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Register a new userA kinematic study of central compact objects and their host supernova remnants
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Authors
Martin G. F. Mayer
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Context. Central compact objects (CCOs) are a peculiar class of neutron stars, primarily encountered close to the center of young supernova remnants (SNRs) and characterized by thermal X-ray emission. Aims. Our goal is to perform a systematic study of the proper motion of all known CCOs with appropriate data available. In addition, we aim to measure the expansion of three SNRs within our sample to obtain a direct handle on their kinematics and age. Methods. We analyze multiple archival Chandra data sets, consisting of HRC and ACIS observations separated by temporal baselines between 8 and 15 years. In order to correct for systematic astrometric uncertainties, we establish a reference frame using X-ray detected sources in Gaia DR2, to provide accurate proper motion estimates for our target CCOs. Complementarily, we use our coaligned data sets to trace the expansion of three SNRs by directly measuring the spatial offset of various filaments and ejecta clumps between different epochs. Results. In total, we present new proper motion measurements for six CCOs, among which we do not find any indication of a hypervelocity object. We tentatively identify direct signatures of expansion for the SNRs G15.9+0.2 and Kes 79, at estimated significance of $2.5sigma$ and $2sigma$, respectively. Moreover, we confirm recent results by Borkowski et al., measuring the rapid expansion of G350.1$-$0.3 at almost $6000,{rm km,s^{-1}}$, which places its maximal age at $600-700$ years. The observed expansion, combined with the rather small proper motion of its CCO, implies the need for a very inhomogeneous circumstellar medium to explain the highly asymmetric appearance of the SNR. Finally, for the SNR RX J1713.7$-$3946, we combine previously published expansion measurements with our measurement of the CCOs proper motion to obtain a constraining upper limit of $1700$ years on the systems age.
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Central Compact Objects (CCOs) are a handful of soft X-ray sources located close to the centers of Supernova Remnants and supposed to be young, radio-quiet Isolated Neutron Stars (INSs). A clear understanding of their physics would be crucial in order to complete our view of the birth properties of INSs. We will review the phenomenologies of CCOs, underlining the most important, recent results, and we will discuss the possible relationships of such sources with other classes of INSs.
Most young neutron stars belonging to the class of Central Compact Objects in supernova remnants (CCOs) do not have known periodicities. We investigated seven such CCOs to understand the common reasons for the absence of detected pulsations. Making use of XMM-Newton, Chandra, and NICER observations, we perform a systematic timing and spectral analysis to derive updated sensitivity limits for both periodic signals and multi-temperature spectral components that could be associated with radiation from hotspots on the neutron star surface. Based on these limits, we then investigated for each target the allowed viewing geometry that could explain the lack of pulsations. We estimate it is unlikely ($< 10^{-6}$) to attribute that we do not see pulsations to an unfavorable viewing geometry for five considered sources. Alternatively, the carbon atmosphere model, which assumes homogeneous temperature distribution on the surface, describes the spectra equally well and provides a reasonable interpretation for the absence of detected periodicities within current limits. The unusual properties of CCOs with respect to other young neutron stars could suggest a different evolutionary path, as that proposed for sources experiencing episodes of significant fallback accretion after the supernova event.
We perform a sub-threshold follow-up search for continuous nearly-monochromatic gravitational waves from the central compact objects associated with the supernova remnants Vela Jr., Cassiopeia A, and SNR G347.3$-$0.5. Across the three targets, we investigate the most promising ~ 10,000 combinations of gravitational wave frequency and frequency derivative values, based on the results from an Einstein@Home search of the LIGO O1 observing run data, dedicated to these objects. The selection threshold is set so that a signal could be confirmed using the newly released O2 run LIGO data. In order to achieve best sensitivity we perform two separate follow-up searches, on two distinct stretches of the O2 data. Only one candidate survives the first O2 follow-up investigation, associated with the central compact object in SNR G347.3-0.5, but it is not conclusively confirmed. In order to assess a possible astrophysical origin we use archival X-ray observations and search for amplitude modulations of a pulsed signal at the putative rotation frequency of the neutron star and its harmonics. This is the first extensive electromagnetic follow-up of a continuous gravitational wave candidate performed to date. No significant associated signal is identified. New X-ray observations contemporaneous with the LIGO O3 run will enable a more sensitive search for an electromagnetic counterpart. A focused gravitational wave search in O3 data based on the parameters provided here should be easily able to shed light on the nature of this outlier. Noise investigations on the LIGO instruments could also reveal the presence of a coherent contamination.
Magnetars are regarded as the most magnetized neutron stars in the Universe. Aiming to unveil what kinds of stars and supernovae can create magnetars, we have performed a state-of-the-art spatially resolved spectroscopic X-ray study of the supernova remnants (SNRs) Kes 73, RCW 103, and N49, which host magnetars 1E 1841-045, 1E 161348-5055, and SGR 0526-66, respectively. The three SNRs are O- and Ne-enhanced and are evolving in the interstellar medium with densities of >1--2 cm$^{-3}$. The metal composition and dense environment indicate that the progenitor stars are not very massive. The progenitor masses of the three magnetars are constrained to be < 20 Msun (11--15 Msun for Kes 73, < 13 Msun for RCW 103, and ~13 --17 Msun for N49). Our study suggests that magnetars are not necessarily made from very massive stars, but originate from stars that span a large mass range. The explosion energies of the three SNRs range from $10^{50}$ erg to ~2$times 10^{51}$ erg, further refuting that the SNRs are energized by rapidly rotating (millisecond) pulsars. We report that RCW 103 is produced by a weak supernova explosion with significant fallback, as such an explosion explains the low explosion energy (~$10^{50}$ erg), small observed metal masses ($M_{rm O}sim 4times 10^{-2}$ Msun and $M_{rm Ne}sim 6times 10^{-3}$ Msun), and sub-solar abundances of heavier elements such as Si and S. Our study supports the fossil field origin as an important channel to produce magnetars, given the normal mass range ($M_{rm ZAMS} < 20$ Msun) of the progenitor stars, the low-to-normal explosion energy of the SNRs, and the fact that the fraction of SNRs hosting magnetars is consistent with the magnetic OB stars with high fields.
Supernova remnants (SNRs) are known to accelerate particles to relativistic energies, on account of their nonthermal emission. The observational progress from radio to gamma-ray observations reveals more and more morphological features that need to be accounted for when modeling the emission from those objects. We use our time-dependent acceleration code RATPaC to study the formation of extended gamma-ray halos around supernova remnants and the morphological implications that arise when the high-energetic particles start to escape from the SNRs. We performed spherically symmetric 1D simulations in which we simultaneously solved the transport equations for cosmic rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma. Our simulations span 25,000 years, thus covering the free-expansion and the Sedov-Taylor phase of the SNRs evolution. We find a strong difference in the morphology of the gamma-ray emission from SNRs at later stages dependent on the emission process. At early times, both the inverse-Compton and the Pion-decay morphology are shell-like. However, as soon as the maximum-energy of the freshly accelerated particles starts to fall, the inverse-Compton morphology starts to become center-filled, whereas the Pion-decay morphology keeps its shell-like structure. Escaping high-energy electrons start to form an emission halo around the SNR at this time. There are good prospects for detecting this spectrally hard emission with the future Cerenkov Telescope Array, as there are for detecting variations in the gamma-ray spectral index across the interior of the SNR. Further, we find a constantly decreasing nonthermal X-ray flux that makes a detection of X-ray unlikely after the first few thousand years of the SNRs evolution. The radio flux is increasing throughout the SNRs lifetime and changes from a shell-like to a more center-filled morphology later on.
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