G. Lo Curto - Academia.edu (original) (raw)

Papers by G. Lo Curto

Astrophysics and Space Science, 2007

Astronomy & Astrophysics, 2006

Aims. We have investigated the structure of the pulsating atmosphere of one of the best studied r... more Aims. We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. Methods. For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. Results. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower than the sound speed. We confirm large pulsation amplitudes (150-400 m s −1) for REE lines and the Hα core, while spectral lines of the other elements (Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.

Astronomy & Astrophysics, 2008

Context. We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1... more Context. We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1810-197, 1RXS J1708-4009, and 1E 1841-045, and the soft gamma-ray repeater SGR 1900+14, taken with the ESO-VLT, the Gemini, and the CFHT telescopes. Aims. This work aims at identifying and/or confirming the IR counterparts of these magnetars, as well as at measuring their possible IR variability. Methods. To perform photometry of objects as faint as K s ∼ 20, we 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 for achieving the sharp spatial accuracy required to pinpoint faint objects in crowded fields. Results. We confirm with high confidence 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 as yet be convincingly associated with this AXP. Conclusions. The IR variability of the AXP XTE J1810-197 does not follow the same monotonic decrease in its post-outburst X-ray emission. Instead, the IR variability appears more like the one observed in radio band, although simultaneous IR and radio observations are crucial for 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.

Astronomy & Astrophysics, 2007

Context. Recent radio observations have unveiled the existence of a number of radio pulsars with ... more Context. 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 appear to be more similar to those of the classical radio pulsars than to the magnetars's ones. Aims. 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. Methods. To investigate this possibility, we have performed deep IR (J, H, K s bands) 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. Results. No IR emission is detected from the pulsar position down to J ∼ 24, H ∼ 23 and K s ∼ 22. Conclusions. 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 rateṀ > ∼ 3 × 10 16 g s −1. 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 andṖ. We have also compared the upper limit on the IR luminosity of PSR J1119-6127 with the IR luminosities of rotation-powered pulsars and magnetars. We found that, while magnetars are intrinsically more efficient IR emitters than rotation-powered pulsars, possibly because of their higher magnetic field, the relatively low IR emission efficiency of PSR J1119-6127 suggests that it is more similar to the latters than to the former.

Astronomy & Astrophysics, 2009

Context. Despite about a decade of observations, very little is known about the optical and infra... more Context. 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. Aims. The aim of this work is to search for a likely candidate counterpart to the recently discovered transient radio AXP 1E 1547.0−5408. Methods. We performed the first deep near-IR (NIR) observations (K s band) of 1E 1547.0−5408 with the Very Large Telescope (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. Results. We detected four objects within, or close to, the 3σ radio position of 1E 1547.0−5408. The faintest of them (object 1) has a brightness Ks = 20.27 ± 0.05, which would yield an unabsorbed X-ray-to-NIR flux ratio F X /F Ks ∼ 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 ΔK s ∼ 0.2 on its NIR variability, which prevented us from searching for correlations with the radio or X-ray flux. We detected no other object at the radio position down to a limit of K s ∼ 21.7 (at 5σ), computed in our deepest VLT image (July 12th). Conclusions. From our observations we cannot 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.

Astronomy & Astrophysics, 2007

Aims. In this work we present a revised measurement of the phase-averaged optical polarisation of... more Aims. In this work we present a revised measurement of the phase-averaged optical polarisation of the Vela pulsar (PSR B0833-45), for which only one value has been published so far (Wagner & Seifert 2000, ASP Conf. Ser., 202, 315). Methods. Our measurement has been obtained through an accurate reanalysis of archival polarisation observations with the FORS1 instrument at the VLT. Results. We measured a phase-averaged linear polarisation degree P L = 9.4% ± 4% and a position angle θ = 146 • ± 11 • , very close to the ones for the axis of symmetry of the X-ray arcs and jets detected by Chandra and of the pulsar proper motion. Conclusions. We compared the measured phase-averaged optical polarisation with the expectations of different pulsars' magnetosphere models. We find that all models consistently predict values of the phase-averaged linear polarisation that are too high with respect to the observed one. This is probably a consequence of present models' limitations that neglect the contributions of various depolarisation effects. Interestingly, for the outer gap model we find that, when assuming synchrotron radiation for the optical emission, the observed polarisation position angle also implies an alignment between the pulsar rotational axis and the axis of symmetry of the X-ray arcs and jets.

Astronomy and Astrophysics, 2007

ESO Astrophysics Symposia European Southern Observatory, 2008

Many detectors, optical CCDs and IR arrays, are currently in operation onboard ESO instruments at... more Many detectors, optical CCDs and IR arrays, are currently in operation onboard ESO instruments at the La Silla Paranal Observatory. A unified scheme for optical detector characterization has been adopted since several years in La Silla, and it is used by the Science Operation team to monitor the 18 CCDs belonging to the eight instruments operated by ESO at the Observatory. This scheme has been proven successful in ensuring a high quality performance of the detectors along the years. In Paranal the science operation team and QC Garching monitor the performance of the detectors using instrument-specific data reduction pipelines.

Modern Technologies in Space- and Ground-based Telescopes and Instrumentation, 2010

The HARPS spectrograph, after its first six years of operations has established itself as the wor... more The HARPS spectrograph, after its first six years of operations has established itself as the worldwide reference for accurate Doppler measurements. The radial velocity precision of the instrument on a single measurement is estimated around 60 cm/sec. One of the main limitations to the radial velocity precision is the variation of the injection illumination function due to tracking errors. A light and fast guiding system has been recently developed for HARPS. In this way it is expected to increase the guiding closed loop bandwidth by a factor of 10 with respect to the standard guiding performed with the telescope.

