Energetic Pulsars Research Articles

Is the Bursting Radio Source GCRT J1745-3009 a Double Neutron Star Binary?

GCRT J1745-3009 is a peculiar transient radio source in the direction of the Galactic center. It was observed to emit a series of ~1 Jy bursts at 0.33 GHz, with a typical duration of ~10 minutes and at apparently regular intervals of ~77 minutes. If the source is indeed at the distance of the Galactic center, as seems likely, we show that its observational properties are compatible with those expected from a double neutron star binary, similar to the double pulsar system J0737-3039. In the picture we propose, the (coherent) radio emission comes from the shock originating in the interaction of the wind of the more energetic pulsar with the magnetosphere of the companion. The observed modulation of the radio signal is the consequence of an eccentric orbit, along which the separation between the two stars varies. This cyclically drives the shock inside the light-cylinder radius of the less energetic pulsar.

γ-rays from cascades in close massive binaries containing energetic pulsars

Some massive binaries should contain energetic pulsars which inject relativistic leptons from their inner magnetospheres and/or pulsar wind regions. If the binary system is compact enough, then these leptons can initiate inverse Compton (IC) $e^\pm$ pair cascades in the anisotropic radiation field of a massive star. $\gamma$-rays can be produced in the IC cascade during its development in a pulsar wind region and above a shock in a massive star wind region where the propagation of leptons is determined by the structure of a magnetic field around the massive star. For a binary system with specific parametres, we calculate phase dependent spectra and fluxes of $\gamma$-rays escaping as a function of the inclination angle of the system and for different assumptions on injection conditions of the primary leptons (their initial spectra and location of the shock inside the binary). We conclude that the features of $\gamma$-ray emission from such massive binaries containing energetic pulsars should allow to obtain important information on the acceleration of particles by the pulsars, and on interactions of a compact object with the massive star wind. Predicted $\gamma$-ray light curves and spectra in the GeV and TeV energy ranges from such binary systems within our Galaxy and Magellanic Clouds should be observed by future AGILE and GLAST satellites and low threshold Cherenkov telescopes such as MAGIC, HESS, VERITAS or CANGAROO III.

Pulsars and their nebulae as EGRET sources

At the end of the EGRET mission, only 6–8 Galactic sources had been identified as young pulsars. Since then, several energetic pulsars have been detected in EGRET error boxes along the Galactic plane, as well as several pulsar wind nebulae from which pulsations have not yet been discovered. Some of these nebulae are associated with moderately variable EGRET sources, suggesting that the γ-ray emission might be coming from the nebula rather than from the pulsar magnetosphere. There is also a population of unidentified EGRET sources at mid-Galactic latitudes which have been proposed to be either nearby middle-aged pulsars or millisecond pulsars. I review the current status of observational studies of pulsars associated with EGRET sources and what they suggest the upcoming AGILE and GLAST γ-ray missions might observe.

A Spin-down Power Threshold for Pulsar Wind Nebula Generation?

A systematic X-ray survey of the most energetic rotation-powered pulsars known, based on spin-down energy loss rate, shows that all energetic pulsars with Ė > Ėc ≍ 4 × 1036 ergs s−1 are X-ray-bright, manifest a distinct pulsar wind nebula (PWN), and are associated with a supernova event, with over half residing in shell-like supernova remnants. Below Ėc, the 2–10 keV flux ratio FPWN/FPSR decreases by an order-of-magnitude. This threshold is consistent with the lower limit on the spectral slope γmin ≍ 0.6 observed for rotation-powered pulsars (Gotthelf 2003). The apparent lack of bright PWNe below Ėc suggests a change in the particle injection spectrum and serves as a constraint on emission models for rotation-powered pulsars. Neither a young age nor a high density environment is found to be a sufficient condition for generating a PWN, as often suggested, instead Ė is likely the key parameter in determining the evolution of a rotation-powered pulsar.

