[Paper Review] ASTRO-H White Paper - Broad-band Spectroscopy and Polarimetry

P. Coppi, Ł. Stawarz+14 more|arXiv (Cornell University)|Dec 3, 2014

Astrophysical Phenomena and Observations4 references3 citations

TL;DR

This white paper proposes ASTRO-H's broad-band X-ray spectroscopy and polarimetry capabilities to advance understanding of high-energy astrophysical phenomena, including jetted AGN, non-thermal emission in novae, and unidentified hard X-ray sources. By enabling precise spectral characterization from 0.3 keV to 80 keV, ASTRO-H will resolve key uncertainties in source energetics, particle acceleration, and source populations in the early Universe.

ABSTRACT

The broad energy range spanned by ASTRO-H instruments, from ~0.3 to 600 keV, with its high spectral resolution calorimeter and sensitive hard X-ray imaging, offers unique opportunities to study black holes and their environments. The ability to measure polarization is particularly novel, with potential sources including blazars, Galactic pulsars and X-ray binaries. In this White Paper, we present an overview of the synergistic instrumental capabilities and the improvements over prior missions. We also show how ASTRO-H fits into the multi-wavelength landscape. We present in more detail examples and simulations of key science ASTRO-H can achieve in a typical 100 ksec observation when data from all four instruments are combined. Specifically, we consider observations of black-hole source (Cyg X-1 and GRS 1915+105), blazars (Mrk 421 and Mrk 501), a quasar (3C 273), radio galaxies (Centaurus A and 3C 120), and active galaxies with a strong starburst (Circinus and NGC 4945). We will also address possible new discoveries expected from ASTRO-H.

Motivation & Objective

• To address the deficit of misaligned, radio-loud AGN at z > 3 by characterizing high-redshift blazars via broad-band X-ray spectroscopy.
• To investigate non-thermal particle acceleration in nova shocks by detecting hard X-ray emission associated with GeV gamma-ray emission.
• To identify and characterize previously unidentified hard X-ray/soft gamma-ray sources using ASTRO-H’s high-energy instruments.
• To improve constraints on the energetics and spectral properties of jetted AGN, particularly those accreting at high Eddington rates.
• To enable precise measurements of thermal and non-thermal X-ray emission in transient sources through simultaneous high-resolution spectroscopy and hard X-ray sensitivity.

Proposed method

• Utilize ASTRO-H’s X-ray Spectrometer (SXS) for high-resolution, high-throughput X-ray spectroscopy across 0.3–12 keV.
• Employ the Hard X-ray Imager (HXI) to extend sensitivity to 80 keV, enabling broad-band spectral coverage of non-thermal emission components.
• Apply the Soft Gamma-ray Detector (SGD) for high-energy response in the 20–60 keV band, crucial for detecting hard X-ray and soft gamma-ray emission.
• Conduct deep 100 ks observations to achieve sensitivity curves that match or exceed those of previous missions like NuSTAR and Fermi.
• Model broad-band spectral energy distributions (SEDs) of high-redshift blazars using multi-wavelength data, including HXI and SGD responses.
• Combine spectral fitting with polarization measurements to disentangle emission components from jets, accretion disks, and scattering regions.

Experimental results

Research questions

• RQ1 What is the true population of jetted AGN at z > 3, and how do their spectral and luminosity properties compare to low-redshift counterparts?
• RQ2 To what extent are nova shocks capable of accelerating electrons to non-thermal energies, as evidenced by hard X-ray and GeV gamma-ray emission?
• RQ3 What are the spectral and timing properties of unidentified hard X-ray/soft gamma-ray sources detected by Swift-BAT, COMPTEL, and INTEGRAL?
• RQ4 How do the X-ray spectra of high-redshift blazars constrain the bulk Lorentz factors and radiative efficiencies of relativistic jets?
• RQ5 Can ASTRO-H detect and characterize a non-thermal hard X-ray component in a nova, providing direct evidence for particle acceleration?

Key findings

• High-redshift blazars with z > 2.345, such as 2149–306, show flat hard X-ray spectra (Γ < 2) and emit most of their radiative power at MeV frequencies, making them ideal targets for broad-band X-ray studies.
• The HXI and SGD instruments on ASTRO-H are expected to achieve sensitivity curves that match or exceed those of NuSTAR for 100 ks exposures, enabling precise spectral fitting of high-energy sources.
• A significant deficit of misaligned, radio-loud AGN at z > 3 suggests either lower jet bulk Lorentz factors or sample incompleteness, which ASTRO-H can help resolve via detailed spectral analysis.
• Only one nova, V2491 Cyg, has shown tentative evidence of non-thermal hard X-ray emission (from Suzaku HXD/PIN), highlighting the need for deeper, simultaneous X-ray and gamma-ray observations.
• The combination of SXS and HXI on ASTRO-H will allow simultaneous high-resolution thermal X-ray spectroscopy and hard X-ray continuum measurements, enabling full characterization of nova shock physics.
• ASTRO-H’s broad-band response from 0.3 keV to 80 keV will enable the identification and spectral characterization of previously unidentified hard X-ray sources, including potential new classes of high-energy emitters.