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We present analysis of multiple Chandra and XMM-Newton spectra, separated by 9-19 years, of four of the youngest central compact objects (CCOs) with ages < 2500 yr: CXOU J232327.9+584842 (Cassiopeia A), CXOU J160103.1-513353 (G330.2+1.0), 1WGA J1713.4-3949 (G347.3-0.5), and XMMU J172054.5-372652 (G350.1-0.3). By fitting these spectra with thermal models, we attempt to constrain each CCO's long-term cooling rate, composition, and magnetic field. For the CCO in Cassiopeia A, 14 measurements over 19 years indicate a decreasing temperature at a ten-year rate of 2.2+/-0.2 or 2.8+/-0.3 percent (1sigma error) for a constant or changing X-ray absorption, respectively. We obtain cooling rate upper limits of 17 percent for CXOU J160103.1-513353 and 6 percent for XMMU J172054.5-372652. For the oldest CCO, 1WGA J1713.4-3949, its temperature seems to have increased by 4+/-2 percent over a ten year period. Assuming each CCO's preferred distance and an emission area that is a large fraction of the total stellar surface, a non-magnetic carbon atmosphere spectrum is a good fit to spectra of all four CCOs. If distances are larger and emission areas are somewhat smaller, then equally good spectral fits are obtained using a hydrogen atmosphere with B <= 7x10^10 G or B >= 10^12 G for CXOU J160103.1-513353, B <= 10^10 G or B >= 10^12 G for XMMU J172054.5-372652, and non-magnetic hydrogen atmosphere for 1WGA J1713.4-3949. In a unified picture of CCO evolution, our results suggest most CCOs, and hence a sizable fraction of young neutron stars, have a surface magnetic field that is low early in their life but builds up over several thousand years., Comment: 15 pages, 9 figures; accepted for publication in MNRAS

We analysed Chandra observations of the bright Fermi pulsar J0633+0632 and found evidence of an absorption feature in its spectrum at $804^{+42}_{-26}$ eV (the errors here and below are at 90% confidence) with equivalent width of $63^{+47}_{-36}$ eV. In addition, we analysed in detail the X-ray spectral continuum taking into account correlations between the interstellar absorption and the distance to the source. We confirm early findings by Ray et al. (2011) that the spectrum contains non-thermal and thermal components. The latter is equally well described by the blackbody and magnetised atmosphere models and can be attributed to the emission from the bulk of the stellar surface in both cases. The distance to the pulsar is constrained in a range of 1--4 kpc from the spectral fits. We infer the blackbody surface temperature of $108^{+22}_{-14}$ eV, while for the atmosphere model, the temperature, as seen by a distant observer, is $53^{+12}_{-7}$ eV. In the latter case J0633+0632 is one of the coldest middle-aged isolated neutron stars with measured temperatures. Finally, it powers an extended pulsar wind nebula whose shape suggests a high pulsar proper motion. Looking backwards the direction of the presumed proper motion we found a likely birthplace of the pulsar -- the Rosette nebula, a 50-Myr-old active star-forming region located at about 1$.\!\!^\circ$5 from the pulsar. If true, this constrains the distance to the pulsar in the range of 1.2--1.8 kpc., Comment: 15 pages, 9 figures, 3 tables

PSR J2021+3651 is a 17 kyr old rotation powered pulsar detected in the radio, X-rays, and $\gamma$-rays. It powers a torus-like pulsar wind nebula with jets, dubbed the Dragonfly, which is very similar to that of the Vela pulsar. The Dragonfly is likely associated with the extended TeV source VER J2019+368 and extended radio emission. We conducted first deep optical observations with the GTC in the Sloan $r'$ band to search for optical counterparts of the pulsar and its nebula. No counterparts were detected down to $r'\gtrsim27.2$ and $\gtrsim24.8$ for the point-like pulsar and the compact X-ray nebula, respectively. We also reanalyzed Chandra archival X-ray data taking into account an interstellar extinction--distance relation, constructed by us for the Dragonfly line of sight using the red-clump stars as standard candles. This allowed us to constrain the distance to the pulsar, $D=1.8^{+1.7}_{-1.4}$ kpc at 90% confidence. It is much smaller than the dispersion measure distance of $\sim$12 kpc but compatible with a $\gamma$-ray "pseudo-distance" of 1 kpc. Based on that and the optical upper limits, we conclude that PSR J2021+3651, similar to the Vela pulsar, is a very inefficient nonthermal emitter in the optical and X-rays, while its $\gamma$-ray efficiency is consistent with an average efficiency for $\gamma$-pulsars of similar age. Our optical flux upper limit for the pulsar is consistent with the long-wavelength extrapolation of its X-ray spectrum while the nebula flux upper limit does not constrain the respective extrapolation., Comment: 17 pages, 4 figures, 1 table, accepted for publication in ApJ