PSR J2021+3651: a new γ-ray pulsar candidate

The COS B high energy γ-ray source 2CG 075+00, also known as GeV J2020+3658 or 3EG J2021+3716, has avoided identification with a low energy counterpart for over 20 years. We present a likely identification with the discovery and subsequent timing of a young and energetic pulsar, PSR J2021+3651, with the Wideband Arecibo Pulsar Processor at the Arecibo Observatory. PSR J2021+3651 has a rotation period P∼104 ms and P ̇ ∼9.6×10 −14 , implying a characteristic age τ c∼17 kyr and a spin-down luminosity E ̇ ∼3.4×10 36 erg s −1. The pulsar is also coincident with the ASCA source AX J2021.1+3651. The implied luminosity of the associated X-ray source suggests the X-ray emission is dominated by a pulsar wind nebula unresolved by ASCA. The pulsar’s unexpectedly high dispersion measure (DM ∼ 371 pc cm −3) and the d⩾10 kpc DM distance pose a new question: is PSR J2021+3651 an extremely efficient γ-ray pulsar at the edge of the Galaxy? This is a question for AGILE and GLAST to answer.

Prospects for high energy studies of pulsars with the AGILE γ-ray telescope

AGILE is a small γ-ray astronomy satellite, with good spatial resolution, excellent timing capabilities and an unprecedented large field of view (∼1/5 of the sky). It will be the next mission dedicated to high energy astrophysics in the range 30 MeV–50 GeV and will be launched in 2005. Pulsars are a major topic of investigation of AGILE and, besides studying the small sample of known objects, AGILE will offer the first possibility of detecting several young and energetic radio pulsars that have been discovered since the end of the CGRO mission. We provide an estimate of the expected number of detectable γ-ray pulsars and present AGILE capabilities for timing analysis with small counting statistics, based on the analysis of data from simulations, from the EGRET archive and from radio pulsar catalogs.

Revealing New Physical Structures in the Supernova Remnant N63A throughChandraImaging Spectroscopy

We present Chandra X-ray observations of the supernova remnant (SNR) N63A in the Large Magellanic Cloud (LMC). N63A, one of the brightest LMC remnants, is embedded in an H II region and is probably associated with an OB association. The optical remnant consists of three lobes of emission contained within the approximately 3 times larger X-ray remnant. Our Chandra data reveal a number of new physical structures in N63A. The most striking of these are the several structures located beyond the main shell that resemble similar features seen in the Vela SNR. In Vela, these have been interpreted as arising from high-speed clumps of supernova ejecta interacting with the ambient medium. Another distinct feature of the remnant is a roughly triangular hole in the X-ray emission near the location of the optical lobes and the brightest radio emission. X-ray spectral analysis shows that this deficit of emission is the result of absorption by an intervening dense cloud with a mass of ~450 M☉ that is currently being engulfed by the remnant's blast wave. We also find that the rim of the remnant, as well as the crescent-shaped features, has considerably softer X-ray spectra than the interior. Limits on hard X-ray emission rule out a young, energetic pulsar in N63A, but the presence of an older or less active one, powering a wind nebula with a luminosity less than ~4 × 1034 ergs s-1, is allowed.

Gamma-rays and neutrinos from the pulsar wind nebulae

We construct the time dependent radiation model for the pulsar wind nebulae (PWNe), assuming that leptons are accelerated in resonant scattering with heavy nuclei, which are injected into the nebula by the pulsar. We calculate the equilibrium spectra of these particles inside the nebula taking into account their radiation and adiabatic energy losses. The spectra of γ-rays are compared with the observations of the PWNe emitting TeV γ-rays and predictions are made for the expected γ-ray fluxes from other PWNe. Expected neutrino fluxes and neutrino event rates in a 1 km 2 neutrino detector from these nebulae are also calculated. It is found that only the Crab Nebula, and possibly the Vela nebula and MSH15-52, can produce detectable neutrino event rates. 1. Introduction Young pulsars create relativistic winds which interact with the supernova envelopes creating the PWNe. These nebulae are filled with energetic particles which emit photons from radio up to TeV γ-rays (e.g. the nebulae around the Crab and Vela pulsars, PSR 1706-44, and PSR 1509-58). It is widely argued that this radiation is produced by leptons in the synchrotron and the inverse Compton (IC) processes. Here we investigate the radiation model for the PWNe in which not only leptons but also heavy nuclei can play important role. 2. The model The energetic pulsar, formated during the supernova explosion, can have strong influence on the evolution of expanding supernova envelope due to the supply of energy in the form of electromagnetic waves and relativistic particles. We construct the simple model for the evolution of such pulsar-supernova envelope system in order to consider the processes which turn to the production of high energy radiation (see for details of the model in [3]). This model allows to determine the main parameters of the expanding nebula as a function of time (the outer radius of the nebula, the radius of the pulsar wind shock, the magnetic field inside the nebula, density of matter).

