arXiv subjects

A. De Luca

Publications and source records attributed to A. De Luca.

At least 109 records · Page 6Linked to original sources

arXiv2007

Using GRO J1655-40 to test Swift/BAT as a monitor for bright hard X-ray sources

While waiting for new gamma-ray burst detections, the Burst Alert Telescope (BAT) on board Swift covers each day ~50% of the sky in the hard X-ray band (``Survey data''). The large field of view (FOV), high sensitivity and good angular resolution make BAT a potentially powerful all-sky hard X-ray monitor, provided that mask--related systematics can be properly accounted for. We have developed and tested a complete procedure entirely based on public Swift/BAT software tools to analyse BAT Survey data, aimed at assessing the flux and spectral variability of bright sources in the 15-150 keV energy range. Detailed tests of the capabilities of our procedure were performed focusing, in particular, on the reliability of spectral measurements over the entire BAT FOV. First, we analyzed a large set of Crab observations, spread over ~7 months. Next, we studied the case of GRO J1655-40, a strongly variable source, which experienced a 9-month long outburst, beginning on February 2005. Such an outburst was systematically monitored with the well-calibrated PCA and HEXTE instruments on board the RXTE mission. Thanks to the good BAT temporal coverage of the source, we have been able to cross-check BAT light-curves with simultaneous HEXTE ones. The Crab tests have shown that our procedure recovers both the flux and the source spectral shape over the whole FOV of the BAT instrument. Moreover, by cross-checking GRO J1655-40 light-curves obtained by BAT and HEXTE, we found the spectral and flux evolution of the outburst to be in very good agreement. Using our procedure, BAT reproduces HEXTE fluxes within a 10-15% uncertainty with a 3sigma sensitivity of ~20 mCrab for an on-axis source, thus establishing its capability to monitor the evolution of relatively bright hard X-rays sources.

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arXiv2007

HST multi-epoch imaging of the PSR B0540-69 system unveils a highly dynamic synchrotron nebula

PSR B0540-69 is the Crab twin in the Large Magellanic Cloud. Age, energetic and overall behaviour of the two pulsars are very similar. The same is true for the general appearance of their pulsar wind nebulae (PWNe). Analysis of Hubble Space Telescope images spanning 10 years unveiled significant variability in the PWN surrounding PSR B0540-69, with a hot spot moving at ~0.04c. Such behaviour, reminiscent of the variability observed in the Crab nebula along the counter-jet direction, may suggest an alternative scenario for the geometry of the system. The same data were used to assess the pulsar proper motion. The null displacement recorded over 10 y allowed us to set a 3sigma upper limit of 290 km/s to the pulsar velocity.

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arXiv2007

VLT observations of the Central Compact Object in the Vela Jr. supernova remnant

X-ray observations have unveiled the existence of enigmatic point-like sources at the center of young (a few kyrs) supernova remnants. These sources, known as Central Compact Objects (CCOs), are thought to be neutron stars produced by the supernova explosion, although their X-ray phenomenology makes them markedly different from all the other young neutron stars discovered so far.The aim of this work is to search for the optical/IR counterpart of the Vela Junior CCO and to understand the nature of the associated Halpha nebula discovered by Pellizzoni et al. (2002).}{We have used deep optical (R band) and IR (J,H,Ks bands) observations recently performed by our group with the ESO VLT to obtain the first deep, high resolution images of the field with the goal of resolving the nebula structure and pinpointing a point-like source possibly associated with the neutron star.Our R-band image shows that both the nebula's flux and its structure are very similar to the Halpha ones, suggesting that the nebula spectrum is dominated by pure Halpha line emission. However, the nebula is not detected in our IR observations, whick makes it impossible to to constrain its spectrum. A faint point-like object (J>22.6, H~21.6, Ks ~ 21.4) compatible with the neutron star's Chandra X-ray position is detected in our IR images (H and Ks) but not in the optical one (R > 25.6), where it is buried by the nebula background. The nebula is most likely a bow-shock produced by the neutron star motion through the ISM or, alternatively, a photo-ionization nebula powered by UV radiation from a hot neutron star.

