arXiv:0809.4029v2 [astro-ph] 26 Sep 2008
The Advanced X-ray Timing Array (AXTAR)
Deepto Chakrabarty∗, Paul S. Ray†, Tod E. Strohmayer∗∗and
the AXTAR Collaboration∗
∗MIT Kavli Institute for Astrophysics and Space Research, Cambridge, MA 02139, USA
†Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA
∗∗Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
Abstract. AXTAR is an X-ray observatory mission concept, currently under study in the U.S.,
that combines very large collecting area, broadband spectral coverage, high time resolution, highly
flexible scheduling, and an ability to respond promptly to time-critical targets of opportunity. It is
optimized for submillisecond timing of bright Galactic X-ray sources in order to study phenomena
at the natural time scales of neutron star surfaces and black hole event horizons, thus probing the
physics of ultradense matter, strongly curved spacetimes, and intense magnetic fields. AXTAR’s
main instrument is a collimated, thick Si pixel detector with 2–50 keV coverageand 8 m2collecting
area.Fortimingobservationsofaccretingneutronstars andblackholes,AXTAR providesatleast an
order of magnitude improvement in sensitivity over both RXTE and Constellation-X. AXTAR also
carries a sensitive sky monitor that acts as a trigger for pointed observations of X-ray transients and
also provides continuous monitoring of the X-ray sky with 20× the sensitivity of the RXTE ASM.
AXTAR builds on detector and electronics technology previously developed for other applications
and thus combines high technical readiness and well understood cost.
Keywords: Neutron stars, Black holes, X-ray timing
PACS: 95.55.Ka, 97.60.Jd, 97.60.Lf, 97.80.Jp
The natural time scales near neutron stars (NSs) and stellar mass black holes (BHs)
are in the millisecond range. These time scales characterize the fundamental physical
properties of compact objects: mass, radius, and angular momentum. For example, the
maximum spin rate of a NS is set by the equation of state of the ultradense matter in
its interior, a fundamental property of matter that still eludes us. Similarly, orbital peri-
ods at a given radius near a BH are set by the BH’s mass, angular momentum, and the
laws of relativistic gravity. Although it was recognized for decades that the measure-
ment of such time scales would provide unique insight into these compact stars and their
extreme physics, it was not until the 1995 launch of the Rossi X-ray Timing Explorer
(RXTE; ) that oscillations on these time scales were actually detected from accreting
NSs and stellar-mass BHs. This conference celebrates RXTE’s discovery of millisecond
oscillationsthat trace the spin rate of accreting NSs. RXTE has also discovered millisec-
ond oscillations from accreting BHs with frequencies that scale inversely with BH mass
and are consistent with the orbital time scale of matter moving in the strongly curved
spacetime near the BH event horizon.
While RXTE revealed the existence of these phenomena, it lacks the sensitivity to
fully exploit them in determining the fundamental properties of NSs and BHs. Here, we
describe the Advanced X-ray Timing Array (AXTAR), a new mission concept boasting
Selected AXTAR Science Topics
Science ObjectiveAXTAR Observations
NS mass, radius, EOSX-ray burst oscillations. kHz QPOs. Accreting ms pul-
sars. Asteroseismology with magnetar oscillations.
BH oscillations. Broad Fe lines. Phase-resolved spec-
troscopy of low-freq QPOs. AGN monitoring.
Thermonuclear X-ray bursts and superbursts.
Accreting msec pulsars. kHz QPOs in NSs.
NS and BH transients.
X-ray pulsar orbital evolution.
Hard X-ray cyclotron lines in high-mass binaries.
Magnetar pulse profiles.
Physics of nuclear burning
Physics of accretion
Physics of jets
Physics of mass transfer
Multipolar magnetic field
components of pulsars
an order of magnitude improvement in sensitivity over RXTE1. AXTAR was initially
proposed as a medium-class probe concept for NASA’s 2007 Astrophysics Strategic
Mission Concept Studies. A modified version of AXTAR is also being studied as a
NASA MIDEX-class mission concept.
