Ⅱ-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 47 (2014)

RIKEN Accel. Prog. Rep. 00 (0)NuSTAR observation of the fast rotating magnetized white dwarf
AE Aquarii†
T. Kitaguchi∗1 on behalf of the NuSTAR team
AE Aquarii (AE Aqr) is a cataclysmic variable classiﬁed as a member of the DQ Herculis or intermediate
polar (IP) class, consisting of a white dwarf (WD) anda K4–5 V star. In the IP class, the WD is generally
thought to possess a magnetic ﬁeld (B ∼ 105−7 G) sufﬁciently strong to channel the accretion ﬂow from thesecondary star to the WD poles. Accordingly, hard
X-rays are produced by the shock-heated gas, whichreaches temperatures of a few tens of keV near theWD surface. The X-ray emission exhibits spin modulation caused by the varying aspect of the accreting
poles with respect to the rotation of the WD1) .
The 33.08 s period makes AE Aqr the fastestspinning magnetic WD with intriguing emission features. In comparison to many IPs, AE Aqr shows athermal soft X-ray spectrum with a very low luminosity, and therefore the mechanism and location of the Xray emission are uncertain. In addition, a Suzaku observation2) showed that AE Aqr may emit non-thermal
hard X-rays with a narrow pulse proﬁle at the spin period, suggesting that the source may accelerate charged
particles in a manner similar to rotation-powered pulsars3) . However, a more recent Suzaku observation2)
did not reproduce the earlier result, leaving the detection of non-thermal X-rays uncertain.
The Nuclear Spectroscopic Telescope Array (NuSTAR) satellite4) , launched in 2012 June, carries theﬁrst focusing hard X-ray (3–79 keV) telescope in orbit. Owing to focusing optics, NuSTAR achieves thehighest sensitivity ever observed in this band, and it
has the capability to detect hard X-ray point sources
with a ﬂux down to sub µCrab. Therefore, NuSTAR
can help measure the maximum temperature of thethermal plasma in AE Aqr and test the presence ofany beamed non-thermal component. We performed along observations of this source with NuSTAR for anexposure of 125 ks in 2012 September.
Spectral analysis shows that hard X-rays are wellﬁtted by an optically thin thermal plasma model with
+6.1
+1.0
three temperatures of 0.8+0.2−0.5 , 2.3−0.8 , and 9.3−2.2 keV,
the highest of which is higher than that previously
observed for this source (3.0 keV)2) . In addition,
the spectrum is also characterized by an opticallythin thermal plasma model with two temperatures of+1.6
1.0+0.3−0.2 and 4.6−0.8 keV in combination with a powerlaw component with index of 2.5±0.2, although the derived index is inconsistent with the Suzaku value (1.1±0.62) ) and is steeper than those found for rotationpowered pulsars (0.6–2.1)3) . Compared with the three†∗1Condensed from the article in ApJ, Vol.782, 3 (2014)
RIKEN Nishina Center

temperature model, the ﬁt with the two-temperaturemodel with the power-law emission is slightly but notsigniﬁcantly preferred. We cannot distinguish whether
the hard X-ray component detected with NuSTAR isthermal or non-thermal emission.
A timing analysis with Z12 -statistic or Rayleigh test5)
shows that the spin period in the 3–10 keV band is33.0769 ± 0.0004 s, which is consistent with previously
measured values2) . The 3–20 keV pulse proﬁle obtained by folding data at the best determined period
is broad and approximately sinusoidal with a pulsed
fraction of 16.6 ± 2.3%. We do not ﬁnd any evidence
for a sharp feature in the pulse proﬁle.
Two energy sources could, in principle, power theobserved X-ray luminosity: liberation of gravitationalenergy of accreting matter and the rotational energy of
the WD. The observed X-ray emission is diﬃcult to explain as a result of rotation-powered emission because
synchrotron radiation, which is observed for rotationpowered pulsars, is expected to be strongly beamed
along the ﬁeld lines, which is inconsistent with the observed broad pulse proﬁle. Instead, accretion-poweredemission is more probable, although the observed spectrum with the highest temperature of 9.3+6.1−2.2 keV issofter than a postshock temperature of ∼ 30 keV predicted by the standard accretion column model6) under the assumption that the WD mass is ∼ 0.7 M⊙ asdetermined in the optical measurement7) .
The standard model assumes a high-accretion column heated by the shock close to the WD surface andcooled by thermal bremsstrahlung. However, the accretion rate in AE Aqr is considerably small, which isa consequence of the low X-ray luminosity. We suggest two modiﬁcations of the standard model to explain the AE Aqr spectrum: the shock temperature
could be low because of a tall accretion column comparable with the WD radius, and cyclotron emission
with B > 106 G could additionally cool down the accretion plasma. Detailed calculations of such models
will hopefully reproduce the spectrum and pulse proﬁleof AE Aqr with the optically determined WD mass.
References
1) J. Patterson: PASP 106, 209 (1994)
2) Y. Terada et al.: PASJ 60, 387 (2008)3) V. M. Kaspi et al.: in Compact Stellar X-Ray Sources,edited by W. Lewin & M. van der Klis (Cambridge Univ.
Press, Cambridge, 2006), p.279
4) F. A. Harrison et al.: ApJ 770, 103 (2013)5) R. Buccheri et al.: A&A 128, 245 (1983)
6) K. Aizu: PThPh 49, 1184 (1973)
7) J. Echevarrı́a et al.: MNRAS 387, 1563 (2008)- 138 -1