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Register a new userSearching for the optical counterparts of two young gamma-ray pulsars
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R. P. Mignani
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We report on the first deep optical observations of two $gamma$-ray pulsars, both among the very first discovered by the {em Fermi} Gamma-ray Space Telescope. The two pulsars are the radio-loud PSR, J1907+0602 in the TeV pulsar wind nebula (PWN) MGRO, J1908+06 and the radio-quiet PSR, J1809$-$2332 in the Taz radio/X-ray PWN. These pulsars are relatively young and energetic and have been both detected in the X-rays by xmm, which makes them viable targets for optical observations. We observed the pulsar fields in the B and V bands with the Very Large Telescope (VLT) in June/July 2015 to search for their optical counterparts. Neither of the two pulsars has been detected down to $3sigma$ limiting magnitudes of $m_{rm v} sim 26.9$ and $m_{rm v} sim 27.6$ for PSR, J1907+0602 and PSR, J1809$-$2332, respectively. We discuss these results in the framework of the multi-wavelength emission properties of pulsars.
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L. Guillemot (for the Fermi LAT Collaboration
, for the Fermi Pulsarn Search Consortium
,
2012
Observations of pulsars with the Large Area Telescope (LAT) on the Fermi satellite have revolutionized our view of the gamma-ray pulsar population. For the first time, a large number of young gamma-ray pulsars have been discovered in blind searches of the LAT data. More generally, the LAT has discovered many new gamma-ray sources whose properties suggest that they are powered by unknown pulsars. Radio observations of gamma-ray sources have been key to the success of pulsar studies with the LAT. For example, radio observations of LAT-discovered pulsars provide constraints on the relative beaming fractions, which are crucial for pulsar population studies. Also, radio searches of LAT sources with no known counterparts have been very efficient, with the discovery of over forty millisecond pulsars. I review radio follow-up studies of LAT-discovered pulsars and unidentified sources, and discuss some of the implications of the results.
Studies of Fermi data indicate an excess of GeV gamma rays around the Galactic center (GC), possibly due to dark matter. We show that young gamma-ray pulsars can yield a similar signal. First, a high concentration of GC supernovae naturally leads to a population of kicked pulsars symmetric about the GC. Second, while very-young pulsars with soft spectra reside near the Galactic plane, pulsars with spectra that have hardened with age accumulate at larger angles. This combination, including unresolved foreground pulsars, traces the morphology and spectrum of the Excess.
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.
The wealth of detections of millisecond pulsars (MSPs) in $gamma$-rays by {em Fermi} has spurred searches for these objects among the several unidentified $gamma$-ray sources. Interesting targets are a sub-class of binary MSPs, dubbed Black Widows (BWs) and Redbacks (RBs), which are in orbit with low-mass non-degenerate companions fully or partially ablated by irradiation from the MSP wind. These systems can be easily missed in radio pulsar surveys owing to the eclipse of the radio signal by the intra-binary plasma from the ablated companion star photosphere, making them better targets for multi-wavelength observations. We used optical and X-ray data from public databases to carry out a systematic investigation of all the unidentified $gamma$-ray sources from the Fermi Large Area Telescope (LAT) Third Source Catalog (3FGL), which have been pre-selected as likely MSP candidates according to a machine-learning technique analysis. We tested our procedure by recovering known binary BW/RB identifications and searched for new ones, finding possible candidates. At the same time, we investigated previously proposed BW/RB identifications and we ruled out one of them based upon the updated $gamma$-ray source coordinates.
In the last few years, over 43 millisecond radio pulsars have been discovered by targeted searches of unidentified gamma-ray sources found by the Fermi Gamma-Ray Space Telescope. A large fraction of these millisecond pulsars are in compact binaries with low-mass companions. These systems often show eclipses of the pulsar signal and are commonly known as black widows and redbacks because the pulsar is gradually destroying its companion. In this paper, we report on the optical discovery of four strongly irradiated millisecond pulsar companions. All four sources show modulations of their color and luminosity at the known orbital periods from radio timing. Light curve modelling of our exploratory data shows that the equilibrium temperature reached on the companions dayside with respect to their nightside is consistent with about 10-30% of the available spin-down energy from the pulsar being reprocessed to increase the companions dayside temperature. This value compares well with the range observed in other irradiated pulsar binaries and offers insights about the energetics of the pulsar wind and the production of gamma-ray emission. In addition, this provides a simple way of estimating the brightness of irradiated pulsar companions given the pulsar spin-down luminosity. Our analysis also suggests that two of the four new irradiated pulsar companions are only partially filling their Roche lobe. Some of these sources are relatively bright and represent good targets for spectroscopic follow-up. These measurements could enable, among other things, mass determination of the neutron stars in these systems.
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