arXiv subjects

G. F. Bignami

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

At least 19 recordsLinked to original sources

arXiv2012

PSR J0357+3205: a fast moving pulsar with a very unusual X-ray trail

The middle-aged PSR J0357+3205 is a nearby, radio-quiet, bright gamma-ray pulsar discovered by the Fermi mission. Our previous Chandra observation revealed a huge, very peculiar structure of diffuse X-ray emission, originating at the pulsar position and extending for > 9' on the plane of the sky. To better understand the nature of such a nebula, we have studied the proper motion of the parent pulsar. We performed relative astrometry on Chandra images of the field spanning a time baseline of 2.2 yr, unveiling a significant angular displacement of the pulsar counterpart, corresponding to a proper motion of 0.165"+/-0.030" yr^(-1). At a distance of ~500 pc, the space velocity of the pulsar would be of ~390 km s^(-1) assuming no inclination with respect to the plane of the sky. The direction of the pulsar proper motion is perfectly aligned with the main axis of the X-ray nebula, pointing to a physical, yet elusive link between the nebula and the pulsar space velocity. No optical emission in the H_alpha line is seen in a deep image collected at the Gemini telescope, which implies that the interstellar medium into which the pulsar is moving is fully ionized.

astro-ph.HE↗

arXiv2011

A time-variable, phase-dependent emission line in the X-ray spectrum of the isolated neutron star RX J0822-4300

RX J0822-4300 is the Central Compact Object associated with the Puppis A supernova remnant. Previous X-ray observations suggested RX J0822-4300 to be a young neutron star with a weak dipole field and a peculiar surface temperature distribution dominated by two antipodal spots with different temperatures and sizes. An emission line at 0.8 keV was also detected. We performed a very deep (130 ks) observation with XMM-Newton, which allowed us to study in detail the phase-resolved properties of RX J0822-4300. Our new data confirm the existence of a narrow spectral feature, best modelled as an emission line, only seen in the `Soft' phase interval - when the cooler region is best aligned to the line of sight. Surprisingly, comparison of our recent observations to the older ones yields evidence for a variation in the emission line component, which can be modelled as a decrease in the central energy from ~0.80 keV in 2001 to ~0.73 keV in 2009--2010. The line could be generated via cyclotron scattering of thermal photons in an optically thin layer of gas, or - alternatively - it could originate in low-rate accretion by a debris disk. In any case, a variation in energy, pointing to a variation of the magnetic field in the line emitting region, cannot be easily accounted for.

astro-ph.HE↗

arXiv2011

Discovery of a faint X-ray counterpart and of a parsec-long X-ray tail for the middle-aged, gamma-ray only pulsar PSR J0357+3205

The Large Area Telescope (LAT) onboard the Fermi satellite opened a new era for pulsar astronomy, detecting gamma-ray pulsations from more than 60 pulsars, ~40% of which are not seen at radio wavelengths. One of the most interesting sources discovered by LAT is PSR J0357+3205, a radio-quiet, middle-aged (tau_C ~0.5 Myr) pulsar standing out for its very low spin-down luminosity (Erot ~6x10^33 erg/s), indeed the lowest among non-recycled gamma-ray pulsars. A deep X-ray observation with Chandra (0.5-10 keV), coupled with sensitive optical/infrared ground-based images of the field, allowed us to identify PSR J0357+3205 as a faint source with a soft spectrum, consistent with a purely non-thermal emission (photon index Gamma=2.53+/-0.25). The absorbing column (NH=8+/-4x10^20 cm^-2) is consistent with a distance of a few hundred parsecs. Moreover, the Chandra data unveiled a huge (9 arcmin long) extended feature apparently protruding from the pulsar. Its non-thermal X-ray spectrum points to synchrotron emission from energetic particles from the pulsar wind, possibly similar to other elongated X-ray tails associated with rotation-powered pulsars and explained as bow-shock pulsar wind nebulae (PWNe). However, energetic arguments, as well as the peculiar morphology of the diffuse feature associated with PSR J0357+3205 make the bow-shock PWN interpretation rather challenging.

astro-ph.HE↗

arXiv2011

Discovery of Powerful Gamma-Ray Flares from the Crab Nebula

The well known Crab Nebula is at the center of the SN1054 supernova remnant. It consists of a rotationally-powered pulsar interacting with a surrounding nebula through a relativistic particle wind. The emissions originating from the pulsar and nebula have been considered to be essentially stable. Here we report the detection of strong gamma-ray (100 MeV-10 GeV) flares observed by the AGILE satellite in September, 2010 and October, 2007. In both cases, the unpulsed flux increased by a factor of 3 compared to the non-flaring flux. The flare luminosity and short timescale favor an origin near the pulsar, and we discuss Chandra Observatory X-ray and HST optical follow-up observations of the nebula. Our observations challenge standard models of nebular emission and require power-law acceleration by shock-driven plasma wave turbulence within a ~1-day timescale.

