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 ( G) isolated radio pulsars in the Galactic plane to search for CCO-like X-ray emission. None of the 12 candidates are detected at X-ray energies, with luminosity limits of erg s. We consider a scenario in which the weak surface fields of CCOs are due to a 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 - parameter space of our candidates at a young age of a few 10 kyr. Hence, the candidates are likely to just be old ordinary pulsars in this case. We suggest that further searches for orphaned CCOs, which are aged CCOs with parent SNRs that have dissipated, should include pulsars with stronger magnetic fields.
Subject headings:pulsars: general — stars: evolution — stars: neutron — X-rays: stars
Before the mid-1990s it was believed that young neutron stars are all fast-spinning objects with high surface magnetic field strengths of G, emitting radio pulses. However, recent discoveries of new populations of neutron stars, including central compact objects (CCOs), magnetars, and X-ray dim isolated neutron stars, have challenged this simple picture (see reviews by Kaspi, 2010; Harding, 2013). CCOs are the most enigmatic class. They are found at the center of supernova remnants (SNRs) and cannot be as easily classified as other types of objects. There are nine confirmed CCOs sharing the following properties: (1) they are located near the centers of young SNRs; (2) they show no radio or optical counterparts; (3) they have no detectable pulsar wind nebulae; and (4) they exhibit a thermal spectrum in the soft X-ray band with high luminosity erg s (see de Luca, 2008; Gotthelf & Halpern, 2008; Gotthelf et al., 2013a, for reviews). Since CCOs are generally associated with very young SNRs, their nature and evolution are highly relevant to the neutron star production rate and the physics underlying the branching ratios of core collapse (Keane & Kramer, 2008). However, the active lifetime and evolution of CCOs are poorly understood due to the small sample. It is also unclear if these neutron stars are intrinsically radio-quiet, or if they are radio pulsars beamed away from us (see e.g., Ho, 2013b).
Only three CCOs have X-ray pulsations firmly detected. They show periods of –0.4 s and long-term timing revealed small period derivatives of (Halpern & Gotthelf, 2010; Gotthelf et al., 2013a). Their spin parameters are plotted in the - diagram in Figure 1. These suggest spin-down luminosities of – erg s, where is the neutron star moment of inertia. These values are nearly two orders of magnitude smaller than the CCO’s X-ray luminosities, implying that the sources cannot be entirely powered by rotation. The characteristic age of the three CCOs is over years, much older than their true ages of a few thousand years, estimated from their associated SNRs. The inferred surface dipole field strengths G are of the order of G. These are much lower than those of young pulsars but still lie well above the radio pulsar “death line” (e.g., G s; Bhattacharya et al., 1992), which is the (uncertain) theoretical limit for producing radio emission. Indeed, radio pulsars have been detected in the CCO range of and (see Figure 1), hence with comparable dipole field strengths. This raises a fundamental question: are CCOs and radio pulsars the same class of objects or do they belong to disjointed sets of neutron stars?
If CCOs are ordinary radio pulsars born with weak magnetic fields (see, e.g., Bonanno et al., 2006; Spruit, 2008), their small values imply very slow spin evolution, such that they take a long time to reach the death line. The detection of nine young CCOs implies that there should be over older ones in the Galaxy (Kaspi, 2010). This is in contrast to the small number of known radio pulsars with low G as shown in the - diagram in Figure 1, given that there is no observational bias against these pulsars in radio surveys (see Kramer et al., 2003; Faucher-Giguère & Kaspi, 2006). The discrepancy could be reconciled if CCOs are strong-field objects but appear to have weak surface fields at a young age. This is supported by modeling of CCOs’ X-ray light curves, which suggest a strong crustal field (Gotthelf et al., 2010; Shabaltas & Lai, 2012; Bogdanov, 2014). It was proposed that the -fields of CCOs could be buried by supernova fallback (Halpern & Gotthelf, 2010; Ho, 2011, 2013a), and hence the radio emission is suppressed (as in the case of accreting millisecond pulsars in quiescence; see e.g., Stappers et al. 2014; Archibald et al. 2015). After accretion stops, the -field is expected to diffuse back to the surface with a timescale depending sensitively on the amount of accreted mass and ranging from years to over years (see Chevalier, 1989; Geppert et al., 1999; Ho, 2011; Viganò & Pons, 2012). The radio emission would then presumably switch on (Muslimov & Page, 1996). This picture predicts that young CCOs should be radio-quiet and aged ones could become ordinary radio pulsars (see Gotthelf et al., 2013b; Bogdanov et al., 2014).
