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Register a new userObservations of one young and three middle-aged $gamma$-ray pulsars with the Gran Telescopio Canarias
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R. P. Mignani
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We used the 10.4m Gran Telescopio Canarias to search for the optical counterparts to four isolated $gamma$-ray pulsars, all detected in the X-rays by either xmm or chan but not yet in the optical. Three of them are middle-aged pulsars -- PSR, J1846+0919 (0.36 Myr), PSR, J2055+2539 (1.2 Myr), PSR, J2043+2740 (1.2 Myr) -- and one, PSR, J1907+0602, is a young pulsar (19.5 kyr). For both PSR, J1907+0602 and PSR, J2055+2539 we found one object close to the pulsar position. However, in both cases such an object cannot be a viable candidate counterpart to the pulsar. For PSR, J1907+0602, because it would imply an anomalously red spectrum for the pulsar and for PSR, J2055+2539 because the pulsar would be unrealistically bright ($r=20.34pm0.04$) for the assumed distance and interstellar extinction. For PSR, J1846+0919, we found no object sufficiently close to the expected position to claim a possible association, whereas for PSR, J2043+2740 we confirm our previous findings that the object nearest to the pulsar position is an unrelated field star. We used our brightness limits ($g approx 27$), the first obtained with a large-aperture telescope for both PSR, J1846+0919 and PSR, J2055+2539, to constrain the optical emission properties of these pulsars and investigate the presence of spectral turnovers at low energies in their multi-wavelength spectra.
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We report the analysis of the first deep optical observations of three isolated $gamma$-ray pulsars detected by the {em Fermi Gamma-ray Space Telescope}: the radio-loud PSR, J0248+6021 and PSR, J0631+1036, and the radio-quiet PSR, J0633+0632. The latter has also been detected in the X rays. The pulsars are very similar in their spin-down age ($tau sim$40--60 kyrs), spin-down energy ($dot{E} sim10^{35}$ erg s$^{-1}$), and dipolar surface magnetic field ($B sim 3$--$5times10^{12}$ G). These pulsars are promising targets for multi-wavelength observations, since they have been already detected in $gamma$ rays and in radio or X-rays. None of them has been detected yet in the optical band. We observed the three pulsar fields in 2014 with the Spanish 10.4m Gran Telescopio Canarias (GTC). We could not find any candidate optical counterpart to the three pulsars close to their most recent radio or {em Chandra} positions down to $3 sigma$ limits of $gsim27.3$, $gsim27$, $gsim27.3$ for PSR, J0248+6021, J0631+1036, and J0633+0632, respectively. From the inferred optical upper limits and estimated distance and interstellar extinction, we derived limits on the pulsar optical luminosity. We also searched for the X-ray counterpart to PSR, J0248+6021 with chan but we did not detect the pulsar down to a 3$sigma$ flux limit of $5 times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$ (0.3--10 keV). For all these pulsars, we compared the optical flux upper limits with the extrapolations in the optical domain of the $gamma$-ray spectra and compared their multi-wavelength properties with those of other $gamma$-ray pulsars of comparable age.
Electrons/positrons produced in a pulsar magnetosphere emit synchrotron radiation, which is widely believed as the origin of the non-thermal X-ray emission detected from pulsars. Particles are produced by curvature photons emitted from accelerated particles in the magnetosphere. These curvature photons are detected as pulsed $gamma$-ray emissions from pulsars with age $lesssim10^6$ yr. Using $gamma$-ray observations and analytical model, we impose severe constraints on the synchrotron radiation as a mechanism of the non-thermal X-ray emission. In most middle-aged pulsars ($sim10^5-10^6$ yr) which photon-photon pair production is less efficient in their magnetosphere, we find that the synchrotron radiation model is difficult to explain the observed non-thermal X-ray emission.
The Canarias InfraRed Camera Experiment (CIRCE) is a near-infrared (1-2.5 micron) imager, polarimeter and low-resolution spectrograph operating as a visitor instrument for the Gran Telescopio Canarias 10.4-meter telescope. It was designed and built largely by graduate students and postdocs, with help from the UF astronomy engineering group, and is funded by the University of Florida and the U.S. National Science Foundation. CIRCE is intended to help fill the gap in near-infrared capabilities prior to the arrival of EMIR to the GTC, and will also provide the following scientific capabilities to compliment EMIR after its arrival: high-resolution imaging, narrowband imaging, high-time-resolution photometry, imaging polarimetry, low resolution spectroscopy. In this paper, we review the design, fabrication, integration, lab testing, and on-sky performance results for CIRCE. These include a novel approach to the opto-mechanical design, fabrication, and alignment.
Radio pulsars are often used as clocks in a wide variety of experiments. Imperfections in the clock, known as timing noise, have the potential to reduce the significance of, or even thwart e.g. the attempt to find a stochastic gravitational wave (GW) background. We measure the timing noise in a group of 129 mostly middle-aged pulsars (i.e. characterstic ages near 1~Myr) observed with the Parkes radio telescope on a monthly basis since 2014. We examine four different metrics for timing noise, but it remains unclear which, if any, provides the best determination. In spite of this, it is evident that these pulsars have significantly less timing noise than their younger counterparts, but significantly more than the (much older) millisecond pulsars (MSPs). As with previous authors, we find a strong correlation between timing noise and the pulsar spin-down rate, $dot{ u}$. However, for a given $dot{ u}$ there is a spread of about a factor 30 in the strength of the timing noise likely indicating that nuclear conditions in the interior of the stars differs between objects. We briefly comment on the implications for GW detection through pulsar timing arrays as the level of timing noise in MSPs may be less than predicted.
Chandra and XMM-Newton resolved extremely long tails behind two middle-aged pulsars, J1509-5850 and J1740+1000. The tail of PSR J1509-5850 is discernible up to 5.6 from the pulsar (6.5 pc at a distance of 4 kpc), with a flux of 2*10^{-13} erg s^{-1} cm^{-2} in 0.5-8 keV. The tail spectrum fits an absorbed power-law (PL) model with the photon index of 2.3pm0.2, corresponding to the 0.5-8 keV luminosity of 1*10^{33} ergs s^{-1}, for n_H= 2.1*10^{22} cm^{-2}. The tail of PSR J1740+1000 is firmly detected up to 5 (2 pc at a 1.4 kpc distance), with a flux of 6*10^{-14} ergs cm^{-2} s^{-1} in 0.4-10 keV. The PL fit yields photon index of 1.4-1.5 and n_H=1*10^{21} cm^{-2}. The large extent of the tails suggests that the bulk flow in the tails starts as mildly relativistic downstream of the termination shock, and then gradually decelerates. Within the observed extent of the J1509-5850 tail, the average flow speed exceeds 5,000 km s^{-1}, and the equipartition magnetic field is a few times 10^{-5} G. For the J1740+1000 tail, the equipartition field is a factor of a few lower. The harder spectrum of the J1740+1000 tail implies either less efficient cooling or a harder spectrum of injected electrons. For the high-latitude PSR J1740+1000, the orientation of the tail on the sky shows that the pulsar is moving toward the Galactic plane, which means that it was born from a halo-star progenitor. The comparison between the J1509 and J1740 tails and the X-ray tails of other pulsars shows that the X-ray radiation efficiency correlates poorly with the pulsar spin-down luminosity or age. The X-ray efficiencies of the ram-pressure confined pulsar wind nebulae (PWNe) are systematically higher than those of PWNe around slowly moving pulsars with similar spin-down parameters.
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