A middle-aged radio-quiet pulsar J0357+3205 was discovered in gamma-rays with $Fermi$ and later in X-rays with $Chandra$ and $XMM$-$Newton$ observatories. It produces an unusual thermally-emitting pulsar wind nebula observed in X-rays. Deep optical observations were obtained to search for the pulsar optical counterpart and its nebula using the Gran Telescopio Canarias (GTC). The direct imaging mode in the Sloan $g'$ band was used. Archival X-ray data were reanalysed and compared with the optical data. No pulsar optical counterpart was detected down to $g'\geq~28_{\cdotp}^{\text{m}}1$. No pulsar nebula was either identified in the optical. We confirm early results that the X-ray spectrum of the pulsar consists of a nonthermal power-law component of the pulsar magnetospheric origin dominating at high energies and a soft thermal component from the neutron star surface. Using magnetised partially ionised hydrogen atmosphere models in X-ray spectral fits we found that the thermal component can come from entire surface of the cooling neutron star with a temperature of 36$^{+9}_{-6}$ eV, making it one of the coldest among cooling neutron stars known. The surface temperature agrees with the standard neutron star cooling scenario. The optical upper limit does not put any additional constraints on the thermal component, however it implies a strong spectral break for the nonthermal component between the optical and X-rays as is observed in other middle-aged pulsars. The thermal emission from the entire surface of the neutron star likely dominates over the nonthermal emission in the UV range. Observations of the PSR J0357+3205 in this range are promising to put more stringent constraints on its thermal properties., Comment: 7 pages, 6 figures, 2 tables, accepted for publication in Astronomy and Astrophysics

According to recent results of Ho & Heinke (2009) and Heinke & Ho (2010), the Cassiopeia A supernova remnant contains a young neutron star which has carbon atmosphere and shows noticeable decline of the effective surface temperature. We report a new (November 2010) Chandra observation which confirms the previously reported decline rate. The decline is naturally explained if neutrons have recently become superfluid (in triplet-state) in the NS core, producing a splash of neutrino emission due to Cooper pair formation (CPF) process that currently accelerates the cooling. This scenario puts stringent constraints on poorly known properties of NS cores: on density dependence of the temperature $T_\mathrm{cn}(\rho)$ for the onset of neutron superfluidity [$T_\mathrm{cn}(\rho)$ should have a wide peak with maximum $\approx (7-9)\times 10^8$ K], on the reduction factor $q$ of CPF process by collective effects in superfluid matter ($q > 0.4$), and on the intensity of neutrino emission before the onset of neutron superfluidity (30--100 times weaker than the standard modified Urca process). This is serious evidence for nucleon superfluidity in NS cores that comes from observations of cooling NSs., Comment: 6 pages, 3 figures, accepted for publication in MNRAS Letters

We explore the thermal state of the neutron star in the Cassiopeia A supernova remnant using the recent result of Ho & Heinke (Nature, 462, 71 (2009)) that the thermal radiation of this star is well-described by a carbon atmosphere model and the emission comes from the entire stellar surface. Starting from neutron star cooling theory, we formulate a robust method to extract neutrino cooling rates of thermally relaxed stars at the neutrino cooling stage from observations of thermal surface radiation. We show how to compare these rates with the rates of standard candles -- stars with non-superfluid nucleon cores cooling slowly via the modified Urca process. We find that the internal temperature of standard candles is a well-defined function of the stellar compactness parameter $x=r_g/R$, irrespective of the equation of state of neutron star matter ($R$ and $r_g$ are circumferential and gravitational radii, respectively). We demonstrate that the data on the Cassiopeia A neutron star can be explained in terms of three parameters: $f_\ell$, the neutrino cooling efficiency with respect to the standard candle; the compactness $x$; and the amount of light elements in the heat blanketing envelope. For an ordinary (iron) heat blanketing envelope or a low-mass ($\lesssim 10^{-13}\,M_\odot$) carbon envelope, we find the efficiency $f_\ell \sim 1$ (standard cooling) for $x \lesssim 0.5$ and $f_\ell \sim 0.02$ (slower cooling) for a maximum compactness $x\approx 0.7$. A heat blanket containing the maximum mass ($\sim 10^{-8}\,M_\odot$) of light elements increases $f_\ell$ by a factor of 50. We also examine the (unlikely) possibility that the star is still thermally non-relaxed., Comment: accepted for publication in MNRAS; 12 pages, 6 figures