THE OUTBURST DECAY OF THE LOW MAGNETIC FIELD MAGNETAR SGR 0418+5729
- Institute of Space Sciences (CSIC-IEEC), Campus UAB, Faculty of Science, Torre C5-parell, E-08193 Barcelona (Spain)
- INAF/Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio Catone (Italy)
- Department de Fisica Aplicada, Universitat d'Alacant, Ap. Correus 99, E-03080 Alacant (Spain)
- Dipartimento di Fisica e Astronomia, Universita di Padova, via F. Marzolo 8, I-35131 Padova (Italy)
- Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT (United Kingdom)
- INAF/Istituto di Astrofisica Spaziale e Fisica Cosmica-Milano, via E. Bassini 15, I-20133 Milano (Italy)
- JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309 (United States)
- INAF/Osservatorio Astronomico di Cagliari, localita Poggio dei Pini, strada 54, I-09012 Capoterra (Italy)
- Sabanc Latin-Small-Letter-Dotless-I University, Orhanl Latin-Small-Letter-Dotless-I -Tuzla, 34956 Istanbul (Turkey)
- NASA Marshall Space Flight Center, Huntsville, AL 35812 (United States)
- AIM (UMR 7158 CEA/DSM-CNRS-Universite Paris Diderot) Irfu/Service d'Astrophysique, Saclay, F-91191 Gif-sur-Yvette Cedex (France)
We report on the long-term X-ray monitoring of the outburst decay of the low magnetic field magnetar SGR 0418+5729 using all the available X-ray data obtained with RXTE, Swift, Chandra, and XMM-Newton observations from the discovery of the source in 2009 June up to 2012 August. The timing analysis allowed us to obtain the first measurement of the period derivative of SGR 0418+5729: P-dot =4(1) Multiplication-Sign 10{sup -15} s s{sup -1}, significant at a {approx}3.5{sigma} confidence level. This leads to a surface dipolar magnetic field of B{sub dip} {approx_equal} 6 Multiplication-Sign 10{sup 12} G. This measurement confirms SGR 0418+5729 as the lowest magnetic field magnetar. Following the flux and spectral evolution from the beginning of the outburst up to {approx}1200 days, we observe a gradual cooling of the tiny hot spot responsible for the X-ray emission, from a temperature of {approx}0.9 to 0.3 keV. Simultaneously, the X-ray flux decreased by about three orders of magnitude: from about 1.4 Multiplication-Sign 10{sup -11} to 1.2 Multiplication-Sign 10{sup -14} erg s{sup -1} cm{sup -2}. Deep radio, millimeter, optical, and gamma-ray observations did not detect the source counterpart, implying stringent limits on its multi-band emission, as well as constraints on the presence of a fossil disk. By modeling the magneto-thermal secular evolution of SGR 0418+5729, we infer a realistic age of {approx}550 kyr, and a dipolar magnetic field at birth of {approx}10{sup 14} G. The outburst characteristics suggest the presence of a thin twisted bundle with a small heated spot at its base. The bundle untwisted in the first few months following the outburst, while the hot spot decreases in temperature and size. We estimate the outburst rate of low magnetic field magnetars to be about one per year per galaxy, and we briefly discuss the consequences of such a result in several other astrophysical contexts.
- OSTI ID:
- 22127119
- Journal Information:
- Astrophysical Journal, Vol. 770, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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