[Paper Review] ASTRO-H White Paper - Accreting Pulsars, Magnetars, and Related Sources

Shunji Kitamoto, Teruaki Enoto+19 more|arXiv (Cornell University)|Dec 3, 2014

Pulsars and Gravitational Waves Research2 references3 citations

TL;DR

This ASTRO-H white paper proposes using the Soft Gamma-ray Detector (SGD) onboard the ASTRO-H X-ray observatory to study magnetars and accreting pulsars with unprecedented sensitivity and time resolution. The SGD's active shield provides fine time resolution (16 ms), high effective area (~800 cm² at 1 MeV), and spectral resolution, enabling detection of MeV photons from short bursts and distinguishing spectral models like blackbody plus power-law, crucial for probing QED effects in strong magnetic fields.

ABSTRACT

As the endpoints of massive star evolution, neutron stars are enigmatic celestial objects characterized by extremely dense and exotic nuclear matter, magnetospheres with positrons (antimatter), rapid rotation and ultra-strong magnetic fields. Such an extreme environment has provided an accessible astrophysical laboratory to study physics under conditions unattainable on Earth and to tackle a range of fundamental questions related to: the aftermath of stellar evolution and the powerful explosions of massive stars, the equation of state and physics of some of the most exotic and magnetic stars in the Universe, the workings of the most powerful particle accelerators in our Galaxy and beyond, and the sources of gravitational waves that are yet to be detected. Recent observations revealed a great diversity of neutron stars, including ultra-strongly magnetized pulsars, referred to as "magnetars", and unusual types of accreting X-ray pulsars. In this white paper, we highlight the prospects of the upcoming X-ray mission, ASTRO-H, in studying these highly magnetized neutron stars.

Motivation & Objective

• To investigate the spectral and temporal properties of short bursts from magnetars, particularly the presence of MeV photons.
• To explore the connection between persistent X-ray emission and burst emission in magnetars.
• To test radiation mechanisms in ultra-strong magnetic fields using high-energy photon detection.
• To improve detection of weak and intermediate flares through enhanced sensitivity and reduced dead time.
• To enable detailed study of photon splitting and other QED effects via high-statistics, fine-time-resolution data from the SGD shield.

Proposed method

• Utilize the Soft Gamma-ray Detector (SGD) onboard ASTRO-H, which features a large active BGO scintillator shield surrounding the main instrument.
• Leverage the SGD’s wide field-of-view (~2π sr) and high effective area (~800 cm² at 1 MeV) to achieve high photon statistics for bright and intermediate bursts.
• Apply fine time resolution (16 ms) to resolve rapid burst features and minimize data loss due to dead time or rollover effects.
• Use spectral modeling with components such as two blackbody (2BB) or blackbody plus power-law (BB+PL) to distinguish emission mechanisms.
• Simulate detectability of polarization and spectral features using realistic burst light curves and flux levels (e.g., 1–1000 Crab).
• Correct for pile-up and counter rollover effects in data analysis to extend observable dynamic range up to ~1000 Crab.

Experimental results

Research questions

• RQ1 Can MeV photons from short magnetar bursts be detected with high time and spectral resolution using the SGD shield?
• RQ2 Do the spectral shapes of weak short bursts evolve to resemble the persistent X-ray emission, as seen in SGR 0501+4516?
• RQ3 What is the detectability of polarization in accumulated short burst events using the SGD shield?
• RQ4 Can the SGD distinguish between competing spectral models (e.g., 2BB vs. BB+PL) in bright bursts like AXP 1E1547−5408?
• RQ5 To what extent can the SGD detect and characterize intermediate-flare and faint bursts, given its high sensitivity and low dead time?

Key findings

• The SGD shield detector can detect up to 2 MeV photons from bright short bursts, enabling study of high-energy emission features.
• The detector achieves a possible detection limit of about 1 Crab flux for MeV emission, with observable flux range extending up to 1000 Crab with corrections.
• Spectral simulations show that the SGD can clearly distinguish between 2BB and BB+PL models in burst spectra, such as those from AXP 1E1547−5408.
• The fine time resolution (16 ms) and reduced dead time allow for immediate data transfer and minimal loss, overcoming limitations of previous instruments like Suzaku WAM.
• The active shield design provides ~800 cm² effective area at 1 MeV—roughly twice that of Suzaku WAM—significantly improving photon statistics.
• Polarization detectability simulations show that accumulated burst events can be studied with high confidence, enabling tests of QED effects in strong magnetic fields.