-ray burst with the highest measured redshift. We performed time resolved X-ray spectroscopy of the late GRB and early afterglow emission. We find robust evidence for a decrease with time of the soft X-ray absorbing column. We model the evolution of the column density due to the flash ionization of the GRB and early afterglow photons. This allows us to constrain the metallicity and geometry of the absorbing cloud. We conclude that the progenitor of GRB050904 was a massive star embedded in a dense metal enriched molecular cloud with Z~0.03 Z_solar. This is the first local measurement of metallicity in the close environment of a GRB and one of the highest redshift metallicity measurements. We also find that the dust associated with the cloud cannot be similar to that of our Galaxy but must be either sizably depleted or dominated by silicate grains. We discuss the implications of these results for GRB progenitors and high redshift star formation., Comment: 4 pages, 2 figures. Accepted for publication on Astrophysical Journal Letters

We analyze high resolution spectroscopic observations of the optical afterglow of GRB050730, obtained with UVES@VLT about hours after the GRB trigger. The spectrum shows that the ISM of the GRB host galaxy at z = 3.967 is complex, with at least five components contributing to the main absorption system. We detect strong CII*, SiII*, OI* and FeII* fine structure absorption lines associated to the second and third component. For the first three components we derive information on the relative distance from the site of the GRB explosion. Component 1, which has the highest redshift, does not present any fine structure nor low ionization lines; it only shows very high ionization features, such as CIV and OVI, suggesting that this component is very close to the GRB site. From the analysis of low and high ionization lines and fine structure lines, we find evidences that the distance of component 2 from the site of the GRB explosion is 10-100 times smaller than that of component 3. We evaluated the mean metallicity of the z=3.967 system obtaining values about 0.01 of the solar metallicity or less. However, this should not be taken as representative of the circumburst medium, since the main contribution to the hydrogen column density comes from the outer regions of the galaxy while that of the other elements presumably comes from the ISM closer to the GRB site. Furthermore, difficulties in evaluating dust depletion correction can modify significantly these values. The mean [C/Fe] ratio agrees well with that expected by single star-formation event models. Interestingly the [C/Fe] of component 2 is smaller than that of component 3, in agreement with GRB dust destruction scenarios, if component 2 is closer than component 3 to the GRB site., Comment: 11 pages, 15 postscript figures, accepted for pubblication in A&A