PSR J1544-2555: Spider Millisecond Pulsar

• PSR J1544-2555 is a black-widow millisecond pulsar in a compact binary system, characterized by rapid spin, strong ablation of its low-mass companion, and multiwavelength variability.
• Multiwavelength campaigns combining gamma-ray, radio, optical, and X-ray observations have precisely determined its orbital period (~2.7 h), spin frequency (~418 Hz), and light curve asymmetry.
• Detailed light-curve modeling and timing analyses have yielded key system parameters such as companion mass (~0.10 M☉), inclination angles, and evidence for intra-binary shock processes.

PSR J1544–2555 is a newly discovered black-widow millisecond pulsar in a compact binary system, associated with the Fermi-LAT gamma-ray source 4FGL J1544.2–2554. It exemplifies the “spider pulsar” category—systems where a rapidly spinning neutron star ablates and heats a low-mass companion, producing strong multiwavelength variability. Recent studies, incorporating gamma-ray, radio, optical, and X-ray observations, have established its fundamental parameters, light curve morphology, timing behaviour, and physical interpretation as a key template for spider binaries.

1. Discovery and Initial Characterization

PSR J1544–2555 was identified through its spatial association with Fermi-LAT source 4FGL J1544.2–2554, whose gamma-ray properties—specifically low long-term variability and a curved spectrum—suggested a pulsar origin. Systematic optical surveys using ULTRACAM on the 3.5-m New Technology Telescope revealed a highly variable source with a single broad peak per orbital cycle and brightness modulations of more than 3 magnitudes, consistent with black-widow systems. Optical periodicity analyses employed Lomb–Scargle periodograms, yielding a precise orbital period: $$P_{\rm orb} = 0.11349520(4)~{\rm d} \approx 2.724~{\rm h}$$ The epoch of the ascending node is determined as: $T_{\rm asc} = {\rm MJD}~59672.32042(7)$ Targeted radio searches, scheduled to avoid eclipse-prone orbital phases, found 2.4-ms pulsations with MeerKAT, which initiated a broad multiwavelength campaign.

2. Multiwavelength Observational Campaigns

Dedicated optical, radio, X-ray, and gamma-ray follow-ups cemented the system’s nature:

• Optical: ULTRACAM photometry established the ~2.7 h period and revealed marked asymmetry in the light curve, hinting at uneven heating of the companion. Light-curve modeling with the Icarus code, including spot models and direct heating prescriptions, constrained temperature contrasts and system inclination.
• Radio: MeerKAT, Effelsberg, Parkes, and Nançay radio telescopes delivered pulsar timing solutions, with radio measurements crucial for pinpointing the orbital ephemeris and measuring dispersion (DM = 25.817 ± 0.060 pc cm⁻³).
• Gamma-ray: Fermi-LAT analysis spanned 16 years, implementing photon weighting and GPU-accelerated acceleration searches. This led to the confirmation of gamma-ray pulsations matching the radio-determined parameters. The gamma-ray pulse profile exhibits two narrow peaks approximately half a rotation apart.
• X-ray: eROSITA detected a faint, nonthermal source at the pulsar’s location, interpreted as emission from an intra-binary shock. The measured X-ray rate was (3.3 ± 0.9) × 10⁻² cnt/s in the 0.2–2.3 keV band.

3. Timing and Spin Parameters

Integration of optical and radio timing data enabled precise determination of spin and orbital parameters:

Parameter	Value	Notes
Spin frequency, $f$	418.39828661948(4) Hz	$P_{\rm spin} \approx 2.4$ ms
Spin-down rate, $\dot{f}$	$-2.0383(2) \times 10^{-15}$ Hz/s
Orbital period, $P_{\rm orb}$	$0.113495141(9)$ d	$\approx2.7$ h
Projected semi-major axis, $x$	$0.128832(5)$ lt–s	Compact binary

This timing solution, achieved via the tempo2 framework with multi-telescope TOAs, revealed short-term orbital period variations, further supporting spider pulsar behaviour.

