Einstein@Home discovers a radio-quiet gamma-ray millisecond pulsar

Abstract

Millisecond pulsars (MSPs) are old neutron stars that spin hundreds of times per second and appear to pulsate as their emission beams cross our line of sight. To date, radio pulsations have been detected from all rotation-powered MSPs. In an attempt to discover radio-quiet gamma-ray MSPs, we used the aggregated power from the computers of tens of thousands of volunteers participating in the Einstein@Home distributed computing project to search for pulsations from unidentified gamma-ray sources in Fermi Large Area Telescope data. This survey discovered two isolated MSPs, one of which is the only known rotation-powered MSP to remain undetected in radio observations. These gamma-ray MSPs were discovered in completely blind searches without prior constraints from other observations, raising hopes for detecting MSPs from a predicted Galactic bulge population.

Fig. 1. Pulsed signals from the two newly discovered MSPs.

Rotational phases of individual gamma-ray photons (bottom) and integrated pulse profiles (top) of the newly detected MSPs. Each photon has been assigned a weight, determined by its energy and arrival direction, representing the probability of it having come from the gamma-ray source in question. These weights are indicated by the color bar. In the top panels, the black dashed line shows the estimated background level, derived from the photon weights as in study of Abdo et al. (9). For PSR J1035−6720, the blue line shows the radio profile as measured by the Parkes radio telescope at 1400 MHz, averaged over four observations after subtracting an estimated baseline (dotted line). Two identical rotations are shown for clarity. MJD, Modified Julian Date.

Fig. 2. Pseudo-luminosities (L₁₄₀₀ = S₁₄₀₀ d²) of known MSPs (including both Galactic field MSPs and those found in globular clusters) with flux density and distance measurements in the ATNF Catalogue (26), version 1.57.

The measured pseudo-luminosities of PSR J1035−6720 according to the NE2001 and YMW16 Galactic dispersion models are shown by solid and dashed vertical lines, respectively. The dotted and dashed-dotted vertical lines show the upper limits on the pseudo-luminosity of PSR J1744−7619, assuming a maximum distance of 1 kpc, from two dedicated follow-up radio observations and (a conservative estimate) from the 10 shorter observations of Camilo et al. (6), respectively.

Fig. 3. Pulse profiles of the two new gamma-ray MSPs, showing the phase definitions of the off-pulse regions (blue dashed-dotted lines) used to search for unpulsed gamma-ray emission.

The estimated background levels are shown by the black dashed lines. No significant off-pulse emission was detected from PSR J1035−6720. Off-pulse emission was detected with a significance of ~ 5σ from PSR J1744−7619.

Fig. 4. Distance and spin-down constraints for the two new MSPs obtained from their gamma-ray flux, radio dispersion measure, and timing measurements of their proper motion.

(Top) constraints on MSP distances, as in figure 11 of the work by Abdo et al. (9). Assuming certain gamma-ray efficiencies, constraints on the pulsar distance are inferred from the gamma-ray flux and spin-down power, after correcting the observed $\stackrel{.}{\mathrm{\nu }}$ for the Shklovskii effect due to the measured proper motion (1σ range given by the gray shaded region). The physically realistic region is to the left of the 100% efficiency line (${\mathit{L}}_{\mathrm{\gamma }}=\stackrel{.}{\mathit{E}}$), although higher apparent efficiencies are possible, depending on the beam correction factor. Contours of constant transverse velocity are shown in gray. For PSR J1035−6720, the green dotted and dashed-dotted vertical lines show the DM distance according to the YMW16 and NE2001 models, respectively. For PSR J1744−7619, the ${\stackrel{.}{\mathrm{\nu }}}_{\text{INT}}=0$ line shows the required distance for the observed spin down to be purely due to the Shklovskii effect, giving a hard upper limit. (Bottom) Spin-down powers after correcting $\stackrel{.}{\mathrm{\nu }}$ for the Shklovskii effect. The shaded region shows the allowed range for $\stackrel{.}{\mathit{E}}$ at each distance, given the observed proper motion. The solid curves show $\stackrel{.}{\mathit{E}}$ as a function of distance at fixed gamma-ray efficiencies. The dashed line shows the maximum $\stackrel{.}{\mathit{E}}$, that is, if there was no proper motion.

Fig. 5. Gamma-ray pulse profiles of the newly detected MSPs.

The overlaying curves are the best-fitting pulse profiles predicted by fits to OG, TPC, and PSPC gamma-ray emission models. The black dashed line is the estimated background level, derived from the photon weights as in the study of Abdo et al. (9). Predicted radio pulse profiles (with arbitrary baseline and normalization) are shown by dashed-dotted lines. The three models are shown in separate rotations for clarity.