No radio emission has yet been observed from any CCO (see Pavlov et al., 2004, and references therein), but given the small number of known pulsating CCOs and the possibility of small beaming fractions, the result is inconclusive and a more systematic study is needed. In addition to deeper radio observations, we can turn the search around to look for CCO-like X-ray emission from selected weak-field radio pulsars. Indeed, if CCOs manifest as radio pulsars, this could be a more efficient way to detect them, especially after their natal SNRs fade away in years. Any detection of X-ray emission from the radio-selected sample will confirm CCOs as a subset of radio pulsars, providing direct evidence to rule out the scenario of ongoing accretion. There were previous attempts to identify CCOs from radio pulsars positionally coincident with SNRs (Bogdanov et al., 2014) and aged CCOs with parent SNRs that have dissipated (so-called orphaned CCOs) from disrupted recycled pulsars (Gotthelf et al., 2013b). However, no new CCOs have been found. To complete the study, here we present an X-ray census of all isolated weak-field radio pulsars near the Galactic plane to search for CCO-like emission. The sample selection and data analysis are described in Section 2 and we report the detection limits and discuss the implications in Section 3.
2. Observations and Data Reduction
We select weak-field radio pulsars from the ATNF catalog (Manchester et al., 2005) with similar spin parameters as those of the known CCOs, according to the following criteria:
weak surface dipole -fields of G inferred from spin-down;
isolated and having periods s to avoid recycled pulsars; and
located pc from the Galactic plane to exclude old objects.
The pulsar height from the plane is calculated from the source’s Galactic latitude and estimated distance. Neutron stars are born at an average distance of 50 pc from the plane with a mean space velocity of km s (Faucher-Giguère & Kaspi, 2006). At this velocity, a pulsar travels only 36 pc in years, which is well below the cut even in the rare case that a pulsar moves exactly perpendicular to the plane.
|Pulsar||Dist.aaAll distance are estimated from the DM, since no parallax measurements are available.||DM||bb values are estimated by (He et al., 2013).||ccThe characteristic age is given by .||ddThe surface magnetic field strength, , is inferred from the spin parameters with G, where is in seconds.|
|(kpc)||(°)||(°)||( pc cm)||( cm)||(s)||( years)||( G)|
|PSR J0609+2130 (catalog J0609+2130)||1.2||189.2||1.0||3.9||0.06||0.24||38||0.37|
|PSR J1107-5907 (catalog J1107$-$5907)||1.3||289.9||1.1||4.0||0.25||9.0||4.5||4.8|
|PSR J1355-6206 (catalog J1355$-$6206)||8.3||310.3||0.2||55||0.28||3.1||14||3.0|
|PSR J1425-5723 (catalog J1425$-$5723)||1.2||315.4||3.2||4.3||0.35||22||2.5||8.9|
|PSR J1650-4341 (catalog J1650$-$4341)||7.5||341.6||0.5||67||0.31||17||2.9||7.3|
|PSR J1653-4315 (catalog J1653$-$4315)||4.6||342.3||0.4||34||0.42||15||4.4||8.0|
|PSR J1702-4217 (catalog J1702$-$4217)||7.1||344.1||0.3||63||0.23||11||3.2||5.2|
|PSR J1739-3951 (catalog J1739$-$3951)||0.8||4.0||4.7||2.5||0.34||20||2.7||8.3|
