Center Of Supernova Remnants Research Articles

Candidate isolated neutron stars in the 4XMM-DR10 catalogue of X-ray sources

ABSTRACT Most isolated neutron stars have been discovered thanks to the detection of their pulsed non-thermal emission, at wavelengths spanning from radio to gamma-rays. However, if the beamed non-thermal radiation does not intercept our line of sight or it is too faint or absent, isolated neutron stars can also be detected through their thermal emission, which peaks in the soft X-ray band and is emitted nearly isotropically. In the past 30 yr, several thermally emitting isolated neutron stars have been discovered thanks to X-ray all-sky surveys, observations targeted at the centre of supernova remnants, or as serendipitous X-ray sources. Distinctive properties of these relatively rare X-ray sources are very soft spectra and high ratios of X-ray to optical flux. The recently released 4XMM-DR10 catalogue contains more than half a million X-ray sources detected with the XMM–Newton telescope in the 0.2–10 keV range in observations carried out from 2000 to 2019. Based on a study of the spectral properties of these sources and on cross-correlations with catalogues of possible counterparts, we have carried out a search of isolated neutron stars, finding four potential candidates. The spectral and long-term variability analysis of these candidates, using Chandra and Swift-XRT data, allowed us to point out the most interesting sources deserving further multiwavelength investigations.

Are pulsars born with a hidden magnetic field?

The observation of several neutron stars in the center of supernova remnants and with significantly lower values of the dipolar magnetic field than the average radio-pulsar population has motivated a lively debate about their formation and origin, with controversial interpretations. A possible explanation requires the slow rotation of the proto-neutron star at birth, which is unable to amplify its magnetic field to typical pulsar levels. An alternative possibility, the hidden magnetic field scenario, considers the accretion of the fallback of the supernova debris onto the neutron star as responsible for the submergence (or screening) of the field and its apparently low value. In this paper we study under which conditions the magnetic field of a neutron star can be buried into the crust due to an accreting, conducting fluid. For this purpose, we consider a spherically symmetric calculation in general relativity to estimate the balance between the incoming accretion flow and the magnetosphere. Our study analyses several models with different specific entropy, composition, and neutron star masses. The main conclusion of our work is that typical magnetic fields of a few times 1e12 G can be buried by accreting only 1e-3 - 1e-2 solar mass, a relatively modest amount of mass. In view of this result, the Central Compact Object scenario should not be considered unusual, and we predict that anomalously weak magnetic fields should be common in very young (< few kyr) neutron stars.

HUNTING FOR ORPHANED CENTRAL COMPACT OBJECTS AMONG RADIO PULSARS

Central compact objects (CCOs) are a handful of young neutron stars found at the center of supernova remnants (SNRs). They show high thermal X-ray luminosities but no radio emission. Spin-down rate measurements of the three CCOs with X-ray pulsations indicate surface dipole fields much weaker than those of typical young pulsars. To investigate if CCOs and known radio pulsars are objects at different evolutionary stages, we carried out a census of all weak-field (<1e11 G) isolated radio pulsars in the Galactic plane to search for CCO-like X-ray emission. None of the 12 candidates is detected at X-ray energies, with luminosity limits of 1e32-1e34 erg/s. We consider a scenario in which the weak surface fields of CCOs are due to rapid accretion of supernova materials and show that as the buried field diffuses back to the surface, a CCO descendant is expected to leave the P-Pdot parameter space of our candidates at a young age of a few times 10kyr. Hence, the candidates are likely to be just old ordinary pulsars in this case. We suggest that further searches for orphaned CCO, which are aged CCOs with parent SNRs dissipated, should include pulsars with stronger magnetic fields.

Central compact objects and their magnetic fields

AbstractCentral compact objects (CCOs) are neutron stars that are found near the center of supernova remnants, and their association with supernova remnants indicates these neutron stars are young (≲ 104 yr). Here we review the observational properties of CCOs and discuss implications, especially their inferred magnetic fields. X-ray timing and spectral measurements suggest CCOs have relatively weak surface magnetic fields (~ 1010 − 1011 G). We argue that, rather than being created with intrinsically weak fields, CCOs are born with strong fields and we are only seeing a weak surface field that is transitory and evolving. This could imply that CCOs are one manifestation in a unified picture of neutron stars.

Evolution of a buried magnetic field in the central compact object neutron stars

The central compact objects are a newly-emerging class of young neutron stars near the centre of supernova remnants. From X-ray timing and spectral measurements, their magnetic fields are determined to be ~ 10^10-10^11 G, which is significantly lower than that found on most pulsars. Using the latest electrical and thermal conductivity calculations, we solve the induction equation to determine the evolution of a buried crustal or core magnetic field. We apply this model of a buried field to explain the youth and low observed magnetic field of the central compact objects. We obtain constraints on their birth magnetic field and depth of submergence (or accreted mass). Measurement of a change in the observed magnetic field strength would discriminate between the crustal and core fields and could yield uniquely the birth magnetic field and submergence depth. If we consider the central compact objects as a single neutron star viewed at different epochs, then we constrain the magnetic field at birth to be ~ (6-9)x10^11 G. A buried magnetic field can also explain their location in an underpopulated region of the spin period-period derivative plane for pulsars.

SEARCHING FOR COMPACT OBJECTS IN SNRs G27.8+0.6 AND G28.8+1.5

We analyzed XMM-Newton observations of two center-filled supernova remnants, G27.8+0.6 and G28.8+1.5, to search for pulsars/pulsar wind nebulae (PWNe) and other types of neutron stars associated with these remnants. We discovered a PWN candidate within the extent of the centrally-peaked radio emission of G27.8+0.6. The X-ray morphology of the PWN candidate and its position with respect to the host supernova remnant suggest that the alleged pulsar might be moving away from the supernova remnant's center with a transverse velocity of around 100-200 km/s. The majority of the detected X-ray point sources in both fields are classified as main-sequence stars, based on their bright optical counterparts and relatively soft X-ray spectra. The remaining medium-hard and hard sources are most probably either AGNs or cataclysmic variables, although we cannot completely rule out the possibility that some of these sources are neutron stars.

Detection of the High-Energy Cosmic Rays from the Monogem Ring

The MAKET-ANI detector reveals significant excess of extensive air showers with arrival directions pointed to the Monogem ring, a supernova remnant located at a distance of ≈300 pc from the Sun with ≈100 kyr old radio pulsar PSR B0656+14 near the center. The chances that this excess is due to the fluctuations of an isotropic flux is 2 per million. For the search of the cosmic-ray source, we use the MAKET-ANI detector data from years 1997 to 2003. The best signal bin coordinates, right ascension 75, declination 14° (750+14), significantly deviate from the ring morphological center, shifted in the direction of the most intensive X-ray emission from the supernova remnant's limb, now located 66 pc from the supernova remnant center and 27 pc from the candidate source.