arXiv subjects

G. F. Bignami

Publications and source records attributed to G. F. Bignami.

At least 37 records · Page 2Linked to original sources

arXiv2004

Phase-resolved spectroscopy of Geminga shows rotating hot spot(s)

Isolated neutron stars are seen in x-rays through their nonthermal and/or surface thermal emissions. XMM-Newton observations of the Geminga pulsar show a 43 electron volt spectrum from the whole neutron star surface, as well as a power-law component above 2 kiloelectron volts. In addition, we have detected a hot (170 electron volts) thermal emission from an 60-meter-radius spot on the pulsar s surface. Such a thermal emission, only visible at selected phase intervals, may be coming from polar hot spot(s), long thought to exist as a result of heating from magnetospheric accelerated particles. It may provide the missing link between the x-ray and gamma-ray emission of the pulsar.

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arXiv2004

A first XMM-Newton look at the relativistic double pulsar PSR J0737-3039

We present the results of a 50 ks long X-ray observation of the relativistic double pulsar system PSR J0737-3039 obtained with the XMM-Newton satellite in March 2004. The source has a soft spectrum (power law photon index = 3.5(+0.5,-0.3)) and a 0.2-10 keV luminosity of ~3x10^30 erg/s (assuming a distance of 500 pc), consistent with the values derived from a previous Chandra observation. No flux variations have been detected, implying the absence of large orbital modulations. The high time resolution of the EPIC instrument has allowed us to perform the first search for X-ray pulsations from this system. The result was negative, with an upper limit of 60% on the pulsed fraction of the 22 ms pulsar.

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arXiv2004

X-ray and optical coverage of 3EG J0616-3310 and 3EG J1249-8330

The limited angular resolution of gamma-ray telescopes prevents the straight identification of the majority of the sources detected so far. This is particularly true for the low latitude, probably galactic ones, only 10 % of which has been identified. Most of the counterparts are Isolated Neutron Stars, both radio-loud and radio-quiet, characterised by an extremely high value of X-ray-to-optical flux ratio. The best way to search for INSs in the error boxes of unidentified EGRET sources is to perform the X-ray coverage of the gamma-ray field, followed by the optical characterization of each X-ray source. We applied this procedure to two EGRET sources, which could belong to a local galactic population: 3EG J0616-3310 and 3EG J1249-8330. Here we report on the analysis of about 300 X-ray objects, as well as on their optical study.

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arXiv2004

1E1207.4-5209 - a Unique Object

The discovery of deep spectral features in the X-ray spectrum of 1E1207.4-5209 focussed the attention of the astronomical community on this radio-quiet NS, making it the most intensively observed INS ever. The harvest of X-ray photons, collected mainly by XMM-Newton, unveiled a number of unique characteristics, raising questions on this source very nature.

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arXiv2004

Geminga: a tale of two tails, and much more

We report on the deep (100 ksec) XMM-Newton/EPIC observation of the field of the Geminga pulsar. The unprecedented throughput of the instrument allowed to detect two elongated parallel X-ray tails trailing the neutron star. They are aligned with the object's supersonic motion, extend for ~2', and have a nonthermal spectrum produced by electron-synchrotron emission in the bow shock between the pulsar wind and the surrounding medium. Electron lifetime against synchrotron cooling matches the source transit time over the X-ray features' length. Such an X-ray detection of a pulsar bow shock allows us to gauge the pulsar electron injection energy and the shock magnetic field while constraining the angle of Geminga's motion and the local matter density. We give also preliminary results on the timing and spectral analysis of the \~63,000 photons collected from the neutron star.

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arXiv2003

XMM-Newton and VLT observations of the isolated neutron star 1E 1207.4-5209

In August 2002, XMM-Newton devoted two full orbits to the observation of 1E 1207.4-5209, making this isolated neutron star the most deeply scrutinized galactic target of the mission. Thanks to the high throughput of the EPIC instrument, ~360,000 photons were collected from the source, allowing for a very sensitive study of the temporal and spectral behaviour of this object. The spectral data, both time-averaged and phase-resolved, yield one compelling interpretation of the observed features: cyclotron absorption from one fundamental (~0.7 keV) and three harmonics, at ~1.4, ~2.1 and ~2.8 keV. Possible physical consequences are discussed, also on the basis of the obvious phase variations of the features' shapes and depths. We also present deep VLT optical data which we have used to search for a counterpart, with negative results down to ~27.