Subaru optical observations of the old pulsar PSR B0950+08

We report the B band optical observations of an old (17.5 Myr) radiopulsar PSR B0950+08 obtained with the Suprime-Cam at the Subaru telescope. We detected a faint object, B=27.07(16). Within our astrometrical accuracy it coincides with the radio position of the pulsar and with the object detected earlier by Pavlov et al. (1996) in UV with the HST/FOC/F130LP. The positional coincidence and spectral properties of the object suggest that it is the optical counterpart of PSR B0950+08. Its flux in the B band is two times higher than one would expect from the suggested earlier Rayleigh-Jeans interpretation of the only available HST observations in the adjacent F130LP band. Based on the B and F130LP photometry of the suggested counterpart and on the available X-ray data we argue in favour of nonthermal origin of the broad-band optical spectrum of PSR B0950+08, as it is observed for the optical emission of the younger, middle-aged pulsars PSR B0656+14 and Geminga. At the same time, the optical efficiency of PSR B0950+08, estimated from its spin-down power and the detected optical flux, is by several orders of magnitude higher than for these pulsars, and comparable with that for the much younger and more energetic Crab pulsar. We cannot exclude the presence of a compact, about 1'', faint pulsar nebula around PSR B0950+08, elongated perpendicular to the vector of its proper motion, unless it is not a projection of a faint extended object on the pulsar position.

PSR J2021+3651: A Young Radio Pulsar Coincident with an Unidentified EGRET γ-Ray Source

We report on a deep search for radio pulsations toward five unidentified ASCA X-ray sources coincident with EGRET gamma-ray sources. This search has led to the discovery of a young and energetic pulsar using data obtained with the new Wideband Arecibo Pulsar Processor. PSR J2021+3651 is likely associated with the X-ray source AX J2021.1+3651, which in turn is likely associated with the COS B high energy gamma-ray source 2CG 075+00, also known as GeV J2020+3658 or 3EG J2021+3716. PSR J2021+3651 has a rotation period P = 104 ms and P_dot = 9.6x10^{-14}, implying a characteristic age ~17 kyr and a spin-down luminosity E_dot ~ 3.4x10^{36}ergs/s. The dispersion measure DM ~ 371 pc cm^{-3} is by far the highest of any observed pulsar in the Galactic longitude range 55 < l < 80. This DM suggests a distance d > 10 kpc, and a high gamma-ray efficiency of \~15%, but the true distance may be closer if there is a significant contribution to the DM from excess gas in the Cygnus region. The implied luminosity of the associated X-ray source suggests the X-ray emission is dominated by a pulsar wind nebula unresolved by ASCA.

Pulse Phase Variations of the X‐Ray Spectral Features in the Radio‐quiet Neutron Star 1E 1207−5209

We present the results of an XMM-Newton observation of the radio-quiet X-ray pulsar 1E 1207� 5209 located at the center of the shell-like supernova remnant G296.5+10.0. The X-ray spectrum is characterized by the presence of two phase-dependent absorption lines at energies of � 0.7 and � 1.4 keV. Moreover, these broad spectral features have significant substructure, suggesting that they are caused by the blending of several narrower lines. We interpret such features as evidence for an atmosphere containing metals and a magnetic field value of a few 10 12 G, consistent with the observed spin-down rate _ P ¼ð 1:98 � 0:83 Þ� 10 � 14 s s � 1 . Since 1E 1207� 5209 is the only X-ray–emitting pulsar showing evidence of such features, we tentatively link them to the unique combination of age and energetics that characterize this object. We suggest that a young age and a low level of magnetospheric activity are favorable conditions for the detection of atomic spectral features from Z > 1 elements in neutron star atmospheres, which would be either blanketed by a thin layer of accreted hydrogen in older objects or masked by nonthermal processes in young energetic pulsars. Subject headings: pulsars: individual (1E 1207� 5209) — stars: neutron — X-rays: stars

