arXiv subjects

R. P. Mignani

Publications and source records attributed to R. P. Mignani.

At least 73 records · Page 4Linked to original sources

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.

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arXiv2011

X-ray and optical observations of the closest isolated radio pulsar

With a parallactic distance of 170 pc, PSR J2144-3933 is the closest isolated radio pulsar currently known. It is also the slowest (P = 8.51 s) and least energetic (Edot = 2.6E28 erg/s) radio pulsar; its radio emission is difficult to account for with standard pulsar models, since the position of PSR J2144-3933 in the period-period derivative diagram is far beyond the typical radio `death lines'. Here we present the first deep X-ray and optical observations of PSR J2144-3933, performed in 2009 with XMM-Newton and European Southern Observatory (ESO)/Very Large Telescope (VLT), from which we derive, assuming a blackbody emission spectrum, a surface temperature upper limit of 2.3E5 K for a 13 km radius neutron star, 4.4E5 K for a 500 m radius hot spot and 1.9E6 K for a 10 m radius polar cap. In addition, our non-detection of PSR J2144-3933 constrains its non-thermal luminosity to be <30 per cent and <2 per cent of the pulsar rotational energy loss in the 0.5-2 keV X-ray band and in the B optical band, respectively.

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arXiv2010

Neutron Star Astronomy with the HST

Since its launch in 1990, HST has played a leading role in optical studies of isolated neutron stars, both radio-loud pulsars and radio-silent ones, paving the way to follow-up observations performed with 8m-class telescopes, like the VLT, the Gemini, and Subaru. Here, I present the last results obtained mostly by the WFPC2, before its de-commissioning during the last refurbishment mission in May 2009, from the observations of the rotation-powered pulsars PSR B0540-69, PSR B1055-52 and of the CCO 1E 1207.4-5209 in the PKS 1209-52 SNR.

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arXiv2010

Discovery of 59ms Pulsations from 1RXS J141256.0+792204 (Calvera)

We report on a multi-wavelength study of the compact object candidate 1RXS J141256.0+792204 (Calvera). Calvera was observed in the X-rays with XMM/EPIC twice for a total exposure time of ~50 ks. The source spectrum is thermal and well reproduced by a two component model composed of either two hydrogen atmosphere models, or two blackbodies (kT_1~ 55/150 eV, kT_2~ 80/250 eV, respectively, as measured at infinity). Evidence was found for an absorption feature at ~0.65 keV; no power-law high-energy tail is statistically required. Using pn and MOS data we discovered pulsations in the X-ray emission at a period P=59.2 ms. The detection is highly significant (> 11 sigma), and unambiguously confirms the neutron star nature of Calvera. The pulse profile is nearly sinusoidal, with a pulsed fraction of ~18%. We looked for the timing signature of Calvera in the Fermi Large Area Telescope (LAT) database and found a significant (~5 sigma) pulsed signal at a period coincident with the X-ray value. The gamma-ray timing analysis yielded a tight upper limit on the period derivative, dP/dt < 5E-18 s/s (dE_rot/dt <1E33 erg/s, B<5E10 G for magneto- dipolar spin-down). Radio searches at 1.36 GHz with the 100-m Effelsberg radio telescope yielded negative results, with a deep upper limit on the pulsed flux of 0.05 mJy. Diffuse, soft (< 1 keV) X-ray emission about 13' west of the Calvera position is present both in our pointed observations and in archive ROSAT all-sky survey images, but is unlikely associated with the X-ray pulsar. Its spectrum is compatible with an old supernova remnant (SNR); no evidence for diffuse emission in the radio and optical bands was found. The most likely interpretations are that Calvera is either a central compact object escaped from a SNR or a mildly recycled pulsar; in both cases the source would be the first ever member of the class detected at gamma-ray energies.