AXTAR will study a broad range of topics in the astrophysics of NSs and BHs as well as
the physics of ultradense matter, strongly curved spacetime, and intense magnetic fields
(see Table 1). The mission design was driven by the requirements for three key efforts:
Neutron Star Radii and the Equation of State of Ultradense Matter. AXTAR will
brightness oscillations near the onset of thermonuclear X-ray bursts. Gravitational self-
lensing by the NS suppresses the amplitude of observed oscillations by allowing an
observer to “see” more than just the facing hemisphere, an effect set by the ratio of mass
to radius, M/R [2, 5, 6]. Additionally, the pulse shape of the oscillations is influenced
by the NS rotational velocity, vrot∝ Rνspin, where the NS spin frequency νspinis known
from the oscillations [7, 8, 9]. The pulsed surface emission encodes information about
the NS structure, and modeling of high signal-to-noise profiles can be used to constrain
M and R separately . Precise measurement of NS radii at the 5% level is the key to
determining the equation of state of ultradense matter , one of the most fundamental
questions arising at the interface of physics and astrophysics.
Black hole oscillations and the physics of extreme gravity. AXTAR will determine
if high-frequency oscillations from BHs are a direct tracer of mass and spin. In several
systems, RXTE has detected high-frequency QPO pairs whose properties suggest such
a relationship . By extending the detection threshold down a 20× to ∼0.05% ampli-
1A similar mission concept was previously discussed by Phil Kaaret and collaborators [3, 4].
AXTAR and several other missions with X-ray timing capabilities.
(a) A notional depiction of the AXTAR mission concept. (b) The effective area curves for
tude, AXTAR will test this prospect in two ways: (1) by detecting additional examples
of rapid QPO pairs in other BHs to test the 1/M scaling of the observed frequencies
with a larger source sample; and (2) by detecting additional weaker QPO modes in the
existing systems, as predicted by models for the BH oscillations [see, e.g., 13-17].
Continuous long-term monitoring of the variable X-ray sky. The AXTAR Sky
Monitor will continuously monitor hundreds of X-ray sources in addition to serving as
a trigger for target-of-opportunity observations of active X-ray transients. A sensitive
sky monitor with nearly continuous all-sky coverage can serve as a primary science
instrument for a wide variety of investigations using an all-sky sample, including: daily
flux and spin monitoring of accretion-powered pulsars, phase-coherent tracking of the
superbursts, X-ray flashes, etc.
The AXTAR mission concept assumes a 3-axis stabilized spacecraft in low-Earth orbit.
A notional illustration of the spacecraft layout is shown in Figure 1a, and the mission
parameters are summarized in Table 2. Like RXTE, AXTAR is optimized for study of
X-ray transients anywhere in the sky by combining a small solar avoidance angle with
the ability to repoint promptly (<1 d) to targets of opportunity (TOOs).
AXTAR’s principal instrument is the Large Area Timing Array (LATA), a collimated
(1◦FWHM), thick (1 mm) Si pixel detector with 2–50 keV coverage and 8 m2collecting
area. Pixelated solid-state detectors constructed out of large Si wafers promise better
performance, higher reliability, and lower cost than gas proportional counters; they also
1000 of the largest detectors that can be made from a single 150 mm diameter Si wafer,
coupledwithelectronics optimizedforextremelylownoiseand lowpower.Theeffective
area curve for AXTAR-LATA is compared to several other missions in Figure 1b. An
improvement of over an order of magnitude over RXTE is achieved.
The AXTAR Sky Monitor (SM) comprises a set of 32 modest-size coded aperture
AXTAR Mission Parameters
LARGE AREA TIMING ARRAY (LATA)
Low E threshold
High E threshold
SKY MONITOR (SM)
Sensitivity (1 day)
Source loc. acc.
Solar avoidance angle
Effective area6–10 m2
1–20% on 10 Crab
10% on 10 Crab
cameras, each consisting of a 2D-pixelated Si detector (essentially identical to those
used in the LATA) with 300 cm2area and 2–25 keV coverage. Each detector will
view the sky through a 2-D coded mask and will have a 40◦×40◦(FWHM) field of
view. The cameras will be mounted with pointing directions chosen to cover 4π sr. The
AXTAR-SM will be able to monitor most celestial locations with a 60–100% duty cycle
(depending on final orbit choice), as compared to the ∼3% duty cycle of RXTE/ASM.
It will also have timing capability as well as a 1-day sensitivity of a few mCrab (20×
better than RXTE/ASM). This will allow early detection (and TOO triggering) of much
fainter transients as well as enabling the continuous monitoring and timing programs
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