Neutrinos from Early‐Phase, Pulsar‐driven Supernovae

Neutron stars, just after their formation, are surrounded by expanding, dense, and very hot envelopes that radiate thermal photons. Iron nuclei can be accelerated in the wind zones of such energetic pulsars to very high energies. These nuclei photodisintegrate, and their products lose energy efficiently in collisions with thermal photons and with the matter of the envelope, mainly via pion production. When the temperature of the radiation inside the envelope of the supernova drops below ~ 3 × 106 K, these pions decay before losing energy and produce high-energy neutrinos. We estimate the flux of muon neutrinos emitted during such an early phase of the pulsar-supernova envelope interaction. We find that a 1 km2 neutrino detector should be able to detect neutrinos above 1 TeV within about 1 yr after the explosion from a supernova in our Galaxy. This result holds if these pulsars are able to efficiently accelerate nuclei to energies ~1020 eV, as postulated recently by some authors for models of Galactic acceleration of the extremely high energy cosmic rays (EHE CRs).

Detection of an X‐Ray Pulsar Wind Nebula and Tail in SNR N157B

We report Chandra X-ray observations of the supernova remnant N157B in the Large Magellanic Cloud, which are presented together with an archival Hubble Space Telescope optical image and a radio continuum map for comparison. This remnant contains the recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidly rotating young pulsar known. Using phase-resolved Chandra imaging, we pinpoint the location of the pulsar at 5h37m4736, -69°10'204 (J2000), with an uncertainty of 1''. PSR J0537-6910 is not detected in any other wavelength band. The X-ray observations resolve three distinct features: the pulsar itself, a surrounding compact wind nebula that is strongly elongated (dimensions of ~0.6 × 1.7 pc), and a feature of large-scale (5 pc long) diffuse emission trailing from the pulsar. This latter comet-tail-shaped feature coexists with enhanced radio emission and is oriented nearly perpendicular to the major axis of the pulsar wind nebula. We propose the scenario below to explain these features. The bright, compact nebula is likely to be powered by a toroidal pulsar wind of relativistic particles that is partially confined by the ram pressure from the supersonic motion of the pulsar. The particles, after being forced out from the compact nebula (the head of the comet), are eventually dumped into a bubble (the tail), which is primarily responsible for the extended diffuse X-ray and radio emission. The ram-pressure confinement also allows a natural explanation for the observed X-ray luminosity of the compact nebula and for the unusually small X-ray to spin-down luminosity ratio of ~0.2%, compared to similarly energetic pulsars. We estimate the pulsar wind Lorentz factor of N157B to be ~4 × 106 (with an uncertainty of a factor of ~2), consistent with that inferred from the modeling of the Crab Nebula.

PSR J1016−5857: A Young Radio Pulsar with Possible Supernova Remnant, X-Ray, and Gamma-Ray Associations

We report the discovery of a young and energetic pulsar in the Parkes multibeam survey of the Galactic plane. PSR J1016-5857 has a rotation period of 107 ms and period derivative of 8.0 × 10-14, implying a characteristic age of 21 kyr and spin-down luminosity of 2.6 × 1036 ergs s-1. The pulsar is located just outside, and possibly interacting with, the shell supernova remnant G284.3-1.8. Archival X-ray data show a source near the pulsar position that is consistent with emission from a pulsar wind nebula. The pulsar is also located inside the error box of the unidentified EGRET source 3EG J1013-5915, for which it represents a plausible counterpart.

