Jefferson Michael Kommers

On the Association of Gamma-Ray Bursts with Supernovae

The recent discovery of a supernova (SN 1998bw) seemingly associated with GRB 980425 adds a new twist to the decades-old debate over the origin of gamma-ray bursts. To investigate the possibility that some (or all) bursts are associated with supernovae, we performed a systematic search for temporal/angular correlations using catalogs of BATSE and BATSE/Ulysses burst locations. We find no associations with any of the precise BATSE/Ulysses locations, which allows us to conclude that the fraction of high-fluence gamma-ray bursts from known supernovae is small (less than 0.2%). For the more numerous weaker bursts, the corresponding limiting fraction of 1.5% is less constraining due to the imprecise locations of these events. This limit (1.5%18 bursts) allows that a large fraction of the recent supernovae used as a comparison data set (18 supernovae 20%) could have associated gamma-ray bursts. Thus, although we find no significant evidence to support a burst/supernova association, the possibility cannot be excluded for weak bursts.

The Intensity Distribution of Faint Gamma-Ray Bursts Detected with BATSE

We have recently completed a search of 6 years of archival BATSE data for gamma-ray bursts (GRBs) that were too faint to activate the real-time burst detection system running on board the spacecraft. These ii nontriggered ˇˇ bursts can be combined with the ii triggered ˇˇ bursts detected on board to produce a GRB intensity distribution that reaches peak —uxes a factor of D2 lower than could be studied pre- viously. The value of the statistic (in Euclidean space) for the bursts we detect is 0.177 ^ 0.006. SV /V max T This surprisingly low value is obtained because we detected very few bursts on the 4.096 s and 8.192 s timescales (where most bursts have their highest signal-to-noise ratio) that were not already detected on the 1.024 s timescale. If allowance is made for a power-law distribution of intrinsic peak luminosities, the extended peak —ux distribution is consistent with models in which the redshift distribution of the gamma-ray burst rate approximately traces the star formation history of the universe. We argue that this class of models is preferred over those in which the burst rate is independent of redshift. We use the peak —ux distribution to derive a limit of 10% (99% con—dence) on the fraction of the total burst rate that could be contributed by a spatially homogeneous (in Euclidean space) subpopulation of burst sources, such as type Ib/c supernovae. These results lend support to the conclusions of previous studies predicting that relatively few faint ii classical ˇˇ GRBs will be found below the BATSE onboard detection threshold. Subject headings: gamma rays: burstsmethods: statistical

A Comparison between the Rapid Burster and GRO J1744–28

Twenty years ago, the Rapid Burster (MXB 1730-335) was discovered. Its most salient feature was the occurrence of rapidly repetitive type II X-ray bursts, the release of gravitational potential energy due to spasmodic accretion onto a compact object. This is almost certainly due to an accretion disk instability whose origin is still not understood. With the recent appearance of GRO J1744-28, the Rapid Burster is no longer the only system to produce such bursts. Both systems are transient low-mass X-ray binaries in which the accretor is a neutron star. The Rapid Burster, located in a globular cluster, also produces type I bursts which are due to thermonuclear flashes on the neutron stars surface; no X-ray pulsations are observed. Its neutron star magnetic field is therefore relatively weak. In contrast, strong X-ray pulsations have been observed in the persistent flux as well as in the type II bursts from GRO J1744-28, but no type I bursts have been observed. Thus, the magnetic field of the neutron star in this system is probably stronger than is the case of the Rapid Burster. The fact that type II bursts occur in both systems may bring us closer to an understanding of the mechanism(s?) that produces them.

On the nature of XTE J0421+560/CI Camelopardalis

We present the results of an analysis of RXTE, BATSE, and optical/infrared data of the 1998 outburst of the X-ray transient system XTE J0421+560 (CI Cam). The X-ray outburst shows a very fast decay (initial e-folding time ~0.5 days, slowing down to ~2.3 days). The X-ray spectrum in the 2-25 keV band is complex, softening considerably during decay and with strongly variable intrinsic absorption. A strong iron emission line is observed. No fast time variability is detected (<0.5% rms in the 1-4096 Hz band at the outburst peak). The analysis of the optical/IR data suggests that the secondary is a B[e] star surrounded by cool dust, and places the system at a distance of 2 kpc. At this distance, the peak at 2-25 keV luminosity is ~4 × 1037 ergs s-1. We compare the properties of this peculiar system with those of the Be/NS LMC transient A 0538-66 and suggest that CI Cam is of similar nature. The presence of strong radio emission during outburst indicates that the compact object is likely to be a black hole or a weakly magnetized neutron star.

The evolution of Rapid Burster outbursts

We describe the evolutionary progression of an outburst of the Rapid Burster. Four outbursts have been observed with the Rossi X-Ray Timing Explorer between 1996 February and 1998 May, and our observations are consistent with a standard evolution over the course of each. An outburst can be divided into two distinct phases. Phase I is dominated by type I bursts, with a strong persistent emission component; it lasts for 15-20 d. Phase II is characterized by type II bursts, which occur in a variety of patterns. The light curves of time-averaged luminosity for the outbursts show some evidence for reflares, similar to those seen in soft X-ray transients. The average recurrence time for Rapid Burster outbursts during this period was 218 d, in contrast to an average similar to 180-d recurrence period observed during 1976-1983.

