arXiv subjects

Toshio Murakami

Publications and source records attributed to Toshio Murakami.

21 records · Page 2Linked to original sources

arXiv2005

A possible observational evidence for $θ^{-2}$ angular distribution of opening half-angle of GRB jets

We propose a method to estimate the pseudo jet opening half-angle of GRBs using the spectral peak energy ($\Ep$)--peak luminosity relation (so called Yonetoku relation) as well as the $\Ep$--collimation-corrected $γ$-ray energy relation (so called Ghirlanda relation). For bursts with known jet break times and redshifts, we compared the pseudo jet opening half-angle with the standard one and found that the differences are within a factor 2. We apply the method to 689 long GRBS. We found that the distribution function of the pseudo jet opening half-angle obeys $f(θ_j)\proptoθ_j^{-2.2 \pm 0.2}$ with possible cutoffs for $θ_j < 0.04$ and $θ_j > 0.3$ although the log-normal fit is also possible. $θ^{-2}$ distribution is compatible with the structured jet model. From the distribution function we found that the beaming correction for the rate of GRBs is $\sim 340$, which means $\sim 10^{-5}$ yr$^{-1}$ galaxy$^{-1}$ or only one in $10^2$ type Ib/c supernovae. We also found the evolution of the distribution function as a function of the redshift.

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arXiv2000

Timing Noise in SGR 1806-20

We have phase connected a sequence of RXTE PCA observations of SGR 1806-20 covering 178 days. We find a simple secular spin-down model does not adequately fit the data. The period derivative varies gradually during the observations between 8.1 and 11.7 * 10^-11 s/s (at its highest, ~40% larger than the long term trend), while the average burst rate as seen with BATSE drops throughout the time interval. The phase residuals give no compelling evidence for periodicity, but more closely resemble timing noise as seen in radio pulsars. The magnitude of the timing noise, however, is large relative to the noise level typically found in radio pulsars. Combining these results with the noise levels measured for some AXPs, we find all magnetar candidates have Δ_8 values larger than those expected from a simple extrapolation of the correlation found in radio pulsars. We find that the timing noise in SGR 1806-20 is greater than or equal to the levels found in some accreting systems (e.g., Vela X-1, 4U 1538-52 and 4U 1626-67), but the spin-down of SGR 1806-20 has thus far maintained coherence over 6 years. Alternatively, an orbital model with a period P_orb = 733 days provides a statistically acceptable fit to the data. If the phase residuals are created by Doppler shifts from a gravitationally bound companion, then the allowed parameter space for the mass function (small) and orbital separation (large) rule out the possibility of accretion from the companion sufficient to power the persistent emission from the SGR.

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arXiv1999

Variable Spin-down in the Soft Gamma Repeater SGR 1900+14 and Correlations with Burst Activity

We have analyzed Rossi X-ray Timing Explorer Proportional Counter Array observations of the pulsed emission from SGR 1900+14 during September 1996, June - October 1998, and early 1999. Using these measurements and results reported elsewhere, we construct a period history of this source for 2.5 years. We find significant deviations from a steady spin-down trend during quiescence and the burst active interval. Burst and Transient Source Experiment observations of the burst emission are presented and correlations between the burst activity and spin-down rate of SGR 1900+14 are discussed. We find an 80 day interval during the summer of 1998 when the average spin-down rate is larger than the rate elsewhere by a factor ~ 2.3. This enhanced spin-down may be the result of a discontinuous spin-down event or ``braking glitch'' at the time of the giant flare on 27 August 1998. Furthermore, we find a large discrepancy between the pulsar period and average spin-down rate in X-rays as compared to radio observations for December 1998 and January 1999.

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