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arXiv2007

Phase resolved spectroscopy of the Vela pulsar with XMM-Newton

The ~10^4 y old Vela Pulsar represents the bridge between the young Crab-like and the middle-aged rotation powered pulsars. Its multiwavelength behaviour is due to the superposition of different spectral components. We take advantage of the unprecedented harvest of photons collected by XMM-Newton to assess the Vela Pulsar spectral shape and to study the pulsar spectrum as a function of its rotational phase. As for the middle-aged pulsars Geminga, PSR B0656+14 and PSR B1055-52 (the "Three Musketeers"), the phase-integrated spectrum of Vela is well described by a three-component model, consisting of two blackbodies (T_bb1=(1.06+/-0.03)x10^6 K, R_bb1=5.1+/-0.3 km, T_bb2=(2.16+/-0.06)x10^6 K, R_bb2=0.73+/-0.08 km) plus a power-law (gamma=2.2+/-0.3). The relative contributions of the three components are seen to vary as a function of the pulsar rotational phase. The two blackbodies have a shallow 7-9% modulation. The cooler blackbody, possibly related to the bulk of the neutron star surface, has a complex modulation, with two peaks per period, separated by ~0.35 in phase, the radio pulse occurring exactly in between. The hotter blackbody, possibly originating from a hot polar region, has a nearly sinusoidal modulation, with a single, broad maximum aligned with the second peak of the cooler blackbody, trailing the radio pulse by ~0.15 in phase. The non thermal component, magnetospheric in origin, is present only during 20% of the pulsar phase and appears to be opposite to the radio pulse. XMM-Newton phase-resolved spectroscopy unveils the link between the thermally emitting surface of the neutron star and its charge-filled magnetosphere, probing emission geometry as a function of the pulsar rotation. This is a fundamental piece of information for future 3-dimensional modeling of the pulsar magnetosphere.

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arXiv2007

Discovery of X-ray emission from the young radio pulsar PSR J1357-6429

We present the first X-ray detection of the very young pulsar PSR J1357-6429 (characteristic age of 7.3 kyr) using data from the XMM-Newton and Chandra satellites. We find that the spectrum is well described by a power-law plus blackbody model, with photon index Gamma=1.4 and blackbody temperature kT=160 eV. For the estimated distance of 2.5 kpc, this corresponds to a 2-10 keV luminosity of about 1.2E+32 erg/s, thus the fraction of the spin-down energy channeled by PSR J1357-6429 into X-ray emission is one of the lowest observed. The Chandra data confirm the positional coincidence with the radio pulsar and allow to set an upper limit of 3E+31 erg/s on the 2-10 keV luminosity of a compact pulsar wind nebula. We do not detect any pulsed emission from the source and determine an upper limit of 30% for the modulation amplitude of the X-ray emission at the radio frequency of the pulsar.

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arXiv2006

IGR J08408--4503: a new recurrent Supergiant Fast X-ray Transient

The supergiant fast X-ray transient IGR J08408-4503 was discovered by INTEGRAL on May 15, 2006, during a bright flare. The source shows sporadic recurrent short bright flares, reaching a peak luminosity of 10^36 erg s^-1 within less than one hour. The companion star is HD 74194, an Ob5Ib(f) supergiant star located at 3 kpc in the Vela region. We report the light curves and broad-band spectra (0.1-200 keV) of all the three flares of IGR J08408-4503 detected up to now based on INTEGRAL and Swift data. The flare spectra are well described by a power-law model with a high energy cut-off at ~15 keV. The absorption column density during the flares was found to be ~10^21 cm^-2, indicating a very low matter density around the compact object. Using the supergiant donor star parameters, the wind accretion conditions imply an orbital period of the order of one year, a spin period of the order of hours and a magnetic field of the order of 10^13 G.

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arXiv2006

A new symbiotic low mass X-ray binary system: 4U 1954+319

4U 1954+319 was discovered 25 years ago, but only recently has a clear picture of its nature begun to emerge. We present for the first time a broad-band spectrum of the source and a detailed timing study using more than one year of monitoring data. The timing and spectral analysis was done using publicly available Swift, INTEGRAL, BeppoSAX, and RXTE/ASM data in the 0.7-150 keV energy band. The source spectrum is described well by a highly absorbed (N_H~10^23 cm^-2) power law with a high-energy exponential cutoff around 15 keV. An additional black body component is needed below 3 keV to account for a soft excess. The derived ~5 hr periodicity, with a spin-up timescale of ~25 years, could be identified as the neutron star spin period. The spectral and timing characteristics indicate that we are dealing both with the slowest established wind-accreting X-ray pulsar and with the second confirmed member of the emerging class dubbed "symbiotic low mass X-ray binaries" to host a neutron star.