astro-ph.HE↗

arXiv2010

X-ray pulsations from the radio-quiet gamma-ray pulsar in CTA 1

Prompted by the Fermi LAT discovery of a radio-quiet gamma-ray pulsar inside the CTA 1 supernova remnant, we obtained a 130 ks XMM-Newton observation to assess the timing behavior of this pulsar. Exploiting both the unprecedented photon harvest and the contemporary Fermi LAT timing measurements, a 4.7 sigma single peak pulsation is detected, making PSR J0007+7303 the second example, after Geminga, of a radio-quiet gamma-ray pulsar also seen to pulsate in X-rays. Phase-resolved spectroscopy shows that the off-pulse portion of the light curve is dominated by a power-law, non-thermal spectrum, while the X-ray peak emission appears to be mainly of thermal origin, probably from a polar cap heated by magnetospheric return currents, pointing to a hot spot varying throughout the pulsar rotation.

astro-ph.HE↗

arXiv2010

Agile Observations of the "Soft" Gamma-Ray Pulsar PSR B1509-58

We present the results of new Agile observations of PSR B1509-58 performed over a period of 2.5 years following the detection obtained with a subset of the present data. The modulation significance of the lightcurve above 30 MeV is at a 5$σ$ confidence level and the lightcurve is similar to those found earlier by Comptel up to 30 MeV: a broad asymmetric first peak reaching its maximum 0.39 +/- 0.02 cycles after the radio peak plus a second peak at 0.94 +/- 0.03. The gamma-ray spectral energy distribution of the pulsed flux detected by Comptel and Agile is well described by a power-law (photon index alpha=1.87+/-0.09) with a remarkable cutoff at E_c=81 +/- 20 MeV, representing the softest spectrum observed among gamma-ray pulsars so far. The pulsar luminosity at E > 1 MeV is $L_γ=4.2^{+0.5}_{-0.2} \times10^{35}$ erg/s, assuming a distance of 5.2 kpc, which implies a spin-down conversion efficiency to gamma-rays of $\sim 0.03$. The unusual soft break in the spectrum of PSR B1509-58 has been interpreted in the framework of polar cap models as a signature of the exotic photon splitting process in the strong magnetic field of this pulsar. In this interpretation our spectrum constrains the magnetic altitude of the emission point(s) at 3 km above the neutron star surface, implying that the attenuation may not be as strong as formerly suggested because pair production can substitute photon splitting in regions of the magnetosphere where the magnetic field becomes too low to sustain photon splitting. In the case of an outer-gap scenario, or the two pole caustic model, better constraints on the geometry of the emission would be needed from the radio band in order to establish whether the conditions required by the models to reproduce Agile lightcurves and spectra match the polarization measurements.

astro-ph.HE↗

arXiv2010

HST and VLT observations of the neutron star 1E 1207.4-5209

1E 1207.4-5209, the peculiar Central Compact object in the G296.5+10.0 supernova remnant, has been proposed to be an "anti-magnetar" - a young neutron star born with a weak dipole field. Accretion, possibly of supernova fallback material, has also been invoked to explain a large surface temperature anisotropy as well as the generation of peculiar cyclotron absorption features superimposed to its thermal spectrum. Interestingly enough, a faint optical/infrared source was proposed as a possible counterpart to 1E 1207.4-5209, but later questioned, based on coarse positional coincidence. Considering the large offset of 1E 1207.4-5209 with respect to the center of its host supernova remnant, the source should move at ~70 mas/yr. Thus, we tested the association by measuring the proper motion of the proposed optical counterpart. Using HST observations spanning 3.75 years, we computed a 3 sigma upper limit of 7 mas/yr. Absolute astrometry on the same HST data set also places the optical source significantly off the 99% confidence Chandra position. This allows us to safely rule out the association. Using the HST data set, coupled to ground-based observations collected at the ESO/VLT, we set the deepest limits ever obtained to the optical/infrared emission from 1E 1207.4-5209. By combining such limits to the constraints derived from X-ray timing, we rule out accretion as the source of the thermal anisotropy of the neutron star.

astro-ph.HE↗

arXiv2010

Detection of Gamma-Ray Emission from the Vela Pulsar Wind Nebula with AGILE

Pulsars are known to power winds of relativistic particles that can produce bright nebulae by interacting with the surrounding medium. These pulsar wind nebulae (PWNe) are observed in the radio, optical, x-rays and, in some cases, also at TeV energies, but the lack of information in the gamma-ray band prevents from drawing a comprehensive multiwavelength picture of their phenomenology and emission mechanisms. Using data from the AGILE satellite, we detected the Vela pulsar wind nebula in the energy range from 100 MeV to 3 GeV. This result constrains the particle population responsible for the GeV emission, probing multivavelength PWN models, and establishes a class of gamma-ray emitters that could account for a fraction of the unidentified Galactic gamma-ray sources.