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arXiv2003

Discovery of X-ray cyclotron absorption lines measures the magnetic field of an isolated neutron star

Isolated neutron stars are higly magnetized, fast rotating end points of stellar evolution. They are now becoming directly observable through X-ray astronomy, owing to their high surface temperatures. In particular, features in their X-ray spectra could reveal the presence of atmospheres, or gauge their unknown magnetic fields through the cyclotron process, as in the classic case of X-ray binaries. All isolated neutron stars spectra observed so far, however, appear as featureless thermal continua. The unique exception is 1E 1207.4-5209. In its spectrum, previous observations had detected two deep absorption features, still too undefined for any unambiguous interpretation. Here we report on a much longer X-ray observation, in which the star's spectrum shows three distinct features, regularly spaced at 0.7, 1.4 and 2.1 keV, plus a fourth, lower significance one, at 2.8 keV. Such features vary in phase with the star rotation. Cyclotron resonant absorption is their logical interpretation, yielding a magnetic field strenght of 8x10^10 Gauss in the case of electrons. This is the first direct measure of an isolated neutron star magnetic field.

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arXiv2002

The XMM-Newton view of the radio-quiet neutron star 1E 1207.4-5209

We have observed the radio-quiet X-ray pulsar 1E 1207.4-5209 with the high throughput EPIC cameras onboard XMM-Newton. The spectrum of this peculiar source is characterized by two broad absorption features which present significant substructures and show a clear phase-dependence. We believe that these features represent a strong evidence for the presence of a magnetized atmosphere containing heavy elements.

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arXiv2002

Pulse phase variations of the X-ray spectral features in the radio-quiet neutron star 1E 1207-5209

We present the results of an XMM-Newton observation of the radio-quiet X-ray pulsar 1E 1207-5209 located at the center of the shell-like supernova remnant G296.5+10.0. The X-ray spectrum is characterized by the presence of two phase-dependent absorption lines at energies ~0.7 keV and ~1.4 keV. Moreover, these broad spectral features have significant substructure, suggesting that they are due to the blending of several narrower lines. We interpret such features as evidence for an atmosphere containing metals and a magnetic field value of a few 10^12 G, consistent with the observed spin-down rate P_dot=(1.98+/-0.83)*10^(-14) s/s. Since 1E 1207-5209 is the only X-ray emitting pulsar showing evidence of such features, we tentatively link them to the unique combination of age and energetics that characterize this object. We suggest that a young age and a low level of magnetospheric activity are favorable conditions for the detection of atomic spectral features from metals in neutron star atmospheres, which would be either blanketed by a thin layer of accreted hydrogen in older objects or masked by non-thermal processes in young energetic pulsars.

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arXiv2002

XMM-Newton and Wide Field Optical Imaging to crack the mystery of unidentified gamma-ray sources

The limited angular resolution of gamma-ray telescopes prevents the straight identification of the majority of the sources detected so far. While the only certified galactic gamma-ray emitters are associated with pulsars, more than 90% of the nearly 100 low latitude sources detected by EGRET on the GRO lack an identification. The best identification strategy devised over the years relies on the use of X-ray and optical data to single out possible counterparts, rating them on the basis of their extreme F_x/F_opt. Here we describe our multiwavelength programme based on the EPIC mapping of selected EGRET sources complemented by the optical coverage of the same fields obtained through the Wide Field Imager (WFI) of the ESO 2.2 m telescope. The field of view of the WFI, comparable to that of EPIC, will allow to directly cross-correlate X and optical data speeding up significantly the selection of interesting candidates worth follow-up optical (and X-ray) studies.

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arXiv2001

The distance to the Vela pulsar gauged with HST parallax oservations

The distance to the Vela pulsar (PSR B0833-45) has been traditionally assumed to be 500 pc. Although affected by a significant uncertainty, this value stuck to both the pulsar and the SNR. In an effort to obtain a model free distance measurement, we have applied high resolution astrometry to the pulsar V~23.6 optical counterpart. Using a set of five HST/WFPC2 observations, we have obtained the first optical measurement of the annual parallax of the Vela pulsar. The parallax turns out to be 3.4 +/- 0.7 mas, implying a distance of 294(-50;+76) pc, i.e. a value significantly lower than previously believed. This affects the estimate of the pulsar absolute luminosity and of its emission efficiency at various wavelengths and confirms the exceptionally high value of the N_e towards the Vela pulsar. Finally, the complete parallax data base allows for a better measurement of the Vela pulsar proper motion (mu_alpha(cos(delta))=-37.2 +/- 1.2 mas/yr; mu_delta=28.2 +/- 1.3 mas/yr after correcting for the peculiar motion of the Sun) which, at the parallax distance, implies a transverse velocity of ~65 km/s. Moreover, the proper motion position angle appears specially well aligned with the axis of symmetry of the X-ray nebula as seen by Chandra. Such an alignment allows to assess the space velocity of the Vela pulsar to be ~81 km/s.

