arXiv subjects

R. Perna

Publications and source records attributed to R. Perna.

At least 55 records · Page 3Linked to original sources

arXiv2007

X-Ray Eclipse Time Delays in 4U2129+47

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.

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arXiv2007

Very deep X-ray observations of the Anomalous X-ray Pulsar 4U 0142+614

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.

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arXiv2007

Deep VLT infrared observations of X-ray Dim Isolated Neutron Stars

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.

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arXiv2007

VLT/NACO observations of the High-Magnetic field radio pulsar PSR J1119-6127

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.

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arXiv2007

The Post-Burst Awakening of the Anomalous X-ray Pulsar in Westerlund 1

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.

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arXiv2007

UVES/VLT high resolution spectroscopy of GRB 050730 afterglow: probing the features of the GRB environment

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.

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arXiv2007

On the dynamical formation of accreting intermediate mass black holes

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.

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arXiv2007

On the maximum efficiency of the propeller mass-ejection mechanism

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 η_{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.

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arXiv2006

A metal rich molecular cloud surrounds GRB 050904 at redshift 6.3

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.

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arXiv2006

Time resolved spectroscopy of GRB 021004 reveals a clumpy extended wind

High resolution spectroscopy of GRB 021004 revealed a wealth of absorption lines from several intermediate ionization species. The velocity structure of the absorber is complex and material with velocity up to >3000 km/s is observed. Since only the blueshifted component is observed, the absorber is very likely to be material closely surrounding the gamma-ray burst. We use a time-dependent photoionization code to track the abundance of the ions over time. Thanks to the presence of absorption from intermediate ionization states at long times, we can estimate the location and mass of the components of the absorber. We interpret those constraints within the hypernova scenario showing that the mass loss rate of the progenitor must have been ~10^{-4} solar masses per year, suggestive of a very massive star. In addition, the wind termination shock must lie at a distance of at least 100 pc, implying a low density environment. The velocity structure of the absorber also requires clumping of the wind at those large distances.

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arXiv2006

Studies of Neutron Stars at Optical/IR Wavelengths

In the last years, optical studies of Isolated Neutron Stars (INSs) have expanded from the more classical rotation-powered ones to other categories, like the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma-ray Repeaters (SGRs), which make up the class of the magnetars, the radio-quiet INSs with X-ray thermal emission and, more recently, the enigmatic Compact Central Objects (CCOs) in supernova remnants. Apart from 10 rotation-powered pulsars, so far optical/IR counterparts have been found for 5 magnetars and for 4 INSs. In this work we present some of the latest observational results obtained from optical/IR observations of different types of INSs.

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arXiv2006

The Chandra X-ray Spectrum of the 10.6 s Pulsar in Westerlund 1: Testing the Magnetar Hypothesis

Two sensitive Chandra X-ray observations of the heavily-reddened galactic starburst cluster Westerlund 1 in May and June 2005 detected a previously unknown X-ray pulsar (CXO J164710.20-455217). Its slow 10.6 s pulsations, moderate X-ray temperature kT $\approx$ 0.5 keV, and apparent lack of a massive companion tentatively suggest that it is an Anomalous X-ray Pulsar (AXP). An isothermal blackbody model yields an acceptable spectral fit but the inferred source radius is much less than that of a neutron star, a result that has also been found for other AXPs. We analyze the X-ray spectra with more complex models including a model that assumes the pulsar is a strongly magnetized neutron star (``magnetar'') with a light element atmosphere. We conclude that the observed X-ray emission cannot be explained as global surface emission arising from the surface of a cooling neutron star or magnetar. The emission likely arises in one or more localized regions (``hot spots'') covering a small fraction of the surface. We discuss these new results in the context of both accretion and magnetar interpretations for the X-ray emission.

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arXiv2006

The variable X-ray light curve of GRB 050713A: the case of refreshed shocks

We present a detailed study of the spectral and temporal properties of the X-ray and optical emission of GRB050713a up to 0.5 day after the main GRB event. The X-ray light curve exhibits large amplitude variations with several rebrightenings superposed on the underlying three-segment broken powerlaw that is often seen in Swift GRBs. Our time-resolved spectral analysis supports the interpretation of a long-lived central engine, with rebrightenings consistent with energy injection in refreshed shocks as slower shells generated in the central engine prompt phase catch up with the afterglow shock at later times. Our sparsely-sampled light curve of the optical afterglow can be fitted with a single power law without large flares. The optical decay index appears flatter than the X-ray one, especially at later times.