|PSR J1755-2725 (catalog J1755$-$2725)||2.4||2.4||1.1||12||0.26||14||3.0||6.1|
|PSR J1810-1820 (catalog J1810$-$1820)||5.6||12.1||0.3||45||0.15||52||0.47||9.1|
|PSR J1819-1510 (catalog J1819$-$1510)||5.3||15.9||0.1||42||0.23||7.9||4.6||4.3|
|PSR B1952+29 (catalog B1952+29)||0.7||65.3||0.8||0.79||0.43||1.7||40||2.7|
|Pulsar||ObsIDaaAll are Chandra observations except for PSR J18191510, which was observed with XMM-Newton MOS.||Effective||Count Rate||ccThe surface temperature () and bolometric luminosity () upper limits are inferred from the count rate limits, assuming uniform blackbody emission with an observed radius of =14.5 km and the DM distance in Table 1. See the text for the details.||ccThe surface temperature () and bolometric luminosity () upper limits are inferred from the count rate limits, assuming uniform blackbody emission with an observed radius of =14.5 km and the DM distance in Table 1. See the text for the details.||Age Lower|
|Exposure||Upper LimitbbThe count rate limits are at a 2 (i.e., 95%) confidence level in the 0.5–7 keV energy range.||LimitddThe age lower limits are inferred from the bolometric luminosity using the minimal neutron star cooling scenario (see Figure 2).|
|(ks)||( cnt s)||(eV)||( erg s)||(kyr)|
|J0609+2130||ADS/Sa.CXO#obs/12687 (catalog 12687)||5||6.3||51||20|
|J11075907||ADS/Sa.CXO#obs/12688 (catalog 12688)||5||14||55||20|
|J13556206||ADS/Sa.CXO#obs/13806 (catalog 13806)||3.4||9.1||134|
|J14255723||ADS/Sa.CXO#obs/12686 (catalog 12686)||5||8.9||53||20|
|J16504341||ADS/Sa.CXO#obs/13777 (catalog 13777)||8||3.9||124|
|J16534315||ADS/Sa.CXO#obs/13774 (catalog 13774)||5||6.3||98||0.4|
|J17024217||ADS/Sa.CXO#obs/13776 (catalog 13776)||8.6||6.4||126|
|J17393951||ADS/Sa.CXO#obs/12685 (catalog 12685)||5||6.3||46||30|
|J17552725||ADS/Sa.CXO#obs/08717 (catalog 8717), ADS/Sa.CXO#obs/08718 (catalog 8718)||4||12||79||1|
|J18101820||ADS/Sa.CXO#obs/13775 (catalog 13775)||5||14||119|
Our sample consists of 12 candidates. They are plotted in the - diagram in Figure 1 and their properties are listed in Table 1. While the pulsars have large characteristic ages of years, we note that their true age could be much younger, particularly if they were born spinning slowly (see Ng et al., 2007; Bogdanov et al., 2014). There is an archival Chandra on-axis ACIS-S observation of PSR J13556206 with 3 ks exposure, and PSR J17552725 was located far off-axis in two Chandra ACIS-I exposures with a total of 4 ks. PSR J18191510 falls onto the edge of the field of view in two XMM-Newton MOS exposures. After filtering the background flaring periods and correcting for vignetting, we obtained an effective total exposure of 72 ks for this source. The XMM-Newton PN data were not used in this study, as the total exposure is only 21 ks. For the remaining nine candidates, we obtained Chandra ACIS-S observations with 5 ks exposure each, except for PSRs J17024217 and J16504341, which have roughly 8 ks each. The observation IDs (ObsIDs) and effective exposures are listed in Table 2. Note that observations of three sources: PSRs J0609+2130, J13556206, and B1952+29 were recently reported in an independent study (Gotthelf et al., 2013b).