Two Young Radio Pulsars Coincident with EGRET Sources

We report the discovery and follow-up timing observations of two young energetic radio pulsars. PSR J1420-6048 has a period P = 68 ms and period derivative = 83 × 10-15, implying a characteristic age τc = 13 kyr and a surface dipole magnetic field strength B = 2.4 × 1012 G. PSR J1837-0604 has P = 96 ms and = 45 × 10-15, implying τc = 34 kyr and B = 2.1 × 1012 G. The two objects have large spin-down luminosities, and, on the basis of an empirical comparison of their properties with those of other young radio pulsars, they are expected to be observable as pulsed γ-ray sources. In fact, they lie within the error circles of γ-ray sources detected by the EGRET instrument on the Gamma Ray Observatory. We show that the pulsars are plausibly associated with the EGRET sources.

A Possible X‐Ray and Radio Counterpart of the High‐Energy Gamma‐Ray Source 3EG J2227+6122

The identity of the persistent EGRET sources in the Galactic plane is largely a mystery. For one of these, 3EG J2227+6122, our complete census of X-ray and radio sources in its error circle reveals a remarkable superposition of an incomplete radio shell with a flat radio spectrum, and a compact, power-law X-ray source with photon index Gamma = 1.5 and with no obvious optical counterpart. The radio shell is polarized at a level of approx. = 25%. The anomalous properties of the radio source prevent us from deriving a completely satisfactory theory as to its nature. Nevertheless, using data from ROSAT, ASCA, the VLA, and optical imaging and spectroscopy, we argue that the X-ray source may be a young pulsar with an associated wind-blown bubble or bow shock nebula, and an example of the class of radio-quiet pulsars which are hypothesized to comprise the majority of EGRET sources in the Galaxy. The distance to this source can be estimated from its X-ray absorption as 3 kpc. At this distance, the X-ray and gamma-ray luminosities would be approx. = 1.7 x 10(exp 33) and approx. = 3.7 x 10(exp 35) erg/s, respectively, which would require an energetic pulsar to power them. If, on the contrary, this X-ray source is not the counterpart of 3EG J2227+6122, then by process of elimination the X-ray luminosity of the latter must be less than 10(exp -4) of its gamma-ray luminosity, a condition not satisfied by any established class of gamma-ray source counterpart. This would require the existence of at least a quantitatively new type of EGRET source, as has been suggested in studies of other EGRET fields.

Limits on radio emission from pulsar wind nebulae

We report on a sensitive survey for radio pulsar wind nebulae (PWN) towards 27 energetic and/or high velocity pulsars. Observations were carried out at 1.4 GHz using the Very Large Array and the Australia Telescope Compact Array, and utilised pulsar-gating to search for off-pulse emission. These observing parameters resulted in a considerably more sensitive search than previous surveys, and could detect PWN over a much wider range of spatial scales (and hence ambient densities and pulsar velocities). However, no emission clearly corresponding to a PWN was discovered. Based on these non-detections we argue that the young and energetic pulsars in our sample have winds typical of young pulsars, but produce unobservable PWN because they reside in low density (n approx 0.003 cm^-3) regions of the ISM. However, non-detections of PWN around older and less energetic pulsars can only be explained if the radio luminosity of their winds is less than 1e-5 of their spin-down luminosity, implying an efficiency at least an order of magnitude smaller than that seen for young pulsars.

Limits on radio emission from pulsar wind nebulae

We report on a sensitive survey for radio pulsar wind nebulae (PWN) towards 27 energetic and/or high-velocity pulsars. Observations were carried out at 1.4 GHz using the Very Large Array and the Australia Telescope Compact Array and utilized pulsar-gating to search for off-pulse emission. These observing parameters resulted in a considerably more sensitive search than previous surveys and could detect PWN over a much wider range of spatial scales (and hence ambient densities and pulsar velocities). However, no emission clearly corresponding to a PWN was discovered. Based on these non-detections we argue that the young and energetic pulsars in our sample have winds which are typical of young pulsars, but produce unobservable PWN because they reside in low-density (n∼0.003 cm−3) regions of the interstellar medium. However, non-detection of PWN around older and less energetic pulsars can only be explained if the radio luminosity of their winds is less than 10−5 of their spin-down luminosity, implying an efficiency at least an order of magnitude smaller than that seen for young pulsars.