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arXiv2010

Neutron Star Astronomy with the E-ELT

So far, 24 Isolated neutron stars (INSs) of different types have been identified at optical wavelengths, from the classical radio pulsars to more peculiar objects, like the magnetars. Most identifications have been obtained in the last 20 years thanks to the deployment of modern technology telescopes, above all the HST, but also the NTT and, later, the 8m-class telescopes like the VLT. The larger identification rate has increased the impact factor of optical observations in the multi-wavelength approach to INS astronomy, opening interesting possibilities for studies not yet possible at other wavelengths. With the HST on the way to its retirement, 8m class telescopes will have the task of bridging neutron star optical astronomy into a new era, characterised by the advent of the generation of extremely large telescopes (ELTs), like the European ELT (E-ELT). This will mark a major step forward in the field, enabling one to identify many more INSs, many of which from follow-ups of observations performed with future radio and X-ray megastruscture facilities like SKA and IXO. Moreover, the E-ELT will make it possible to carry out observations, like timing, spectroscopy, and polarimetry, which still represent a challenge for 8m-class telescopes and are, in many respects, crucial for studies on the structure and composition of the neutron star interior and of its magnetosphere. In this contribution, I briefly summarise the current status of INS optical observations, describe the main science goals for the E-ELT, and their impact on neutron star physics.

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arXiv2010

A kinematic study of Open Clusters: implications for their origin

The Galactic population of open clusters provides an insight into star formation in the Galaxy. The open cluster catalogue by Dias et al.(2002b) is a rich source of data, including kinematic information. This large sample made it possible to carry out a systematic analysis of 481 open cluster orbits, using parameters based on orbit eccentricity and separation from the Galactic plane. These two parameters may be indicative of origin, and we find them to be correlated. We also find them to be correlated with metallicity, another parameter suggested elsewhere to be a marker for origin in that high values of any of these two parameters generally indicates a low metallicity ([Fe/H] Solar$<-$0.2 dex). The resulting analysis points to four open clusters in the catalogue being of extra-Galactic origin by impact of high velocity cloud on the disk: Berkeley21, 32, 99, and Melotte66, with a possible further four due to this origin (NGC2158, 2420, 7789, IC1311). A further three may be due to Galactic globular cluster impact on the disk i.e of internal Galactic origin (NGC6791, 1817, and 7044).

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arXiv2010

Optical timing studies of isolated neutron stars: Current Status

Being fast rotating objects, Isolated Neutron Stars (INSs) are natural targets for high-time resolution observations across the whole electromagnetic spectrum. With the number of objects detected at optical (plus ultraviolet and infrared) wavelengths now increased to 24, high-time resolution observations of INSs at these wavelengths are becoming more and more important. While classical rotation-powered radio pulsars, like the Crab and Vela pulsars, have been the first INSs studied at high-time resolution in the optical domain, observations performed in the last two decades have unveiled potential targets in other types of INSs which are not rotation powered, although their periodic variability is still related to the neutron star rotation. In this paper I review the current status of high-time resolution observations of INSs in the optical domain for different classes of objects: rotation-powered pulsars, magnetars, thermally emitting neutron stars, and rapid radio transients, I describe their timing properties, and I outline the scientific potentials of their optical timing studies.

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.

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arXiv2010

Optical-uv spectrum and proper motion of the middle-aged pulsar b1055-52

PSRB1055-52 is a middle-aged (~535 kyr) radio, X-ray, and gamma-ray pulsar showing X-ray thermal emission from the neutron star (NS) surface. A candidate optical counterpart to PSRB1055-52 was proposed by Mignani and coworkers based on Hubble Space Telescope (HST) observations performed in 1996, in one spectral band only. We report on HST observations of this field carried out in 2008, in four spectral bands. The astrometric and photometric analyses of these data confirm the identification of the proposed candidate as the pulsar's optical counterpart. Similarly to other middle-aged pulsars, its optical-UV spectrum can be described by the sum of a power-law (PLO) component, presumably emitted from the pulsar magnetosphere, and a Rayleigh-Jeans (RJ) component emitted from the NS surface. The spectral index of the PLO component, alpha_O=1.05+/-0.34, is larger than for other pulsars with optical counterparts. The RJ component, with the brightness temperature TO=(0.66+/-0.10) d_350**2 R_O,13**-2 MK (where d_350 and R_O,13 are the distance to the pulsar in units of 350 pc and the radius of the emitting area in units of 13 km), shows a factor of 4 excess with respect to the extrapolation of the X-ray thermal component into the UV-optical. This hints that the RJ component is emitted from a larger, colder area, and suggests that the distance to the pulsar is smaller than previously thought. From the absolute astrometry of the HST images we measured the pulsar coordinates with a position accuracy of 0.15". From the comparison with previous observations we measured the pulsar proper motion, mu = 42+/-5 mas/yr, which corresponds to a transverse velocity V_t = (70+/-8) d_350 km/s.