We carried out standard Chandra data reduction with CIAO v4.5 and CALDB v4.5.8. We reprocessed the data using the task chandra_repro and checked that there was no background flaring occuring during the Chandra observations; all data are therefore used in the analysis. The XMM-Newton data reduction was performed with SAS v13.5111XMM-Newton SAS is developed and maintained by the Science Operations Centre at the European Space Astronomy Centre and the Survey Science Centre at the University of Leicester.. We used the task emchain to reprocess the MOS data, then applied filtering using standard flags and rejected periods with high background. The analysis is restricted to events with PATTERN . Finally, we generated X-ray images in the 0.5–7 keV energy range for source detection.
3. Results and Discussion
The X-ray images show no obvious X-ray sources at the radio pulsar positions. To establish the pulsar flux limits, we extracted events from 5″ and 70″ radius source and background regions, respectively, for the 10 on-axis Chandra observations. The 5″ radius was chosen to ensure that the region encircles over 95% of the flux. A larger source region of 10″ radius was used for PSR J17552725, due to the large off-axis angle. For the XMM-Newton observations of PSR J18191510, we used a 60″ radius source region and excluded a background source inside. We follow Bogdanov et al. (2014) to compute the detection limits at a 2 confidence level, using the task aprates in CIAO, which is based on Bayesian statistics. The results are reported in Table 2.
The count rate limits above can be converted to luminosity limits using a spectral model. Following Gotthelf et al. (2013b), we assume uniform blackbody emission from the entire star with an observed radius of km (which corresponds to a neutron star radius of 11.6 km for a gravitational redshift of 1.25). The pulsar dispersion measure (DM) distance is adopted and the X-ray absorption column density () is inferred from DM using the empirical relation (He et al., 2013). We generated spectral response files for individual observations and determined the flux limits in the Sherpa environment (Freeman et al., 2001) by comparing the expected count rates with the observed limits. The surface temperature and bolometric luminosity limits are listed in Table 2. The detection limits of PSRs J0609+2130, J13556206, and B1952+29 are compatible with those reported in the previous study (Gotthelf et al., 2013b). As shown in the table, the luminosity limits are erg s for nearby ( kpc) sources and a few times erg s for the rest. In Figure 2, these results are compared with the observed bolometric luminosities of known CCOs. The latter are generally above erg s, suggesting that our candidates are unlikely to be young CCOs.
As mentioned, the characteristic age is not a good indicator of the true age of a pulsar. We therefore attempt to constrain the age by comparing the luminosity limits above with standard neutron star cooling curves. We employed a cooling code based on Gnedin et al. (2001) with some modifications as described in Ho et al. (2015). The temperature evolution of an isolated neutron star is determined by the relativistic equations of energy balance and heat flux (see, e.g., Yakovlev & Pethick, 2004). Two important factors affecting the evolution are the composition of the outer layer (i.e., the envelope) and whether the stellar interior is superfluid and/or superconducting. For the former, the envelope serves as a heat blanket (Gudmundsson et al., 1982). If it is composed of light elements, which have higher thermal conductivity, it would be more heat transparent (Potekhin et al., 2003). However, the exact composition of the envelope is uncertain. For the second factor, superfluidity and superconductivity suppress heat capacities and some emission mechanisms, such as modified Urca processes. On the other hand, emission due to the Cooper pairing of nucleons would be enhanced when the temperature decreases just below the critical value (see Yakovlev & Pethick, 2004; Page et al., 2006, for reviews).
Here we consider a and 11.6 km radius (corresponding to an observed radius km) star built with the Akmal-Pandharipande-Ravenhall (APR) equation of state (EOS) (Akmal et al., 1998) and the standard (also known as minimal) cooling scenario, in which neutrino emission by fast direct Urca processes does not take place (see, e.g., Yakovlev & Pethick, 2004; Page et al., 2006). We show in Figure 2 two shaded regions, one for a maximally thick hydrogen envelope and one for an iron envelope. The upper boundary of each region is determined by a cooling model with only superconducting protons (gap model CCDK) and the lower boundary accounts for superfluid neutrons as well (singlet gap model SFB and triplet gap model TToa), which induce more rapid cooling (see Ho et al., 2015, for details). To illustrate the effect of different masses, we plot in the figure inset cooling curves of and stars for models with only superconducting protons.