astro-ph.HE↗

arXiv2009

Discovery of new gamma-ray pulsars with AGILE

Using gamma-ray data collected by the Astrorivelatore Gamma ad Immagini LEggero (AGILE) satellite over a period of almost one year (from 2007 July to 2008 June), we searched for pulsed signals from 35 potentially interesting radio pulsars, ordered according to $F_γ\propto \sqrt{\dot{E}} d^{-2}$ and for which contemporary or recent radio data were available. AGILE detected three new top-ranking nearby and Vela-like pulsars with good confidence both through timing and spatial analysis. Among the newcomers we find pulsars with very high rotational energy losses, such as the remarkable PSR B1509-58 with a magnetic field in excess of 10^13 Gauss, and PSR J2229+6114 providing a reliable identification for the previously unidentified EGRET source 3EG 2227+6122. Moreover, the powerful millisecond pulsar B1821-24, in the globular cluster M28, is detected during a fraction of the observations. Four other promising gamma-ray pulsar candidates, among which is the notable J2043+2740 with an age in excess of 1 million years, show a possible detection in the timing analysis only and deserve confirmation.

astro-ph.HE↗

arXiv2008

Deep infrared observations of the puzzling central X-ray source in RCW103

1E 161348-5055 (1E 1613) is a point-like, soft X-ray source originally identified as a radio-quiet, isolated neutron star, shining at the center of the 2000 yr old supernova remnant RCW103. 1E 1613 features a puzzling 6.67 hour periodicity as well as a dramatic variability over a time scale of few years. Such a temporal behavior, coupled to the young age and to the lack of an obvious optical counterpart, makes 1E 1613 a unique source among all compact objects associated to SNRs. It could either be the first low-mass X-ray binary system discovered inside a SNR, or a peculiar isolated magnetar with an extremely slow spin period. Analysis of archival IR observations, performed in 2001 with the VLT/ISAAC instrument, and in 2002 with the NICMOS camera onboard HST unveils a very crowded field. A few sources are positionally consistent with the refined X-ray error region that we derived from the analysis of 13 Chandra observations. To shed light on the nature of 1E 1613, we have performed deep IR observations of the field with the NACO instrument at the ESO/VLT, searching for variability. We find no compelling reasons to associate any of the candidates to 1E 1613. On one side, within the frame of the binary system model for the X-ray source, it is very unlikely that one of the candidates be a low-mass companion star to 1E 1613. On the other side, if the X-ray source is an isolated magnetar surrounded by a fallback disc, we cannot exclude that the IR counterpart be hidden among the candidates. If none of the potential counterparts is linked to the X-ray source, 1E 1613 would remain undetected in the IR down to Ks>22.1. Such an upper limit is consistent only with an extremely low-mass star (an M6-M8 dwarf) at the position of 1E 1613, and makes rather problematic the interpretation of 1E 1613 as an accreting binary system.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv2006

XMM-Newton and ESO observations of the two unidentified gamma-ray sources 3EG J0616-3310 and 3EG J1249-8330

The limited angular resolution of gamma-ray telescopes prevents a direct identification of the majority of sources detected so far. This is particularly true for the low latitude, probably galactic, ones only 10 % of which has been identified. Most counterparts of the identified low-latitude gamma-ray sources are Isolated Neutron Stars (INS), both radio-loud and radio-quiet (Geminga-like) objects, which are characterised by an extremely high value of the X-ray-to-optical flux ratio f_X/f_opt. Therefore, the systematic X-ray and optical coverage of low-latitude unidentified gamma-ray sources aiming at high f_X/f_opt sources seems one of the most promising ways to spot INS candidate counterparts. Since low latitude sources are heavily affected by the interstellar absorption at both X-ray and optical wavelengths, we have focussed on two middle-latitude, probably galactic, GRO/EGRET sources: 3EG J0616-3310 and 3EG J1249-8330. These two sources, which could belong to a local galactic population, have been selected owing to their relatively good positional accuracy, spectral shape and lack of candidate extragalactic radio counterparts. Here we report on X-ray observations of the two gamma-ray error boxes performed with XMM-Newton and on their optical follow-up carried on with the Wide Field Imager at the ESO/MPG 2.2m telescope. Less than half of the ~300 sources detected by the X-ray coverage have no optical counterparts. Among those, we have selected few interesting sources with f_X/f_opt > 100, which we consider promising INS candidates.

astro-ph↗

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.

astro-ph↗

arXiv2006

On the nature of the radio quiet X-ray neutron star 1E 1207.4-5209

The strange timing property of X-ray pulsar 1E 1207.4-5209 can be explained by the hypothesis that it is a member of an ultra-compact binary system. This paper confronts the ultra-compact assumption with the observed properties of this pulsar. The gravitational potential well of an ultra-compact binary can enlarge the corotation radius and thus make it possible for accreting material to reach the surface of the NS in the low accretion rate case. Thus the generation of the absorption features should be similar to the case of accreting pulsars. The close equality of the energy loss by fast cooling of the postsupernova neutron star and the energy dissipation needed for a wide binary evolving to an ultra-compact binary demonstrates that the ultra-compact binary may be formed in 10-100yr after the second supernova explosion. Moreover, the ultra-compact binary hypothesis can well explain the the absence of optical counterpart and the observed two black body emissions. We suggest a simple method which can test the binary nature directly with XMM-Newton and Chandra observations. We further predict that the temperature of the two black bodies should vary at different pulse periods.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