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arXiv2010

HST/WFPC2 observations of the LMC pulsar PSR B0540-69

The study of the younger, and brighter, pulsars is important to understand the optical emission properties of isolated neutron stars. PSRB0540-69, the second brightest (V~22) optical pulsar, is obviously a very interesting target for these investigations. The aim of this work is threefold: constraining the pulsar proper motion and its velocity on the plane of the sky through optical astrometry, obtaining a more precise characterisation of the pulsar optical spectral energy distribution (SED) through a consistent set of multi-band, high-resolution, imaging photometry observations, measuring the pulsar optical phase-averaged linear polarisation, for which only a preliminary and uncertain measurement was obtained so far from ground-based observations. We performed high-resolution observations of PSRB0540-69 with the WFPC2 aboard the HST, in both direct imaging and polarimetry modes. From multi-epoch astrometry we set a 3sigma upper limit of 1 mas/yr on the pulsar proper motion, implying a transverse velocity <250 km/s at the 50 kpc LMC distance. Moreover, we determined the pulsar absolute position with an unprecedented accuracy of 70 mas. From multi-band photometry we characterised the pulsar power-law spectrum and we derived the most accurate measurement of the spectral index (0.70+/-0.07) which indicates a spectral turnover between the optical and X-ray bands. Finally, from polarimetry we obtained a new measurement of the pulsar phase-averaged polarisation degree (16+/-4%),consistent with magnetosphere models depending on the actual intrinsic polarisation degree and depolarisation factor, and we found that the polarisation vector (22+/-12deg position angle) is possibly aligned with the semi-major axis of the pulsar-wind nebula and with the apparent proper motion direction of its bright emission knot.

astro-ph.SR↗

arXiv2010

Early X-ray and optical observations of the soft gamma-ray repeater SGR 0418+5729

Emission of two short hard X-ray bursts on 2009 June 5 disclosed the existence of a new soft gamma-ray repeater, now catalogued as SGR 0418+5729. After a few days, X-ray pulsations at a period of 9.1 s were discovered in its persistent emission. SGR 0418+5729 was monitored almost since its discovery with the Rossi X-ray Timing Explorer (2-10 keV energy range) and observed many times with Swift (0.2-10 keV). The source persistent X-ray emission faded by a factor 10 in about 160 days, with a steepening in the decay about 19 days after the activation. The X-ray spectrum is well described by a simple absorbed blackbody, with a temperature decreasing in time. A phase-coherent timing solution over the 160 day time span yielded no evidence for any significant evolution of the spin period, implying a 3-sigma upper limit of 1.1E-13 s/s on the period derivative and of 3E+13 G on the surface dipole magnetic field. Phase-resolved spectroscopy provided evidence for a significant variation of the spectrum as a function of the stellar rotation, pointing to the presence of two emitting caps, one of which became hotter during the outburst. Finally, a deep observation of the field of SGR 0418+5729 with the new Gran Telescopio Canarias 10.4-m telescope allowed us to set an upper limit on the source optical flux of i'>25.1 mag, corresponding to an X-ray-to-optical flux ratio exceeding 10000, consistent with the characteristics of other magnetars.

