Discovery of Fast X-Ray Oscillations during the 1998 Giant Flare from SGR 1900+14

By T. Strohmayer, A. Watts. 2005

195 citations recorded by the source corpus.

We report the discovery of complex high-frequency variability during the 1998 August 27 giant flare from SGR 1900+14 using the Rossi X-Ray Timing Explorer (RXTE). We detect an ≈84 Hz quasi-periodic oscillation (QPO) during a 1 s interval beginning approximately 1 minute after the initial hard spike. The amplitude is energy-dependent, reaching a maximum of 26% (rms) for photons above 30 keV and is not detected below 11 keV, with a 90% confidence upper limit of 14% (rms). Remarkably, additional QPOs are detected in the average power spectrum of data segments centered on the rotational phase at which the 84 Hz signal was detected. Two signals, at 53.5 and 155.1 Hz, are strongly detected, while a third feature at 28 Hz is found with lower significance. These QPOs are not detected at other rotational phases. The phenomenology seen in the SGR 1900+14 flare is similar to that of QPOs recently reported by Israel et al. from the 2004 December 27 flare from SGR 1806-20, suggesting that they may have a common origin, perhaps torsional vibrations of the neutron star crust. Indeed, an association of the four frequencies (in increasing order) found in SGR 1900+14 with l = 2, 4, 7, and 13 toroidal modes appears plausible. We discuss our findings in the context of this model and show that if the stars have similar masses, then the magnetic field in SGR 1806-20 must be about twice as large as in SGR 1900+14, broadly consistent with estimates from pulse timing. We briefly discuss how mode identifications could lead to constraints on the nuclear equation of state.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/c117863c68403047d16bd569ef205c369ec6b492

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In Astronomy and Astrophysics

1. P. Lasky
2. A. Watts
3. T. Strohmayer
4. Sandro Mereghetti
5. N. Andersson
6. L. Samuelsson
7. J. Lattimer

Citations in the corpus

• Neutron star asteroseismology. Axial crust oscillations in the Cowling approximation (2006, background): “It may, for instance, be noted in table 1 that we arrive at a different multipole identification compared to Strohmayer & Watts (2005) for SGR 1900+14 and Israel et al. (2005) for SGR 1806−20.”
• Detection with RHESSI of High-Frequency X-Ray Oscillations in the Tailof the 2004 Hyperflare from SGR 1806–20 (2005, methodology): “To be an l = 8 mode, the neutron star mass would have to exceed that of SGR 1900+14 by > 0.3M⊙ (see the discussion in Strohmayer & Watts (2005)).”
• Gravitational Waves from Neutron Stars: A Review (2015, background): “Observations of quasi-periodic oscillations (QPOs) in the tails of two giant flares (Israel et al. 2005; Strohmayer & Watts 2005) has engendered excitement about the potential new field of neutron star asteroseismology.”
• The strongest cosmic magnets: soft gamma-ray repeaters and anomalous X-ray pulsars (2008, background): “This led to the detection of QPOs at frequencies of 28, 54, 84 and 155 Hz (Strohmayer and Watts 2005).”
• The 2004 Hyperflare from SGR 1806–20: Further Evidence for Global Torsional Vibrations (2006, background): “The strong correlation of power contours with the falling edge of peak 2 is rather similar to the behavior seen in the 84 Hz oscillation from SGR 1900+14 (Strohmayer & Watts 2005, see their Figure 3), and these similarities suggest it may be connected with the X-ray modulation mechanism.”