[Paper Review] Observations of Isolated Neutron Stars with the ESO Multi-Conjugate Adaptive Optics Demonstrator

R. Mignani, R. Falomo+11 more|arXiv (Cornell University)|Jun 25, 2008

Pulsars and Gravitational Waves Research2 references4 citations

TL;DR

This study used the VLT/MAD instrument to conduct deep near-infrared (Ks-band) observations of two isolated neutron stars, RX J0420.0−5022 and RX J1856.5−3754, searching for NIR counterparts to test a proposed evolutionary link between X-ray dim isolated neutron stars (XDINSs) and magnetars. Despite high sensitivity, neither source was detected, with 3σ limits of Ks ~ 21.5 and Ks ~ 20, respectively, suggesting that deeper observations are needed to constrain NIR emission mechanisms and test the magnetar-XDINS connection.

ABSTRACT

High-energy observations have unveiled peculiar classes of isolated neutron stars which, at variance with radio pulsars, are mostly radio silent and not powered by the star rotation. Among these objects are the magnetars, hyper-magnetized neutron stars characterized by transient X-ray/gamma-ray emission, and neutron stars with purely thermal, and in most cases stationary, X-ray emission (a.k.a., X-ray dim isolated neutron stars or XDINSs). While apparently dissimilar in their high-energy behavior and age, both magnetars and XDINSs have similar periods and unusually high magnetic fields. This suggests a tantalizing scenario where the former evolve into the latter.Discovering so far uninvestigated similarities between the multi-wavelength properties of these two classes would be a further step forward to establish an evolutionary scenario. A most promising channels is the near infrared (NIR) one, where magnetars are characterized by a distinctive spectral flattening with respect to the extrapolation of the soft X-ray spectrum.We observed the two XDINSs RX J0420.0-5022 and RX J1856.5-3754 with the Multi-Conjugate Adaptive Optics Demonstrator (MAD) at the Very Large Telescope (VLT) as part of the instrument guaranteed time observations program, to search for their NIR counterparts. Both RX J1856.5-3754 and RX J0420.0-5022 were not detected down to K_s ~20 and Ks ~21.5, respectively. In order to constrain the relation between XDINSs and magnetars it would be of importance to perform deeper NIR observations. A good candidate is 1RXS J214303.7+065419 which is the XDINS with the highest inferred magnetic field.

Motivation & Objective

• To investigate whether X-ray dim isolated neutron stars (XDINSs) exhibit near-infrared (NIR) spectral flattening similar to magnetars, which could support an evolutionary link between the two classes.
• To test the hypothesis that magnetospheric or thermal emission from fallback disks in XDINSs could produce detectable NIR fluxes.
• To constrain the NIR emission efficiency of XDINSs relative to rotation-powered pulsars and magnetars.
• To identify the most promising candidate for future deep NIR studies, particularly among high-magnetic-field XDINSs.

Proposed method

• Conducted Ks-band observations using the Multi-Conjugate Adaptive Optics Demonstrator (MAD) on the Very Large Telescope (VLT) at Paranal Observatory.
• Employed adaptive optics to achieve diffraction-limited image quality, improving sensitivity and source detection in crowded fields.
• Performed photometric analysis using aperture photometry and source extraction with source-fitting algorithms to determine flux upper limits.
• Calculated 3σ detection limits based on background noise and source signal-to-noise ratios, validated against prior ISAAC H-band data.
• Compared observed flux upper limits with theoretical models, including blackbody emission from the neutron star surface and fallback disk models.
• Evaluated NIR emission efficiency (ηNIR = L_NIR / Ė) and compared it with values from rotation-powered pulsars and magnetars.

Experimental results

Research questions

• RQ1 Do XDINSs such as RX J0420.0−5022 and RX J1856.5−3754 exhibit NIR spectral flattening similar to magnetars, indicating a common emission mechanism?
• RQ2 Is the non-detection of these sources in the Ks band consistent with thermal emission from the neutron star surface or fallback disk models?
• RQ3 What is the upper limit on the NIR emission efficiency (ηNIR) of XDINSs, and how does it compare to that of magnetars and rotation-powered pulsars?
• RQ4 Could the anomalously high optical emission in 1RXS J214303.7+065419 be explained by non-thermal magnetospheric emission or a fallback disk, and is it detectable in the NIR?

Key findings

• No significant NIR counterpart was detected for RX J0420.0−5022 down to a 3σ limiting magnitude of Ks ~ 21.5 in the Ks band.
• No significant NIR counterpart was detected for RX J1856.5−3754 down to a 3σ limiting magnitude of Ks ~ 20 in the Ks band.
• The Ks-band flux upper limits are substantially above the Rayleigh-Jeans extrapolation of the X-ray and optical spectra, indicating that the observed optical flux cannot be explained by simple blackbody extrapolation.
• The NIR emission efficiency (ηNIR) of these XDINSs cannot be ruled out to be comparable to that of rotation-powered pulsars or magnetars, based on current upper limits.
• The observed flux upper limits are consistent with a blackbody spectrum from the neutron star surface or a fallback disk, depending on disk size and accretion rate.
• 1RXS J214303.7+065419 remains the most promising candidate for future deep NIR observations due to its high inferred magnetic field and anomalous optical emission.