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arXiv2009

VLT optical observations of the isolated neutron star RX J0420.0-5022

X-ray observations performed with ROSAT led to the discovery of 7 radio-silent isolated neutron stars (INSs) which are detected only through the relatively dim and purely thermal X-ray emission from the cooling star surface. A few of these INSs (a.k.a. X-ray Dim INSs, or XDINSs) have been also detected at optical wavelengths where they seem to feature thermal spectra. Optical studies of XDINSs thus play a crucial role in mapping the temperature distribution on the neutron star surface and in investigating the existence of an atmosphere around the neutron star. The aim of this work is to investigate the optical identification of the XDINS RX J0420.0-5022, tentatively proposed by Haberl et al. (2004) based on VLT observations. We re-analysed the original observations of Haberl et al. to assess the detection significance of the proposed counterpart and we performed deeper VLT observations aiming at a higher confidence detection. With a ~2 sigma detection significance and a re-computed flux of B=27.52+/-0.61, we can not rule out that the proposed counterpart was spurious and produced by the halo of a very bright nearby star. While we could not detect the proposed counterpart in our deeper VLT observations, we found evidence for a marginally significant (~3.9 sigma) detection of a similarly faint object (B= 27.5+/-0.3), ~ 0.5" north of it and coincident with the updated Chandra position of RX J0420.0-5022. Interestingly, the angular separation is consistent with the upper limit on the RX J0420.0-5022 proper motion (Motch et al. 2009), which suggests that we might have actually detected the Haberl et al. proposed counterpart. From the flux of the putative RX J0420.0-5022 counterpart we can rule out a > 7 optical excess with respect to the extrapolation of the XMM-Newton spectrum.

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arXiv2009

Deep optical observations of the central X-ray source in the Puppis A supernova remnant

X-ray observations reveiled a group of radio-silent isolated neutron stars (INSs) at the centre of young supernova remnants (SNRs), dubbed central compact objects or CCOs, with properties different from those of classical rotation-powered pulsars. In at least three cases, evidence points towards CCOs being low-magnetized INSs, born with slow rotation periods, and possibly accreting from a debris disc of material formed out of the supernova event. Understanding the origin of the diversity of the CCOs can shed light on supernova explosion and neutron star formation models. Optical/infrared (IR) observations are crucial to test different CCO interpretations. The aim of our work is to perform a deep optical investigation of the CCO RX J0822.0-4300 in the Puppis A SNR, one of the most poorly understood in the CCO family. By using as a reference the Chandra X-ray coordinates of RX J0822.0-4300, we performed deep optical observations in the B, V and I bands with the Very Large Telescope (VLT). We found no candidate optical counterpart within 3 sigma of the computed Chandra X-ray position down to 5 sigma limits of B~27.2, V~26.9, and I~25.6, the deepest obtained in the optical band for this source. These limits confirm the non-detection of a companion brighter than an M5 dwarf. At the same time, they do not constrain optical emission from the neutron star surface, while emission from the magnetosphere would require a spectral break in the optical/IR.

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arXiv2009

A Deep XMM-Newton Serendipitous Survey of a middle-latitude area. II. New deeper X-ray and optical observations

The radio-quiet neutron star 1E1207.4-5209 has been the target of several XMM-Newton observations, with a total exposure of ~350 ks. The source is located at intermediate galactic latitude (b~10 degrees), i.e. in a sky region with an extremely interesting mix of both galactic and extra-galactic X-ray sources. The aim of our work is to investigate the properties of both the intermediate-latitude galactic and extra-galactic X-ray source populations in the 1E1207.4-5209 field. We performed a coherent analysis of the whole XMM-Newton observation data set to build a catalogue of serendipitous X-ray sources detected with high confidence and to derive information on the source flux, spectra, and time variability. In addition, we performed a complete multi-band (UBVRI) optical coverage of the field with the Wide Field Imager (WFI) of the ESO/MPG 2.2m telescope (La Silla) to search for candidate optical counterparts to the X-ray sources, down to a V-band limiting magnitude of ~24.5. We detected a total of 144 serendipitous X-ray sources. Thanks to the refined X-ray positions and to the WFI observations, we found candidate optical counterparts for most of the X-ray sources in our compilation. For most of the brightest ones we proposed a likely classification based on both the X-ray spectra and the optical colours. Our results indicate that at intermediate galactic latitude the X-ray source population is dominated by the extra-galactic component, but with a significant contribution from the galactic component in the soft energy band, below 2 keV.