ASCAObservations of the Young Rotation‐powered Pulsars PSR B1046−58 and PSR B1610−50

We present X-ray observations of two young energetic radio pulsars, PSRs B1046-58 and B1610-50, and their surroundings, using archival data from the Advanced Satellite for Cosmology and Astrophysics (ASCA). The energetic pulsar PSR B1046-58 is detected in X-rays with a significance of 4.5 sigma. The unabsorbed flux, estimated assuming a power-law spectrum and a neutral hydrogen column density N_H of 5E21 cm^-2 is (2.5 +/- 0.3) x 10E-13 ergs/cm^2/s in the 2-10 keV band. Pulsed emission is not detected; the pulsed fraction is less than 31% at the 90% confidence level for a 50% duty cycle. We argue that the emission is best explained as originating from a pulsar-powered synchrotron nebula. The X-ray counterpart of the pulsar is the only hard source within the 95% error region of the previously unidentified gamma-ray source 3EG J1048-5840. This evidence supports the results of Kaspi et al. (1999), who in a companion paper, suggest that PSR B1046-58 is the counterpart to 3EG J1048-5840. X-ray emission from PSR B1610-50 is not detected. Using similar assumptions as above, the derived 3 sigma upper limit for the unabsorbed 2-10 keV X-ray flux is 1.5E-13 ergs/cm^2/s. We use the flux limit to estimate the pulsar's velocity to be less than ~170 km/s, casting doubt on a previously reported association between PSR B1610-50 and supernova remnant Kes 32. Kes 32 is detected, as is evident from the correlation between X-ray and radio emission. The ASCA images of PSR B1610-50 are dominated by mirror-scattered emission from the X-ray-bright supernova remnant RCW 103, located 33' away. We find no evidence for extended emission around either pulsar, in contrast to previous reports of large nebulae surrounding both pulsars.

High‐Energy Gamma‐Ray Observations of Two Young, Energetic Radio Pulsars

We present results of Compton Gamma-Ray Observatory EGRET observations of the unidentified high-energy gamma-ray sources 2EG J1049-5847 (GEV J1047-5840, 3EG J1048-5840) and 2EG J1103-6106 (3EG J1102-6103). These sources are spatially coincident with the young, energetic radio pulsars PSRs B1046-58 and J1105-6107, respectively. We find evidence for an association between PSR B1046-58 and 2EG J1049-5847. The gamma-ray pulse profile, obtained by folding time-tagged photons having energies above 400 MeV using contemporaneous radio ephemerides, has probability of arising by chance of 1.2E-4 according to the binning-independent H-test. A spatial analysis of the on-pulse photons reveals a point source of equivalent significance 10.2 sigma. Off-pulse, the significance drops to 5.8 sigma. Archival ASCA data show that the only hard X-ray point source in the 95% confidence error box of the gamma-ray source is spatially coincident with the pulsar within the 1' uncertainty (Pivovaroff, Kaspi & Gotthelf 1999). The double peaked gamma-ray pulse morphology and leading radio pulse are similar to those seen for other gamma-ray pulsars and are well-explained in models in which the gamma-ray emission is produced in charge-depleted gaps in the outer magnetosphere. The inferred pulsed gamma-ray flux above 400 MeV, (2.5 +/- 0.6) x 10E-10 erg/cm^2/s, represents 0.011 +/- 0.003 of the pulsar's spin-down luminosity, for a distance of 3 kpc and 1 sr beaming. For PSR J1105-6107, light curves obtained by folding EGRET photons using contemporaneous radio ephemerides show no significant features. We conclude that this pulsar converts less than 0.014 of its spin-down luminosity into E > 100 MeV gamma-rays beaming in our direction (99% confidence), assuming a distance of 7 kpc, 1 sr beaming and a duty cycle of 0.5.