4. Physical System Parameters and Light Curve Modeling

Light curve analysis employed direct heating and asymmetric spot models to extract companion and system parameters. The direct heating model interprets the pulsar as the irradiating source, with the companion emitting as a variable-temperature black body. Key fitted results include:

Parameter	Value
Orbit inclination, $i$	$\approx$83° (direct heating); $\approx$65° (spot model)
Companion mass, $M_c$	$\approx0.10~M_\odot$
Mass ratio, $q$	$\approx18.9$
Roche-lobe filling factor, $f$	$\approx0.65$
Companion night-side temperature, $T_n$	$\approx3060$ K (direct heating); $\approx2300$ K (spot model)
Day-side temperature, $T_d^{\rm max}$	$\approx7200$ K (direct heating); $\approx6100$ K (spot model); spot $T_{\rm spot}\approx6600$ K
Distance, $D$	$\approx2.13$ kpc

The Roche-lobe filling factor is defined as $f = R / R_{L1}$, where $R$ is the companion’s effective radius and $R_{L1}$ is the distance to the inner Lagrange point. The observed temperature contrast $\Delta T \approx 4000$ K aligns with known black widow/redback systems, while the light curve asymmetry is best described by a spot model—suggesting uneven surface heating possibly due to localised intra-binary shock or magnetic field effects.

5. X-ray, Gamma-ray, and Optical Variability

X-ray luminosity inferred for PSR J1544–2555 is $L_X\approx4.0\times10^{31}$ erg/s, assuming $N_H\approx7.8\times10^{20}$ cm$^{-2}$ and a power-law index $\Gamma_X=2.0$. Gamma-ray luminosity is $\approx4.0\times10^{33}$ erg/s, resulting in $L_\gamma/L_X\approx100$, a value commonly observed in MSP systems.

Optical spectra evidenced Ca II, CH, and weak Balmer lines, typical of heated low-mass companions. The bluer colour near light curve minima likely originates from nonthermal emission processes, consistent with intra-binary shock scenarios suggested by X-ray detection. However, the current optical spectra have insufficient S/N to measure radial velocities and thus to tightly constrain the neutron star mass.

6. Significance and Implications for Pulsar Population Studies

The coordinated cross-wavelength identification and characterization of PSR J1544–2555 underscore the efficacy of Fermi-LAT catalogue cross-referencing with variable-source optical surveys. These strategies allow for the systematic uncovering of millisecond pulsar binaries, which may be initially obscured by radio eclipses.

This system exemplifies the established framework for MSP identification, consisting of:

• Multi-band photometry to capture variability and infer orbital phase,
• Period analyses (Lomb–Scargle, robust sinusoidal fits) for ephemeris determination,
• Radio timing for spin/orbit solutions and DM measurement,
• Gamma-ray pulsation detection using photon weighting and advanced acceleration searches,
• X-ray confirmation of nonthermal components via imaging or timing,
• Detailed light-curve synthesis (direct heating, spot models) to model the surface physics of the companion.

A plausible implication is that continuing optical and X-ray monitoring, together with more sensitive spectroscopic campaigns, will allow the discovery of additional systems, more stringent mass measurements, and deeper inquiry into intra-binary shock physics and magnetospheric interactions.

7. Prospects for Future Research

Key future directions include:

• High signal-to-noise optical spectroscopy to enable radial velocity measurement, critical for precise neutron star mass determination and for constraining the equation-of-state of dense matter.
• Targeted phase-resolved X-ray observations to isolate pulsar and shock emission components.
• Long-term radio monitoring to investigate further orbital period variations and eclipse behaviour, advancing understanding of low-mass companion dynamics.
• Expanded optical surveys and variability analyses for identification of further spider pulsar candidates in Fermi-LAT’s unidentified source catalogue.

The discovery of PSR J1544–2555 contributes a benchmark system to the spider pulsar population, enhances the multiwavelength detection methodology, and provides a natural laboratory for investigating evolutionary processes, binary interactions, and extreme physical conditions in neutron star environments.