The model cooling curves are compared with the candidates’ luminosity limits in Figure 2 to constrain the source age. The result slightly depends on the neutron star mass in the model, but the main uncertainty comes from the whether or not neutrons in the core become superfluid. The inferred age lower limits are listed in Table 2. The results are not very constraining. For nearby objects that have good luminosity limits, we deduce age limits of years, but no useful limits are obtained for the rest, and deeper X-ray observations are needed for further investigation. Unlike the CCO candidates in our previous study (Bogdanov et al., 2014), none of the sources here coincide with known SNRs, and we do not find evidence of remnant emission from the X-ray data. This indeed gives a hint that the sources are likely older than years, such that the SNRs have already dissipated in the interstellar environment.
The non-detection of X-ray emission from our sample of weak-field radio pulsars seems to support the idea that the two are not evolutionary connected classes of neutron stars, similar to what previous studies have suggested (e.g., Gotthelf et al. 2013b, Bogdanov et al. 2014). It was proposed that the magnetic fields of CCOs could be buried by accretion from supernova materials at birth (Halpern & Gotthelf, 2010; Ho, 2011, 2013a), resulting in the suppression of radio emission. As the field diffuses back to the surface, the radio emission would subsequently turn on so that a CCO becomes a radio pulsar. To investigate the effect of magnetic field diffusion on the neutron star spin evolution, we applied a model described in Ho (2011) to solve the induction equation. We assumed that the -field is confined in the crust and is initially buried at a high density of g cm, which corresponds to a depth of m or an accreted mass of (see Ho, 2011). We consider different initial surface -field strengths from G to G and the resulting spin evolution is overplotted in the - diagram in Figure 1. We found that even at this large burial depth, the field emerges with a relatively short timescale of a few years, resulting in a vertically upward track in the diagram. More importantly, a CCO with an emerging surface field is expected to pass through the - parameter space of our candidates (i.e., – G) at a young age of the order of yr, such that it should remain hot and luminous, with the parent SNR probably still visible. If this model is true, our candidates are likely to just be old ordinary pulsars rather than orphaned CCOs. Moreover, we expect that when an aged CCO turns on as a radio pulsar, it will have a stronger -field than those of our candidates.
For further study, identifying CCO descendants would require X-ray observations of more radio pulsars to search for thermal emission (see also Gotthelf et al., 2013b). In particular, it would be useful to extend the survey to include sources with G. On the other hand, it is more difficult to detect an orphan CCO if the radio emission never turns on (see Halpern et al., 2013). This is best done with all-sky surveys in X-rays, such as the upcoming eROSITA mission (Merloni et al., 2012). Finally, pulsar braking index measurements from long-term timing could reveal a growing dipole field (e.g., Espinoza et al., 2011), which could indicate -field diffusion to the surface (see Ho, 2011).
We present an X-ray study of all isolated radio pulsars within 100 pc of the Galactic plane, which has spin-down-inferred -fields weaker than G, with the aim of searching for CCO-like thermal X-ray emission. None of our 12 candidates were detected in the Chandra and XMM-Newton observations and we obtained bolometric luminosity limits of – erg s. The limits were compared with a standard neutron star cooling model to constrain the source ages. Together with the lack of known associated SNRs, we conclude that all candidates are unlikely to be young CCOs. This result supports the idea that young CCOs and currently known weak-field radio pulsars are not connected through evolution. This could be a consequence of field burial by supernova materials. We model the pulsar spin evolution in this scenario and show that the surface -field could reemerge rapidly with a timescale shorter than 100 kyr. It could therefore be fruitful to include radio pulsars with slightly stronger fields ( G) in future searches for CCO descendants.
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