arXiv subjects
Publications and source records attributed to P. A. Caraveo.
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.
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.
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.
The discovery of deep spectral features in the X-ray spectrum of 1E1207.4-5209 has pushed this Isolated Neutron Star (INS) out of the chorus line, since no other INS has shown significant features in its X-ray continuum. On August 2002, XMM-Newton devoted a two-orbit TOO observation to this target with the aim to better understand the nature of such spectral features, using much improved statistics. Indeed, the 260 ksec observation yielded 360,000 photons from 1E1207.4-5209, allowing for a very sensitive study of the temporal and spectral behaviour of this object.
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.
Observations of the Vela pulsar region with the Chandra X-ray observatory have revealed the fine structure of its synchrotron pulsar-wind nebula (PWN), which showed an overall similarity with the Crab PWN. However, contrary to the Crab, no firm detection of the Vela PWN in optical has been reported yet. To search for the optical counterpart of the X-ray PWN, we analyzed deep optical observations performed with different telescopes. We compared the optical images with those obtained with the Chandra ACIS to search for extended emission patterns which could be identified as counterparts of the X-ray nebula elements. Although some features are seen in the optical images, we find no correlation with the X-ray structure. Thus, we conclude that the diffuse optical emission is more likely associated with filaments in the host Vela SNR. The derived upper limits on the optical flux from the PWN are compatibile, within the uncertainties, with the values expected on the basis of the extrapolations of the X-ray data.
New results of HST and VLT observations of isolated pulsars and their environments are presented. We present the first deep optical observations of the nearby pulsar PSR J0108-1431, the identification evidence of PSR 1929+10 based on the proper motion measurement of its optical counterpart and a deep investigation at optical wavelengths of the X-ray PWN around the Vela pulsar.
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.
We have measured the proper motion of the candidate optical counterpart of the old, nearby pulsar PSR B1929+10, using a set of HST/STIS images collected in 2001, 7.2 years after the epoch of the original FOC detection (Pavlov et al. 1996). The yearly displacement, mu=107.3+/-1 mas/yr along a position angle of 64.6+/-0.6 deg, is fully consistent with the most recent VLBA radio measurement. This result provides a robust confirmation of the identification of PSR B1929+10 in the optical band.
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.
The 3rd EGRET catalogue lists 271 sources and provides an identification for 101 of them. Thus, the unidentified sources are the majority in the gamma-ray sky, as we currently know it. Solving the mystery of the unidentified gamma-ray sources is a major challenge for all fields of astronomy. Here we shall review the different approaches towards source identification as well as the results achieved so far.
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.
Cir X-1 is one of the most intriguing galactic X-ray sources. It is a ~16.6 days variable X/radio source, a type-I X-ray burster and a QPO emitter. In spite of an uncertain optical counterpart classification, all these properties identify the source as an LMXB. The morphology of the surrounding radio nebula has suggested an association with the nearby (~25 arcmin) SNR G321.9-0.3, implying that Cir X-1 is a runaway binary originated from the supernova explosion 10^5 years ago. To investigate this hypothesis, we carried out a proper motion measurement of the Cir X-1 m ~19 optical counterpart using a set of HST/WFC and WFPC2 observations taken ~8.6 years apart. We obtained a 3 sigma upper limit of ~5 mas/yr on the source proper motion. Since the runaway hypothesis would have implied a proper motion due North ranging between 15 and 75 mas/yr, depending on the actual age of the SNR, our result definitively rules out the association between Cir X-1 and SNR G321.9-0.3.
The vast majority of the high-energy gamma-ray sources discovered by EGRET are still unidentified. Percentages range from 50% at high galactic latitudes, where blazars are responsible for almost all identified sources, to more than 90% near the galactic plane, where isolated neutron stars appear to be the only certified class of sources of high energy gamma-rays. In spite of all the efforts devoted to the identification problem, the only success story, so far, appears to be the chase for Geminga, where X-rays led the way to eventual optical identification. Similar searches are now starting to produce encouraging results, although none has reached, as yet, a certified identification.
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.
Multicolor photometry of the Vela pulsar (PSR B0833-45), updated by recent HST/WFPC2 observations obtained in the 555W, 675W and 814W filters, is presented. The available data provide the best characterization so far of the pulsar spectral shape, which is dominated by a flat power law continuum with spectral index α= -0.2 +/- 0.2, consistent with the extrapolation in the optical domain of the power law component of the X-ray spectrum detected by Chandra. In addition, a marginally significant dip (~ 3 sigma) seems to be present at about 6500 AA. Spectroscopic observations with the VLT, now in progress, will undoubtly provide a much better assessment of the reality and characteristics of this feature.
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).
Given their intrinsic faintness, isolated neutron stars represent an elusive target for optical astronomy. Up to date, an optical counterpart has been identified only for a very tiny fraction of the overall population of more than 1000 radio pulsars. The HST contributions to the optical studies of isolated neutron stars have been recently reviewed by Mignani et al. (2000) and by Caraveo et al. (these proceedings). Here, we review its specific contribution in the field of pulsar astrometry and kinematics.