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arXiv2006

The frontier of darkness: the cases of GRB 040223, GRB 040422, GRB 040624

Understanding the reasons for the faintness of the optical/near-infrared afterglows of the so-called dark bursts is essential to assess whether they form a subclass of GRBs, and hence for the use of GRBs in cosmology. With VLT and other ground-based telescopes, we searched for the afterglows of the INTEGRAL bursts GRB 040223, GRB 040422 and GRB 040624 in the first hours after the triggers. A detection of a faint afterglow and of the host galaxy in the K band was achieved for GRB 040422, while only upper limits were obtained for GRB 040223 and GRB 040624, although in the former case the X-ray afterglow was observed. A comparison with the magnitudes of a sample of afterglows clearly shows the faintness of these bursts, which are good examples of a population that an increasing usage of large diameter telescopes is beginning to unveil.

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arXiv2006

Optical emission from GRB 050709: a short/hard GRB in a star forming galaxy

We present optical observations of the short/hard gamma-ray burst GRB 050709, the first such event with an identified optical counterpart. The object is coincident with a weak X-ray source and is located inside a galaxy at redshift z = 0.1606 +- 0.0002. Multiband photometry allowed us to study the broad-band spectral energy distribution. Late-time monitoring places strong limits on any supernova simultaneous with the GRB. The host galaxy is not of early type. Spectra show that the dominant stellar population is relatively young (~ 1 Gyr), and that ongoing star formation is present at a level of 2-3 (L/L*) Msun/yr. This is at least 2 orders of magnitude larger than that observed in the elliptical hosts of the short GRB 050509B and GRB 050724. This shows that at least some short GRBs originate in a young population. Short/hard GRB models based on the merger of a binary degenerate system are compatible with the host galaxy characteristics, although there is still the possibility of a connection between young stars and at least a fraction of such events.

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arXiv2005

Neutrino Cooled disk in GRB central engine

At the extreme densities and temperatures typical of the central engine of GRBs, the accreting torus is cooled mainly by advection and by neutrino emission. The latter process is dominated by electron and positron capture onto nucleons ($β$ reactions). We calculate the reaction rates and the nuclear composition of matter, assuming that the torus consists of helium, eletron-positron pairs, free neutrons and protons. After determining the equation of state and solving for the disk structure for a given initial accretion rate, we subsequently follow its time evolution. We find that, for accretion rates of the order of $10 M_{\odot}$/s, likely typical for the early stages of the accretion event, the disk becomes unstable, giving rise to variable energy output. This instability may play an important role for producing internal shocks.

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arXiv2005

GRB environment properties through X and optical afterglow observations

We present the spectral analysis of 14 gamma-ray bursts (GRB) X-ray afterglows in order to investigate the properties of interstellar matter (ISM) along the line of sight of GRB. We carried out a simultaneous analysis of the NIR-optical and X-band for those afterglows with an optical counterpart too, in order to evaluate and strongly constrain the absorption effect on the spectral energy distribution due to dust extinction from GRB environment. We evaluated the equivalent hydrogen column density Nh from X-ray spectroscopy and rest frame visual extinction Av by assuming different type of ISM composition and dust grain size distribution. From our analysis we obtained a distribution of the GRB rest frame consistent with the one expected if GRB were embedded in a Galactic-like molecular cloud. Moreover, values of the visual extinction estimated from the simultaneous analysis of NIR-to-X band favour an environment where small dust grain are destroyed by the interaction with the X-ray and UV photons from GRB.

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arXiv2005

Extinction properties of the X-ray bright/optically faint afterglow of GRB 020405

We present an optical-to-X-ray spectral analysis of the afterglow of GRB 020405. The optical spectral energy distribution not corrected for the extragalactic extinction is significantly below the X-ray extrapolation of the single powerlaw spectral model suggested by multiwavelength studies. We investigate whether considerable extinction could explain the observed spectral ``mismatch'' by testing several types of extinction curves. For the first time we test extinction curves computed with time-dependent numerical simulations of dust grains destruction by the burst radiation. We find that an extinction law weakly depen dent on wavelength can reconcile the unabsorbed optical and X-ray data with the expected synchrotron spectrum. A gray extinction law can be provided by a dust grain size distribution biased toward large grains.

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