[Paper Review] The gamma-ray millisecond pulsar deathline, revisited - New velocity and distance measurements

L. Guillemot, D. A. Smith+7 more|OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information)|Jan 22, 2016

Pulsars and Gravitational Waves ResearchPhysics and Astronomy44 references18 citations

TL;DR

This study revisits the gamma-ray millisecond pulsar (MSP) deathline by measuring improved spin-down luminosities ($\dot{E}$) and $\gamma$-ray luminosities ($L_\gamma$) using precise proper motions and timing parallaxes from Nançay and Westerbork radio telescopes. It identifies PSR J1730$-$2304 as the least energetic $\gamma$-ray MSP known, setting the empirical deathline at $\dot{E}_{\text{death}} \sim 8 \times 10^{32}$ erg s$^{-1}$, with no clear correlation between $\dot{E}$ and $L_\gamma$ at low luminosities despite improved measurements.

ABSTRACT

Millisecond pulsars (MSPs) represent nearly half of the more than 160 currently known $γ$-ray pulsars detected by the Large Area Telescope on the extit{Fermi} satellite, and a third of all known MSPs are seen in $γ$ rays. The least energetic $γ$-ray MSPs enable us to probe the so-called deathline for high-energy emission, i.e., the spin-down luminosity limit under which pulsars (PSRs) cease to produce detectable high-energy radiation. Characterizing the MSP luminosity distribution helps to determine their contribution to the Galactic diffuse $γ$-ray emission. We made use of the high-quality pulsar timing data recorded at the Nançay Radio Telescope over several years to characterize the properties of a selection of MSPs. For one of the pulsars, the dataset was complemented with Westerbork Synthesis Radio Telescope observations. The rotation ephemerides derived from this analysis were also used to search the LAT data for new $γ$-ray MSPs. For the MSPs considered in this study, we obtained new transverse proper motion measurements or updated the existing ones, and placed new distance constraints for some of them, with four new timing parallax measurements. We discovered significant GeV $γ$-ray signals from four MSPs, i.e., PSRs J0740+6620, J0931$-$1902, J1455$-$3330, and J1730$-$2304. The latter is now the least energetic $γ$-ray pulsar found to date. Despite the improved $\dot E$ and $L_γ$ estimates, the relationship between these two quantities remains unclear, especially at low $\dot E$ values.

Motivation & Objective

• To improve the accuracy of spin-down luminosity ($\dot{E}$) estimates for $\gamma$-ray MSPs by correcting for the Shklovskii effect using precise proper motion and distance measurements.
• To determine the empirical deathline for $\gamma$-ray emission from MSPs by identifying the lowest $\dot{E}$ MSPs detectable in $\gamma$ rays.
• To investigate the relationship between $\dot{E}$ and $L_\gamma$ at low luminosities, especially in the context of emission geometry and energy budget variations.
• To discover new $\gamma$-ray MSPs through deep analysis of Pass 8 Fermi LAT data using refined timing ephemerides.
• To assess the contribution of MSPs to the Galactic diffuse $\gamma$-ray background by characterizing their luminosity distribution.

Proposed method

• Used high-precision pulsar timing data from the Nançay Radio Telescope over multiple years to derive rotation ephemerides and measure proper motions for selected $\gamma$-ray MSPs.
• Complemented Nançay data with Westerbork Synthesis Radio Telescope observations for one pulsar to improve timing solution accuracy.
• Applied timing parallax techniques to obtain direct distance estimates for four MSPs, reducing uncertainty in $\dot{E}$ and $L_\gamma$ calculations.
• Corrected spin-down luminosity estimates for the Shklovskii effect using measured transverse velocities and distances.
• Analyzed over six years of Pass 8 Fermi LAT $\gamma$-ray data using the refined ephemerides to search for pulsed $\gamma$-ray emission.
• Used the $\dot{E}_{\text{ff}} \simeq \frac{3}{2}\dot{E}_{\text{vac}}(1 + \sin^2\alpha)$ correction to account for force-free magnetosphere effects on $\dot{E}$.

Experimental results

Research questions

• RQ1 What is the lowest spin-down luminosity ($\dot{E}$) at which a millisecond pulsar can still produce detectable $\gamma$-ray emission?
• RQ2 How does the Shklovskii effect impact the accuracy of $\dot{E}$ and $L_\gamma$ estimates for $\gamma$-ray MSPs?
• RQ3 Is there a clear correlation between spin-down luminosity ($\dot{E}$) and $\gamma$-ray luminosity ($L_\gamma$) at low $\dot{E}$ values?
• RQ4 Can new $\gamma$-ray MSPs be discovered through deep analysis of Pass 8 Fermi LAT data using improved timing models?
• RQ5 To what extent do emission geometry and magnetospheric parameters affect the observed $\dot{E}$–$L_\gamma$ relationship?

Key findings

• Four new $\gamma$-ray MSPs were discovered: PSRs J0740+6620, J0931$-$1902, J1455$-$3330, and J1730$-$2304, with the latter being the least energetic $\gamma$-ray pulsar known.
• The empirical $\gamma$-ray MSP deathline was revised to $\dot{E}_{\text{death}} \sim 8 \times 10^{32}$ erg s$^{-1}$, significantly lower than the previous estimate of $3 \times 10^{33}$ erg s$^{-1}$.
• Despite improved $\dot{E}$ and $L_\gamma$ estimates, no clear correlation was found between $\dot{E}$ and $L_\gamma$ at low luminosities, indicating complex emission physics.
• Four timing parallax measurements were obtained, enabling direct distance estimates and reducing uncertainty in $\dot{E}$ and $L_\gamma$ calculations.
• PSR J1730$-$2304 has a spin-down luminosity of $\dot{E} \sim 8 \times 10^{32}$ erg s$^{-1}$, marking the new lower limit for detectable $\gamma$-ray emission.
• The study confirms that $\gamma$-ray MSPs are preferentially the most energetic and nearest objects in the Galactic disk, with 75% of those above $5 \times 10^{33}$ erg s$^{-1}$ kpc$^{-2}$ in $\dot{E}/d^2$ being detected in $\gamma$ rays.