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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

A long-period, violently-variable X-ray source in a young SNR

Observations with the Newton X-ray Multimirror Mission (XMM) show a strong periodic modulation at 6.67+/-0.03 hours of the X-ray source at the centre of the 2,000-year-old supernova remnant RCW 103. No fast pulsations are visible. If genetically tied to the supernova remnant, the source could either be an X-ray binary, comprising a compact object and a low-mass star in an eccentric orbit, or an isolated neutron star. In the latter case, its age-period combination would point to a peculiar magnetar, dramatically slowed-down, possibly by a supernova debris disc. Both scenarios require non-standard assumptions on the formation and evolution of compact objects in supernova explosions.

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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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arXiv2005

Swift and XMM observations of the dark GRB 050326

We present Swift and XMM observations of GRB 050326, detected by Swift-BAT. The fluence was 7.7x10^-6 erg cm^-2 (20-150 keV), and its spectrum was hard, with a power law photon index 1.25. The afterglow light curve did not show any break nor flares between ~1 hr and ~6 d after the burst, and decayed with a slope 1.70. The afterglow spectrum is well fitted by a power-law model, suffering absorption both in the Milky Way and in the host galaxy. The rest-frame Hydrogen column density is significant, N_H_z > 4x10^21 cm^-2, and the redshift of the absorber is z > 1.5. There was good agreement between the Swift-XRT and XMM results. By comparing the prompt and afterglow fluxes, we found that an early break occurred before the XRT observation. The properties of the GRB 050326 afterglow are well described by a spherical fireball expanding in a uniform external medium, so a further steepening is expected at later times. The lack of such a break constrains the jet angle to be >7 deg. Using the redshift constraints provided by the X-ray analysis, we also estimated that the beaming-corrected gamma-ray energy was >3x10^51 erg, at the high end of GRB energies. Despite the brightness in X rays, only deep limits could be placed by Swift-UVOT at optical/UV wavelengths. Thus, this GRB was "truly dark", with the optical-to-X-ray spectrum violating the synchrotron limit. The optical and X-ray observations are consistent either with an absorbed event or with a high-redshift one. To obey the Ghirlanda relation, a moderate/large redshift z>4.5 is required. (abridged)

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arXiv2005

Evidence for intrinsic absorption in the Swift X-ray afterglows

Gamma-ray burst (GRB) progenitors are observationally linked to the death of massive stars. X-ray studies of the GRB afterglows can deepen our knowledge of the ionization status and metal abundances of the matter in the GRB environment. Moreover, the presence of local matter can be inferred through its fingerprints in the X-ray spectrum, i.e. the presence of absorption higher than the Galactic value. A few studies based on BeppoSAX and XMM-Newton found evidence of higher than Galactic values for the column density in a number of GRB afterglows. Here we report on a systematic analysis of 17 GRBs observed by Swift up to April 15, 2005. We observed a large number of GRBs with an excess of column density. Our sample, together with previous determinations of the intrinsic column densities for GRBs with known redshift, provides evidence for a distribution of absorption consistent with that predicted for randomly occurring GRB within molecular clouds.

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arXiv2005

The weak INTEGRAL bursts GRB040223 and GRB040624: an emerging population of dark afterglows

We report here gamma-ray, X-ray and near-infrared observations of GRB040223 along with gamma-ray and optical observations of GRB040624. GRB040223 was detected by INTEGRAL close to the Galactic plane and GRB040624 at high Galactic latitude. Analyses of the prompt emission detected by the IBIS instrument on INTEGRAL are presented for both bursts. The two GRBs have long durations, slow pulses and are weak. The gamma-ray spectra of both bursts are best fit with steep power-laws, implying they are X-ray rich. GRB040223 is among the weakest and longest of INTEGRAL GRBs. The X-ray afterglow of this burst was detected 10 hours after the prompt event by XMM-Newton. The measured spectral properties are consistent with a column density much higher than that expected from the Galaxy, indicating strong intrinsic absorption. We carried out near-infrared observations 17 hours after the burst with the NTT of ESO, which yielded upper limits. Given the intrinsic absorption, we find that these limits are compatible with a simple extrapolation of the X-ray afterglow properties. For GRB040624, we carried out optical observations 13 hours after the burst with FORS 1 and 2 at the VLT, and DOLoRes at the TNG, again obtaining upper limits. We compare these limits with the magnitudes of a compilation of promptly observed counterparts of previous GRBs and show that they lie at the very faint end of the distribution. These two bursts are good examples of a population of bursts with dark or faint afterglows that are being unveiled through the increasing usage of large diameter telescopes engaged in comprehensive observational programmes.

