Discovery of a strong 6.6 keV emission feature from EXO 1745$-$248 after the superburst in 2011 October

Abstract

We discover an unidentified strong emission feature in the X-ray spectrum of EXO 1745$-$248 obtained by RXTE at 40 hr after the peak of a superburst. The structure was centered at 6.6 keV and significantly broadened with a large equivalent width of 4.3 keV, corresponding to a line photon flux of 4.7 $\times$ 10${}^{-3}$ ph cm${}^{-2}$ s${}^{-1}$. The 3-20 keV spectrum was reproduced successfully by a power law continuum with narrow and broad (2.7 keV in FWHM) Gaussian emission components. Alternatively, the feature can be described by four narrow Gaussians, centered at 5.5 keV, 6.5 keV, 7.5 keV and 8.6 keV. Considering the strength and shape of the feature, it is unlikely to have originated from reflection of the continuum X-rays by some optically thick materials, such as an accretion disk. Moreover, the intensity of the emission structure decreased significantly with an exponential time scale of 1 hr. The feature was not detected in an INTEGRAL observation performed 10 h before the RXTE observation with a line flux upper limit of 1.5 $\times$ 10${}^{-3}$ ph cm${}^{-2}$ s${}^{-1}$. The observed emission structure is consistent with gravitationally redshifted charge exchange emission from Ti, Cr, Fe, and Co. We suggest that the emission results from a charge exchange interaction between a highly metal-enriched fall back ionized burst wind and an accretion disk, at a distance of $\sim$60 km from the neutron star. If this interpretation is correct, the results provide new information on the understanding of nuclear burning processes during thermonuclear X-ray bursts.

arXiv comment13 pages, 4 figures, Accepted for publication in PASJ

References in corpus (35)

• The MAXI Mission on the ISS: Science and Instruments for Monitoring All Sky X-Ray Images
• Calibration of the Rossi X-ray Timing Explorer Proportional Counter Array
• The endpoint of the rp process on accreting neutron stars
• X-ray reflected spectra from accretion disk models. III. A complete grid of ionized reflection calculations
• Photospheric radius expansion X-ray bursts as standard candles
• Models for Type I X-Ray Bursts with Improved Nuclear Physics
• A Remarkable Three Hour Thermonuclear Burst From 4U 1820-30
• Carbon Flashes in the Heavy Element Ocean on Accreting Neutron Stars
• Relativistic Lines and Reflection from the Inner Accretion Disks Around Neutron Stars
• Long Type I X-ray Bursts and Neutron Star Interior Physics
• Exposing the Nuclear Burning Ashes of Radius Expansion Type I X-ray Bursts
• Low-level accretion in neutron-star X-ray binaries
• Truncated disc versus extremely broad iron line in XTE J1650-500
• What ignites on the neutron star of 4U 0614+091?
• Distances of the bulge globular clusters Terzan 5, Liller 1, UKS 1 and Terzan 4 based on HST NICMOS photometry
• Evidence of heavy-element ashes in thermonuclear X-ray bursts with photospheric superexpansion
• The hard quiescent spectrum of the neutron-star X-ray transient EXO 1745-248 in the globular cluster Terzan 5
• Modelling the high mass accretion rate spectra of GX 339-4: Black hole spin from reflection?
• Plasma code for astrophysical charge exchange emission at X-ray wavelengths
• A Chandra Study of the Dense Globular Cluster Terzan 5
• Strong X-ray variability in the quiescent state of the neutron star low-mass X-ray binary EXO 1745-248
• NuSTAR observation of a Type I X-ray burst from GRS 1741.9-2853
• Detection of burning ashes from thermonuclear X-ray bursts
• A superburst candidate in EXO 1745--248 as a challenge to thermonuclear ignition models
• Accretion Geometry of the Low-Mass X-ray Binary Aquila X-1 in the Soft and Hard States
• Suzaku Studies of Luminosity-Dependent Changes in the Low-Mass X-ray Binary Aquila X-1
• Very hard states in neutron star low-mass X-ray binaries
• Super-Eddington winds from Type I X-ray bursts
• Simplified Picture of Low Mass X-ray Binaries based on Data from Aql X-1 and 4U1608-52
• Superburst with Outburst from EXO 1745-248 in Terzan 5 with MAXI
• Maxi observations of long X-ray bursts
• An XMM-Newton and INTEGRAL view on the hard state of EXO 1745-248 during its 2015 outburst
• Extreme Quiescent Variability of the Transient Neutron Star Low-mass X-ray Binary EXO 1745-248 in Terzan 5
• A Hard-to-Soft State Transition of Aquila X-1 Observed with Suzaku
• An enigmatic hump around 30 keV in Suzaku spectra of Aquila X-1 in the Hard State

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• Impacts of the direct URCA and Superfluidity inside a Neutron Star on Type-I X-Ray Bursts and X-Ray Superbursts