[Paper Review] The HST contribution to neutron star astronomy

R. Mignani|ArXiv.org|Oct 29, 2007

Pulsars and Gravitational Waves Research4 citations

TL;DR

This paper reviews the Hubble Space Telescope's (HST) pivotal role in advancing neutron star astronomy, particularly in identifying isolated neutron stars (INSs) through high-sensitivity optical and near-UV observations. HST enabled nearly all INS identifications since the 1990s, revealing thermal and magnetospheric emission components, constraining neutron star cooling models, and resolving synchrotron nebulae, with STIS and WFPC2 playing key roles in near-UV timing and astrometry despite instrument failures.

ABSTRACT

While isolated neutron stars (INSs) are among the brightest gamma-ray sources, they are among the faintest ones in the optical, and their study is a challenging task which require the most powerful telescopes. HST has lead neutron star optical astronomy yielding nearly all the identifications achieved since the early 1990s. Here, the major HST contributions in the optical studies of INSs and their relevance for neutron stars' astronomy are reviewed.

Motivation & Objective

• To review HST's contributions to optical and near-UV studies of isolated neutron stars (INSs) since the 1990s.
• To assess how HST observations advanced understanding of neutron star emission mechanisms, cooling, and magnetospheric physics.
• To evaluate the impact of HST instruments—especially STIS, WFPC2, and FOC—on INS identification and spectral characterization.
• To analyze how HST data resolved inconsistencies in neutron star cooling models through precise distance and temperature measurements.
• To advocate for HST instrument repairs to maintain its leading role in neutron star research.

Proposed method

• Utilized archival HST observations from WFPC2, STIS, FOC, and NICMOS to conduct multi-band photometry and spectroscopy of INSs.
• Performed spectral energy distribution (SED) fitting across optical, near-UV, and near-IR bands to distinguish thermal and non-thermal emission components.
• Applied astrometry to measure proper motions and parallactic distances, enabling surface temperature and age constraints.
• Conducted phase-resolved timing and polarimetry using STIS and WFPC2 to study pulsar emission directivity and magnetic field geometry.
• Analyzed variability in pulsar wind nebulae using long-term WFPC2 monitoring to detect expanding structures.
• Combined X-ray and optical data to test neutron star cooling models and assess the role of magnetic field topology and composition.

Experimental results

Research questions

• RQ1 How has HST advanced the identification and characterization of isolated neutron stars (INSs) in the optical and near-UV?
• RQ2 What is the origin of the near-UV thermal emission detected in middle-aged INSs like Geminga and PSR B0656+14?
• RQ3 How do HST observations of pulsar wind nebulae compare with X-ray morphologies and support models of relativistic wind expansion?
• RQ4 To what extent do HST-derived surface temperatures and distances resolve discrepancies in neutron star cooling curves?
• RQ5 What role do HST polarimetry and timing observations play in constraining neutron star magnetosphere models?

Key findings

• HST enabled 8 new isolated neutron star (INS) identifications, boosting the identification rate by a factor of 4 compared to ground-based telescopes like VLT and Keck.
• Near-UV observations with STIS revealed the first phase-resolved pulsations from Geminga, PSR B0656+14, and the Vela pulsar, showing spectral dependence of light curves.
• The detection of a Rayleigh-Jeans spectral component (T ~ 10^5 K) in Geminga and PSR B0656+14 provided direct evidence of thermal emission from the neutron star surface.
• HST astrometry of XTINS (thermally emitting INSs) yielded space velocities too high for interstellar medium accretion, favoring younger ages and natural cooling over re-heating scenarios.
• WFPC2 observations resolved the structure of synchrotron nebulae around the Crab and B0540-69 pulsars, showing morphological similarity to X-ray nebulae and evidence for expanding equatorial winds.
• Phase-averaged optical polarimetry with WFPC2 and ACS is ongoing for PSR B0540-69 and the Vela pulsar, with polarization data expected to constrain magnetic field geometry and magnetospheric models.