The study of neutron star magnetospheres with LOFT

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of magnetospheres of isolated neutron stars. For a summary, we refer to the paper.

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.

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.

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.

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.

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

Observations of Isolated Neutron Stars with the ESO Multi-Conjugate Adaptive Optics Demonstrator

High-energy observations have unveiled peculiar classes of isolated neutron stars which, at variance with radio pulsars, are mostly radio silent and not powered by the star rotation. Among these objects are the magnetars, hyper-magnetized neutron stars characterized by transient X-ray/gamma-ray emission, and neutron stars with purely thermal, and in most cases stationary, X-ray emission (a.k.a., X-ray dim isolated neutron stars or XDINSs). While apparently dissimilar in their high-energy behavior and age, both magnetars and XDINSs have similar periods and unusually high magnetic fields. This suggests a tantalizing scenario where the former evolve into the latter.Discovering so far uninvestigated similarities between the multi-wavelength properties of these two classes would be a further step forward to establish an evolutionary scenario. A most promising channels is the near infrared (NIR) one, where magnetars are characterized by a distinctive spectral flattening with respect to the extrapolation of the soft X-ray spectrum.We observed the two XDINSs RX J0420.0-5022 and RX J1856.5-3754 with the Multi-Conjugate Adaptive Optics Demonstrator (MAD) at the Very Large Telescope (VLT) as part of the instrument guaranteed time observations program, to search for their NIR counterparts. Both RX J1856.5-3754 and RX J0420.0-5022 were not detected down to K_s ~20 and Ks ~21.5, respectively. In order to constrain the relation between XDINSs and magnetars it would be of importance to perform deeper NIR observations. A good candidate is 1RXS J214303.7+065419 which is the XDINS with the highest inferred magnetic field.

A possible optical counterpart to the old nearby pulsar J0108-1431

The multi-wavelength study of old (>100 Myr) radio pulsars holds the key to understanding the long-term evolution of neutron stars, including the advanced stages of neutron star cooling and the evolution of the magnetosphere. Optical/UV observations are particularly useful for such studies because they allow one to explore both thermal and non-thermal emission processes. In particular, studying the optical/UV emission constrains temperature of the bulk of the neutron star surface, too cold to be measured in X-ray observations.Aim of this work is to identify the optical counterpart of the very old (166 Myr) radio pulsar J0108-1431. We have re-analyzed our original VLT observations (Mignani et al. 2003), where a very faint object was tentatively detected close to the radio position, near the edge of a field galaxy. We found that the backward extrapolation of the PSR J0108-1431 proper motion recently measured by CHANDRA(Pavlov et al. 2008) nicely fits the position of this object. Based on that, we propose it as a viable candidate for the optical counterpart to PSR J0108-1431. The object fluxes (U =26.4+/-0.3; B =27.9; V >27.8) are consistent with a thermal spectrum with a brightness temperature of 9X10^4 K (for R = 13 km at a distance of 130 pc), emitted from the bulk of the neutron star surface. New optical observations are required to confirm the optical identification of PSR J0108-1431 and measure its spectrum.

VLT observations of Compact Central Objects

We present the first results of our VLT observation campaign of the Central Compact Objects (CCOs) in SNRs RX J085201.4-461753 (Vela Jr), 1E 1648-5051 (RCW 103) and RX J171328.4-394955 (G347.3-0.5). For Vela Jr., we found that the source is embedded in a compact optical nebulosity, possibly a bow-shock or a photo-ionization nebula, and we identified a candidate IR counterpart to the CCO. For RCW 103, we found no convincing evidence neither for 6 hrs IR modulation nor for variability on any time scale from the proposed counterpart, as well as for the other candidates close to the revised Chandra position. For G347.3-0.5, we identified few possible IR counterparts but none of them is apparently associated with the CCO.

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.