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arXiv2009

The Brera Multi-scale Wavelet Chandra Survey. The serendipitous source catalogue

We present the Brera Multi-scale Wavelet Chandra (BMW-Chandra) source catalogue drawn from essentially all Chandra ACIS-I pointed observations with an exposure time in excess of 10ks public as of March 2003 (136 observations). Using the wavelet detection algorithm developed by Lazzati et al. (1999) and Campana et al. (1999), which can characterise both point-like and extended sources, we identified 21325 sources. Among them, 16758 are serendipitous, i.e. not associated with the targets of the pointings. This makes our catalogue the largest compilation of Chandra sources to date. The 0.5-10keV absorption corrected fluxes of these sources range from 3E-16 to 9E-12 erg/cm2/s with a median of 7E-15 erg/cm2/s. The catalogue consists of count rates and relative errors in three energy bands (total, 0.5-7keV; soft, 0.5-2keV; and hard, 2-7keV), where the detection was performed, and source positions relative to the highest signal-to-noise detection among the three bands. The wavelet algorithm also provides an estimate of the extension of the source. We also extracted source counts in four additional energy bands, SB1 (0.5-1keV), SB2 (1-2keV), HB1 (2-4keV), and HB2 (4-7keV).

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arXiv2009

Virtual Observatory studies of Planetary Nebulae

Starting from the Strasbourg ESO Catalogue (SEC) of Planetary Nebulae (PNe), the largest PNe compilation available with ~ 1500 objects, we undertook a comprehensive study of the whole PN population, never carried out so far, only using on-line catalogues and data from public imaging surveys. The study includes the PN dynamics through their measured proper motions (PMs), the study of their galactocentric orbits, the study of their interactions with the interstellar medium (ISM), and the study of their UV-to-IR spectral energy distribution (SED). As a preliminary step required to perform cross-correlations with on-line catalogues, we first went through a systematic reassessment of the PN coordinates (Kerber et al. 2003a).

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arXiv2009

The BMW-Chandra survey. Serendipitous Source Catalogue

We present the BMW-Chandra source catalogue derived from Chandra ACIS-I observations (exposure time >10ks) public as of March 2003 by using a wavelet detection algorithm (Lazzati et al. 1999; Campana et al. 1999). The catalogue contains a total of 21325 sources, 16758 of which are serendipitous. Our sky coverage in the soft band (0.5-2 keV, S/N =3) is ~8 deg^2 for F_X > 10^-13 erg cm^-2 s-1, and ~2 deg^2 for F_X >10^-15 erg cm^-2 s^-1. The catalogue contains information on positions, count rates (and errors) in three energy bands. (total, 0.5-7 keV; soft, 0.5-2 keV; and hard, 2-7keV), and in four additional energy bands, SB1 (0.5-1keV), SB2 (1-2 keV), HB1 (2-4 keV), and HB2 (4-7keV), as well as information on the source extension, and cross-matches with the FIRST, IRAS, 2MASS, and GSC-2 catalogues.

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arXiv2009

The Crab optical and ultraviolet polarimetry

Polarisation measurements of pulsars and of their pulsar wind nebulae (PWNe) are uniquely able to provide deep insights into the highly magnetised relativistic environment of young, rotation-powered isolated neutron stars (INSs). Besides the radio band, optical observations are primarily suited to providing such insights. The first INS for which optical polarisation observations were performed is the Crab pulsar which is also the brightest one (V=16.5). For this reason, the Crab pulsar is also the only INS for which repeated, phase-resolved polarisation measurements have been performed through the years. Moreover, it is the only case, together with the much fainter and distant PSR B0540-69 in the Large Magellanic Cloud (LMC), of an optical pulsar embedded in an optical PWN. Thus, the Crab is a perfect test case to study the optical polarisation properties of pulsars and of their PWNe. In this paper, we review the polarisation properties of the Crab pulsar and of its PWN in the optical and ultraviolet domains, we summarise the state of the art of the polarisation observations of other INSs, and we outline perspectives for INS polarisation studies with present and future generations of optical telescopes

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