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Register a new userCentral Compact Objects in Supernova Remnants
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Authors
Andrea De Luca
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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.
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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.
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.
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.
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.
In a failed supernova, partial ejection of the progenitors outer envelope can occur due to weakening of the cores gravity by neutrino emission in the protoneutron star phase. We consider emission when this ejecta sweeps up the circumstellar material, analogous to supernova remnants (SNRs). We focus on failed explosions of blue supergiants, and find that the emission can be bright in soft X-rays. Due to its soft emission, we find that sources in the Large Magellanic Cloud (LMC) are more promising to detect than those in the Galactic disk. These remnants are characteristic in smallness ($lesssim 10$ pc) and slowness (100s of ${rm km s^{-1}}$) compared to typical SNRs. Although the expected number of detectable sources is small (up to a few by eROSITA 4-year all-sky survey), prospects are better for deeper surveys targeting the LMC. Detection of these failed SNRs will realize observational studies of mass ejection upon black hole formation.
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