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arXiv2000

Pulsar Astronomy: the HST Contribution

HST observations have contributed significantly to our knowledge on the behaviour of Isolated Neutron Stars (INSs) as optical emitters. First, HST has been instrumental both to discover new optical counterparts (PSR B1055-52, PSR B1929+10, PSR B0950+08) and to confirm proposed identifications (PSR B0656+14). Second, HST multicolor photometry provided useful information to characterize the optical emission mechanism(s) at work in middle-aged INSs like PSR B0656+14 and Geminga. Last, but not least, the superior angular resolution of the HST allowed both to perform a very accurate morphological study of the plerionic environments of young pulsars (e.g. the Crab and PSR B0540-69) and to perform very accurate astrometric measurements yielding proper motions (Crab, Vela, Geminga, PSR B0656+14) and parallaxes (Geminga).

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arXiv2000

SNR0540-69 -the Crab twin- revisited: Chandra vs. HST

High resolution images of the Crab-like SNR SNR0540-69 have been recently obtained by the HRC, on board the Chandra X-ray Observatory, and by the FORS1 instrument, attached to ESO/VLT1. A detailed comparison of these images with an archival Planetary Camera one will be presented.

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arXiv1999

An Atmospheric Model for the Ion Cyclotron Line of Geminga

Recent optical/UV data of Geminga (Mignani et al. 1998) have strengthen the significance of the emission feature present over the thermal continuum best fitting the EUV/soft X-ray data. Here we present a phenomenological model interpreting the data in terms of an ion cyclotron feature, arising from a thin outer layer of hot plasma covering Geminga's polar caps. The width of the feature favours for Geminga the strongly oblique rotator configuration, as already deduced from the soft X-ray data (Halpern & Ruderman 1993). Our interpretation implies a magnetic field of 3 - 5 10d11 G, consistent with the value deduced from the dynamical parameters of the pulsar. This would represent the first case of an independent measurement of the surface magnetic field of an isolated neutron star. The expected time modulation of Geminga's feature is also discussed.

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arXiv1998

Optical observations of PSR1706$-$44 with the Test Camera of the VLT-UT1

We report on the optical observations of the gamma-ray pulsar PSR1706-44 performed with the Test Camera of the VLT-UT1 during the telescope Science Verification programme. With a limiting magnitude of V~ 27.5, these are the deepest optical observations ever performed for this pulsar, for which, however, no likely counterpart has been detected. The non-detection of PSR1706-44 sets an upper limit on its optical luminosity varying from 2 10^28 to 5 10^29 erg/ s, depending on the exact amount of interstellar absorption.

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arXiv1998

HST/FOC observations confirm the presence of a spectral feature in the optical spectrum of Geminga

New optical and near-UV HST observations of Geminga are presented. When compared with previous ground-based and HST imaging, the data confirm and better define the presence of an emission feature centered at 6,000 Angstrom and superimposed on the thermal continuum best fitting the extreme-UV/soft X-ray data. This feature may be interpreted in terms of cyclotron emission originated from a mixture of H/He ions in the neutron star's atmosphere. In the case of pure Hydrogen, the feature wavelength would imply a magnetic field of order 3-5 10d11 G, consistent with the value deduced from the dynamical parameters of the pulsar. If due to a cyclotron emission, the observation of this feature would represent the first case of an in situ measurement of the surface magnetic field of an isolated neutron star.

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arXiv1997

Pulsar Counterparts of Gamma-Ray Sources

The EGRET catalogue of unidentified X-ray sources has more objects along the galactic disk than at high galactic latitude, where identifications are comparatively easier. On the other hand, the Egret/GRO mission has already identified several known radio pulsars as gamma-ray sources as well as discovering Geminga's nature as a pulsar. If Geminga is not a unique case, as it is very likely not to be, than other galactic sources could, in fact, be radio quiet isolated neutron stars. For these, the identification work is extremely difficult and should anyway start from high resolution X-ray/optical data.

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arXiv1997

Is the Fuzziness of GRB970228 constant?

In view of the data gathered in September 1997, we review the flux values collected so far for the "fuzziness" seen in the optical counterpart of GRB970228. Comparison between the ground based data collected in March and the data of September 1997 suggests a fading of the fuzz. Given the diversity of the data in hand, the magnitude of the effect and its significance are not easy to quantify. Only new images, both from the ground and with the Space Telescope, directly comparable to the old ones could settle this problem.

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