Among many different manifestations of neutron stars, the one of the central compact objects (CCOs) is among the most enigmatic class. In this thesis, we have examined the physical properties of two CCOs located in the supernova remnants Puppis-A as well as Vela-Junior. These CCOs are generally accepted as the young neutron stars formed in core-collapsed supernova explosions. However, their emission properties are found to be very different from those observed in young canonical pulsars in many aspects.
While the X-ray emission of young rotation-powered pulsars are generally dominated by magnetospheric emission which is characterized by a power-law spectrum, the X-ray spectra of these CCOs can be well-modeled with a double blackbody model or a blackbody plus a steep power-law model with photon index >3. Utilizing the superior spatial resolution of Chandra, we found no evidence of any plerionic emission, which are typically powered by Crab-like pulsars, around the CCO in Puppis-A down to an accuracy of sub-arcsecond. Moreover, making use of two Chandra HRC-I observations with an epoch separation of somewhat more than five years we have probed the proper motion of this CCO. This is the first case that a CCO’s proper motion being measured directly. Our result suggests that
the transverse velocity of this object is at a magnitude of ∼ 1000 km/s which is about 4
times higher than the average velocity observed in ordinary pulsars.
Our spectral analysis of the CCOs suggest the presence of high temperature hot spots on the stellar surfaces. Under a favorable viewing geometry, low-amplitude periodic signals produced by the rotating hot spots can possibly be observed. Searching for possible pul- sations, we have revealed an interesting periodicity candidate from the CCO in Puppis-A. A periodic signal at ∼ 0.22 s has been marginally detected. Although the significance of
the detection is rather low, the signal is found with comparable properties (e.g. pulsed fraction, shape of the pulse profile, signal strength) in two independent XMM-Newton observations which were taken about 7 months apart from each other. The period deriva- tive calculated from the separation of the epochs of the two XMM-Newton observations is ˙P = (2.112±0.002)×10−10 s s−1. This is comparable with ˙P = (0.8−4.7)×10−10 s
s−1 which is the largest spin-down rate known for a neutron star and has been observed
in the soft gamma-ray repeater (SGR) SGR 1806-20 (Kouveliotou et al. 1998; Woods et al. 2002). If the periodic signal found in RX J0822-4300 is confirmed it suggests a non- steady spin-down behavior for this source as well as making it joining the exclusive group of SGR 1806-20 and SGR 1900+14 which both show large changes in the spin-down torque up to a factor of ∼4 (Woods et al. 2002).
Similar to the other SGRs in quiescence, a recent XMM-Newton observation of SGR 1627-41 has found that its spectrum is rather soft (Mereghetti et al. 2006). Both, a steep power-law and a high temperature blackbody model are able to fit the SGR spectrum. This is similar to the spectral behavior seen in many CCOs. Taken this similarity and the putative periodicity together it may suggest that the CCO in Puppis-A resembles a SGR in some way. This makes the further examination of the rotational dynamics of this CCO extremely interesting. An identification of a CCO with the group of magnetars would be a milestone in neutron star research. The recent XMM-Newton observations of the CCO in the SNR RCW 103 indicate that it could also be a peculiar magnetar (de Luca et al. 2006; Li 2007). This demonstrates indeed that such association is not too speculative.
On the other hand, Li (2007) has also pointed out that fallback disk accretion can be the other possible origin for the CCOs’ X-ray emission. During the core collapse of massive stars, a considerable amount of the stellar material can fall onto the collapsed stellar remnants. The existence of fallback disks can be examined by optical/IR observations since the irradiation of the debris disks by the CCOs is expected to result in IR excess. Wang et al. (2007) has searched for the optical/IR counterparts of a number of CCOs in supernova remnants, including Puppis-A. No conclusive evidence of fallback disks around these CCOs have been reported.
We would also like to point out that the class of CCO only has a sample size of about ten objects. In order to improve our knowledge of them, the sample size has to be enlarged. Systematic searches have been conveyed to look for extended X-ray sources as supernova remnant candidates in the ROSAT all-sky survey (RASS) data base (Busser 1998; Schaudel 2002). A number of diffuse structures have already been identified as promising X-ray counterparts of supernova remnants (Schaudel 2002). For those having compact features identified within the diffuse X-ray emission, observations with Chandra can possibly detect point sources as new neutron star candidates. We have identified one promising candidate in the supernova remnant G67.7+1.8. It was firstly noticed in a ROSAT PSPC image (Fig. 10.1) by matching the geometric center of the remnant with the peak emission in the diffuse X-ray structures. We have subsequently observed the source with Chandra. Figure 10.2 shows the false color image of the SNR as obtained by the ACIS-I detector. Using the wavelet source detection algorithm, we have detected 36 point-like sources in the field of view. Four point sources (labeled with # 9, 10, 11, 35 in Fig. 10.2) are located within 1.5
arcmin from the geometrical center of G67.7+1.8, they all appear to be harder than the surrounding remnant emission. Hence they are considered to be the likely CCO candidates associated with this SNR. Detailed analysis of these X-ray sources will be published soon.