Sidebands due to Quasi-periodic Oscillations in 4U 1626–67

The low-mass X-ray binary pulsar 4U 1626-67 shows 0.048 Hz quasi-periodic oscillations (QPOs) and red noise variability, as well as coherent pulsations at the 0.130 Hz neutron star spin frequency. Power density spectra of observations made with the Rossi X-Ray Timing Explorer show significant sidebands separated from the pulsar spin frequency (and its harmonics) by the QPO frequency. These show that the instantaneous amplitude of the coherent pulsations is modulated by the amplitude of the QPOs. This phenomenon is expected in models such as the magnetospheric beat frequency model, where the QPOs originate near the polar caps of the neutron star. In the 4-8 keV energy range, however, the lower frequency sidebands are significantly stronger than their higher frequency complements; this is inconsistent with the magnetospheric beat frequency model. We suggest that the 0.048 Hz QPOs are produced instead by a structure orbiting the neutron star at the QPO frequency. This structure crosses the line of sight once per orbit and attenuates the pulsar beam, producing the symmetric (amplitude modulation) sidebands. It also reprocesses the pulsar beam at the beat frequencies between the neutron star spin frequency and the QPOs, producing the excess variability observed in the lower frequency sidebands. Quite independently, we find no evidence that the red noise variability modulates the amplitude of the coherent pulsations. This is also in contrast to the expectations of the magnetospheric beat frequency model and differs from the behavior in some high-mass X-ray binary pulsars.

Properties of the Second Outburst of the Bursting Pulsar (GRO J1744-28) as Observed with BATSE

One year after its discovery, the Bursting Pulsar (GRO J1744(28) went into outburst again, dis- playing the hard X-ray bursts and pulsations that make this source unique. We report on BATSE obser- vations of both the persistent and burst emission for this second outburst and draw comparisons with the —rst. The second outburst was smaller than the —rst in both duration and peak luminosity. The per- sistent —ux, burst peak —ux, and burst —uence were all reduced in amplitude by a factor of D1.7. Despite these diUerences, the two outbursts were very similar with respect to the burst occurrence rate, the dura- tions and spectra of bursts, the absence of spectral evolution during bursts, and the evolution of the ratio a of average persistent to burst luminosity. Although no spectral evolution was found within individual bursts, we —nd evidence for a small (20%) variation of the spectral temperature during the course of the second outburst. Subject headings: pulsars: individual (GRO J1744(28) ¨ X-rays: burstsX-rays: stars

Precise interplanetary network localization of the bursting pulsar GRO J1744-28

We analyze 426 observations of the bursting pulsar GRO J1744(28 by Ulysses and BATSE. Triangu- lating each burst and statistically combining the triangulation annuli, we obtain a 3 p error ellipse whose area is 532 arcsec2. The accuracy of this statistical method has been independently veri—ed with obser- vations of the soft gamma repeater SGR 1900)14. The ellipse is fully contained within the 1@ radius ASCA error circle of the soft X-ray counterpart and partially overlaps the 10@@ radius ROSAT error circle of a source which may also be the soft X-ray counterpart. A variable source which has been pro- posed as a possible IR counterpart lies at the edge of the 3 p error ellipse, making it unlikely from a purely statistical point of view to be associated with the bursting pulsar. Subject headings: pulsars: individual (GRO J1744(28) ¨ stars: neutronX-rays: stars

Pulse Delay Observations of GRO J1744–28

The bursting pulsar GRO J1744-28 exhibits a unique combination of persistent X-ray pulsations (with a pulse period Ppulse ≈ 0.467 s) and X-ray bursts. The pulsations are also present (at an enhanced amplitude) during the bursts, but the arrival times of the pulses during the burst are delayed with respect to those of the persistent emission. We present the results of a detailed study of the pulse delays using data obtained with the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory. We find that the average delay, as measured during a 1.5 s interval at the peak of the burst, is independent of energy in the energy range ~25-75 keV and has a magnitude Δt = 74±13 ms. We also find that the phase delay measured near the peak of the bursts remained approximately constant throughout the first outburst of the source, although the peak flux of the bursts varied by a factor of ~3.3.

A search for non-triggered events in the BATSE data base

The archival data from BATSE permit a search for transients that did not activate the onboard burst trigger. Examples of such non-triggered events include faint gamma-ray bursts (GRBs), emission from soft gamma-ray repeaters (SGRs), and bursts and flares from X-ray binaries. A GRB may fail to trigger onboard because it is too faint, because it occurs while the onboard trigger is disabled, or because it biases the onboard background estimation. We describe a search of the BATSE archival data that is sensitive to GRBs with peak fluxes fainter by a factor of ∼2 than those detected with the onboard burst trigger (on the 1.024 s time scale).