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arXiv2005

On the complex X-ray structure tracing the motion of Geminga

A deep (100 ks) XMM-Newton observation of Geminga has shown two faint tails of diffuse X-ray emission, extending for ~2' behind the pulsar, well aligned with the proper motion (PM) direction. We report here on a recent ~20 ks Chandra observation, which unveils a new structure, ~25'' long and ~5'' thick, starting at the pulsar position and perfectly aligned with the PM direction, with a surface brightness ~40 times higher than that of the XMM Tails. The Chandra comet-like feature has a remarkably hard spectrum (photon index 0.9-1.4) and a luminosity of ~5.5x10^28 erg s^-1, comparable to the energetics of the larger XMM one. Geminga is thus the first neutron star to show a clear X-ray evidence of a large-scale, outer bow-shock as well as a short, inner cometary trail.

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arXiv2005

A deep XMM-Newton serendipitous survey of a middle-latitude area

The radio quiet neutron star 1E1207.4-5209 has been the target of a 260 ks XMM-Newton observation, which yielded, as a by product, an harvest of about 200 serendipitous X-ray sources above a limiting flux of 2E-15 erg/cm2/s, in the 0.3-8 keV energy range. In view of the intermediate latitude of our field (b~10 deg), it comes as no surprise that the logN-logS distribution of our serendipitous sources is different from those measured either in the Galactic Plane or at high galactic latitudes. Here we shall concentrate on the analysis of the brightest sources in our sample, which unveiled a previously unknown Seyfert-2 galaxy.

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arXiv2005

Open issues in gamma-ray bursts: polarimetry and dark GRBs

We review some open problems in the physics of afterglows, namely their polarization properties and the existence of dark/faint bursts. Polarization studies yield precious insights in the physical structure and dynamical evolution of GRB jets, revealing their magnetization properties and their energy profile. Polarimetric observations of GRB 020813 already allowed to exclude a homogeneous jet for this event. We then present observations of faint/dark bursts, showing that some of them may be obscured by dust, while others are possibly just intrinsically dim.

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arXiv2005

XMM-Newton and VLT observations of the afterglow of GRB040827

The field of the Gamma-Ray Burst GRB 040827 was observed with XMM-Newton and with the ESO/VLT starting ~6 and ~12 hours after the burst, respectively. A fading X-ray afterglow is clearly detected with the XMM-Newton/EPIC instrument, with a time decay t^(-delta), with delta=1.41+/-0.10. Its spectrum is well described by a power law (photon index Gamma=2.3+/-0.1) affected by an absorption largely exceeding (by a factor ~5) the expected Galactic one, requiring the contribution of an intrinsic, redshifted absorber. In the optical/NIR range, the afterglow emission was observed in the Ks band, as a weak source superimposed to the host galaxy, with magnitude Ks=19.44+/-0.13 (12 hours after the GRB, contribution from the host subtracted); in other bands the flux is dominated by the host galaxy. Coupling constraints derived from X-ray spectral fitting and from photometry of the host, we estimated a gas column density in the range (0.4-2.6)x10^22 cm^-2 in the GRB host galaxy, likely located at a redshift 0.5<z<1.7. GRB 040827 stands out as the best example of an X-ray afterglow with intrinsic absorption.

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arXiv2004

On the polar caps of the 3 Musketeers

XMM-Newton observations of PSR B0656+14, PSR B1055-52 and Geminga have substantially increased the statistics available for these three isolated neutron stars, so apparently similar to deserve the nickname of "Three Musketeers" (Becker & Truemper, 1997). Here we shall take advantage of the EPIC statistics to perform phase resolved spectroscopy for all three objects. The phase-averaged spectrum of the three musketeers is best described by a three component model. This includes two blackbody components, a cooler one, possibly originating from the bulk of the star surface, and a hotter one, coming from a smaller portion of the star surface (a "hot spot"), plus a power law. The relative contributions of the three components are seen to vary as a function of phase, as the stars' rotation bring into view different emitting regions. The hot spots, which have very different apparent dimensions (in spite of the similarity of the three neutron stars polar cap radii) are responsible for the bulk of the phase variation. The amplitude of the observed phase modulation is also markedly different for the three sources. Another striking aspect of our phase-resolved phenomenology is the apparent lack of any common phase alignement between the observed modulation patterns for the two blackbody components. They are seen to vary in phase in the case of PSR B1055-52, but in anti-phase in the case of PSR B0656+14. These findings do not support standard and simplistic models of neutron star magnetic field configuration and surface temperature distribution.

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