[Paper Review] Blackbody excess in persistent Be pulsars

N. La Palombara, S. Mereghetti+3 more|arXiv (Cornell University)|Jan 22, 2013

Astrophysical Phenomena and Observations3 citations

TL;DR

This study identifies a common, high-temperature blackbody (kT_BB > 1 keV) spectral component with small emitting radius (R_BB < 0.5 km) in four persistent low-luminosity Be X-ray pulsars observed with XMM-Newton. The component, contributing 24–42% of flux below 10 keV, is interpreted as thermal emission from neutron star polar caps, indicating a distinct class of low-luminosity, long-period Be pulsars with unique accretion physics.

ABSTRACT

We report on the main results obtained thanks to an observation campaign, performed with XMM-Newton, of four persistent, low-luminosity (Lx ~ 10^34 erg/s) and long-period (P &gt; 200 s) Be accreting pulsars. We found that all sources considered here are characterized by a spectral excess that can be described with a blackbody component of high temperature (kT &gt; 1 keV) and small area (R &lt; 0.5 km). We show that: 1) this feature is a common property of several low-luminosity X-ray binaries; 2) for most sources the blackbody parameters (radius and temperature) are within a narrow range of values; 3) it can be interpreted as emission from the NS polar caps.

Motivation & Objective

• To investigate the spectral properties of persistent, low-luminosity Be X-ray pulsars with long pulse periods (P > 200 s).
• To determine whether a common spectral feature exists across these systems that cannot be explained by standard accretion models.
• To test the hypothesis that the observed spectral excess arises from thermal emission from neutron star polar caps.
• To compare the blackbody parameters with theoretical accretion column radii to validate the physical interpretation.
• To classify these pulsars as a distinct subclass based on their unique spectral and luminosity characteristics.

Proposed method

• Performed XMM-Newton observations of four persistent Be pulsars: 4U 0352+309, RX J0146.9+6121, RX J1037.5-5647, and RX J0440.9+4431.
• Fitted X-ray spectra using a power-law plus blackbody (PL+BB) model, rejecting single-component models due to large residuals.
• Extracted blackbody temperature (kT_BB) and radius (R_BB) from spectral fits, with uncertainties derived from χ² minimization.
• Estimated accretion rates and theoretical accretion column radii (R_col) using standard neutron star parameters (M_NS = 1.4 M☉, R_NS = 10⁶ cm, B_NS = 10¹² G).
• Compared R_BB with R_col to test consistency with polar cap emission models.
• Compiled data from other low-luminosity high-mass X-ray binaries to assess the generality of the hot BB component.

Experimental results

Research questions

• RQ1 Is there a common spectral component across low-luminosity, long-period Be pulsars that is not accounted for by standard accretion models?
• RQ2 Can the observed high-temperature, small-radius blackbody component be physically interpreted as emission from neutron star polar caps?
• RQ3 How do the blackbody parameters (temperature and radius) correlate with X-ray luminosity and pulse period in these systems?
• RQ4 To what extent does the observed blackbody emission contribute to the total X-ray flux in these sources?
• RQ5 Do these sources form a distinct subclass of Be pulsars based on their spectral and luminosity characteristics?

Key findings

• All four persistent Be pulsars exhibit a significant blackbody spectral component with kT_BB > 1 keV and R_BB < 0.5 km, contributing 24–42% of the flux below 10 keV.
• The blackbody temperature and radius are tightly clustered, with kT_BB ≈ 1.1–1.4 keV and R_BB ≈ 128–361 m, indicating a common physical origin.
• The inferred accretion column radius (R_col) is consistent with the blackbody radius (R_BB) across all sources, supporting the polar cap emission interpretation.
• The hot blackbody component is absent in soft-excess pulsars, which instead show low-temperature (kT_SE < 0.5 keV) and large-emitting-region (R_SE > 100 km) components.
• The hot BB feature is a defining characteristic of low-luminosity (L_X ~ 10³⁴–10³⁵ erg s⁻¹), long-period Be pulsars, distinguishing them from other pulsar subclasses.
• The feature is not unique to these four sources; similar hot BB components are observed in other low-luminosity high-mass X-ray binaries, confirming its broader relevance.