BURST FLUENCE DISTRIBUTIONS OF SOFT GAMMA REPEATERS 1806–20 AND 1900+14 IN THEROSSI X-RAY TIMING EXPLORERPCA ERA

Abstract

We study the fluence distributions of over 3040 bursts from SGR 1806−20 and over 1963 bursts from SGR 1900+14 using the complete set of observations available from the Rossi X-Ray Timing Explorer/Proportional Counter Array through 2011 March. Cumulative event distributions are presented for both sources and are fitted with single and broken power laws as well as an exponential cutoff. The distributions are best fitted by a broken power law with exponential cutoff; however the statistical significance of the cutoff is not high and the upper portion of the broken power law can be explained as the expected number of false bursts due to random noise fluctuations. Event distributions are also examined in high and low burst rate regimes and power-law indices are found to be consistent, independent of the burst rate. The contribution function of the event fluence is calculated. This distribution shows that the energy released in the soft gamma repeater (SGR) bursts is dominated by the most powerful events for both sources. The power-law nature of these distributions combined with the dominant energy dissipation of the system occurring in the large, less frequent bursts is indicative of a self-organized critical system, as suggested by Goǧus et al. in 1999.

Learning about the magnetar Swift J1834.9−0846 from its wind nebula

“…This may naturally occur if the mean energy in such outflows is comparable to the radiative energy observed from these bursts. For the latter, the distribution is compatible with dN/dE ∝ E −5/3 (where N is the number of bursts and E is their radiated energy) expected for self-organized criticality (e.g., Cheng et al 1996;Gögüş et al 1999Gögüş et al , 2000Prieskorn & Kaaret 2012). This might suggest that the total energy output (which scales as E 2 dN/dE) is dominated by the largest events -the rare giant flares (although it is possible that such giant flares may comprise a separate component that is not the high energy tail of the self-organized critical phenomenon and are isolated events).…”

Self-organized criticality in a spherically closed cellular automaton: Modeling soft gamma repeater bursts driven by magnetic reconnection

“…In fact, the cutoff energy of the truncated icosahedron model with n = 60 nodes is higher than that of the regular dodecahedron model with n = 20. A cutoff feature was also found in the observations of SGR 1806−20 and SGR 1900+14 [9]. While the statistical significance of the observed cutoff is not high, bursts with the highest energy may correspond to avalanches enclosing the central neutron star.…”

“…This value is consistent with the observations. According to Prieskorn and Kaaret [9], there is no difference between the power-law indices observed in high-and low-burst-rate regimes. This result may appear to contradict our CA model with ∆B dependence, but we found that cases with the same values of ∆B /B c are equivalent.…”

“…The power-law index, γ, of the differential distributions dN ∝ E −γ dE is related to the power-law fit of the cumulative distributions as N (> E) ∝ E 1−γ . According to the observations of SGR 1806−20 and SGR 1900+14, the energy distributions of bursts have been fitted with γ = 1.43-1.76 [7,8] and γ ∼ 1.55 [9]. In our CA simulation, γ depends on the value of ∆B and it is 1.44, 1.57 and 1.74 for ∆B = 0.1, 0.2 and 0.4, respectively.…”

“…The recurrent bursts of soft gamma repeaters (SGRs) have event energy distributions well fitted with a power law [7][8][9]. It is currently thought that SGRs are associated with ultrastrongly magnetized neutron stars ( 10 14 G).…”