Astrophysics and Space Science, 2007

Astronomy & Astrophysics, 2006

Aims. We have investigated the structure of the pulsating atmosphere of one of the best studied r... more Aims. We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. Methods. For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. Results. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower than the sound speed. We confirm large pulsation amplitudes (150-400 m s −1) for REE lines and the Hα core, while spectral lines of the other elements (Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.

Astronomy & Astrophysics, 2008

Context. We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1... more Context. We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1810-197, 1RXS J1708-4009, and 1E 1841-045, and the soft gamma-ray repeater SGR 1900+14, taken with the ESO-VLT, the Gemini, and the CFHT telescopes. Aims. This work aims at identifying and/or confirming the IR counterparts of these magnetars, as well as at measuring their possible IR variability. Methods. To perform photometry of objects as faint as K s ∼ 20, we 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 for achieving the sharp spatial accuracy required to pinpoint faint objects in crowded fields. Results. We confirm with high confidence 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 as yet be convincingly associated with this AXP. Conclusions. The IR variability of the AXP XTE J1810-197 does not follow the same monotonic decrease in its post-outburst X-ray emission. Instead, the IR variability appears more like the one observed in radio band, although simultaneous IR and radio observations are crucial for 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.

Astronomy & Astrophysics, 2007

Context. Recent radio observations have unveiled the existence of a number of radio pulsars with ... more Context. 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 appear to be more similar to those of the classical radio pulsars than to the magnetars's ones. Aims. 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. Methods. To investigate this possibility, we have performed deep IR (J, H, K s bands) 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. Results. No IR emission is detected from the pulsar position down to J ∼ 24, H ∼ 23 and K s ∼ 22. Conclusions. 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 rateṀ > ∼ 3 × 10 16 g s −1. 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 andṖ. We have also compared the upper limit on the IR luminosity of PSR J1119-6127 with the IR luminosities of rotation-powered pulsars and magnetars. We found that, while magnetars are intrinsically more efficient IR emitters than rotation-powered pulsars, possibly because of their higher magnetic field, the relatively low IR emission efficiency of PSR J1119-6127 suggests that it is more similar to the latters than to the former.

Astronomy & Astrophysics, 2009

Context. Despite about a decade of observations, very little is known about the optical and infra... more Context. 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. Aims. The aim of this work is to search for a likely candidate counterpart to the recently discovered transient radio AXP 1E 1547.0−5408. Methods. We performed the first deep near-IR (NIR) observations (K s band) of 1E 1547.0−5408 with the Very Large Telescope (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. Results. We detected four objects within, or close to, the 3σ radio position of 1E 1547.0−5408. The faintest of them (object 1) has a brightness Ks = 20.27 ± 0.05, which would yield an unabsorbed X-ray-to-NIR flux ratio F X /F Ks ∼ 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 ΔK s ∼ 0.2 on its NIR variability, which prevented us from searching for correlations with the radio or X-ray flux. We detected no other object at the radio position down to a limit of K s ∼ 21.7 (at 5σ), computed in our deepest VLT image (July 12th). Conclusions. From our observations we cannot 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.

Astronomy & Astrophysics, 2007

Aims. In this work we present a revised measurement of the phase-averaged optical polarisation of... more Aims. In this work we present a revised measurement of the phase-averaged optical polarisation of the Vela pulsar (PSR B0833-45), for which only one value has been published so far (Wagner & Seifert 2000, ASP Conf. Ser., 202, 315). Methods. Our measurement has been obtained through an accurate reanalysis of archival polarisation observations with the FORS1 instrument at the VLT. Results. We measured a phase-averaged linear polarisation degree P L = 9.4% ± 4% and a position angle θ = 146 • ± 11 • , very close to the ones for the axis of symmetry of the X-ray arcs and jets detected by Chandra and of the pulsar proper motion. Conclusions. We compared the measured phase-averaged optical polarisation with the expectations of different pulsars' magnetosphere models. We find that all models consistently predict values of the phase-averaged linear polarisation that are too high with respect to the observed one. This is probably a consequence of present models' limitations that neglect the contributions of various depolarisation effects. Interestingly, for the outer gap model we find that, when assuming synchrotron radiation for the optical emission, the observed polarisation position angle also implies an alignment between the pulsar rotational axis and the axis of symmetry of the X-ray arcs and jets.

Astronomy and Astrophysics, 2007

ESO Astrophysics Symposia European Southern Observatory, 2008

Many detectors, optical CCDs and IR arrays, are currently in operation onboard ESO instruments at... more Many detectors, optical CCDs and IR arrays, are currently in operation onboard ESO instruments at the La Silla Paranal Observatory. A unified scheme for optical detector characterization has been adopted since several years in La Silla, and it is used by the Science Operation team to monitor the 18 CCDs belonging to the eight instruments operated by ESO at the Observatory. This scheme has been proven successful in ensuring a high quality performance of the detectors along the years. In Paranal the science operation team and QC Garching monitor the performance of the detectors using instrument-specific data reduction pipelines.

Modern Technologies in Space- and Ground-based Telescopes and Instrumentation, 2010

The HARPS spectrograph, after its first six years of operations has established itself as the wor... more The HARPS spectrograph, after its first six years of operations has established itself as the worldwide reference for accurate Doppler measurements. The radial velocity precision of the instrument on a single measurement is estimated around 60 cm/sec. One of the main limitations to the radial velocity precision is the variation of the injection illumination function due to tracking errors. A light and fast guiding system has been recently developed for HARPS. In this way it is expected to increase the guiding closed loop bandwidth by a factor of 10 with respect to the standard guiding performed with the telescope.