Support ATel At Patreon

[ Previous | Next | ADS ]

More RXTE PCA observations of SWIFT J1626.6-5156: a peculiar pulsating object

ATel #687; T. Belloni (INAF-OAB), J. Homan (MIT), S. Campana (INAF-OAB), C. B. Markwardt (U. Maryland & NASA/GSFC), N. Gehrels (NASA/GSFC)
on 12 Jan 2006; 15:58 UT
Credential Certification: Tomaso Belloni (belloni@merate.mi.astro.it)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

We observed the transient pulsar SWIFT J1626.6-5156 (Palmer et al., ATEL #678, Markwardt & Swank, ATEL #679) eleven times, including the observations reported in ATEL #679, from 2005 Dec 19 to 2005 Dec 31. Strong pulsations are detected at the ~15.37s period from all observations, with a peak-to-peak pulsed amplitude of about 50%, weakly dependent on energy. The pulse shape is roughly rectangular and it shows little changes with energy. A more detailed timing analysis to look for period variations and evidence for orbital doppler shifts is under way. Power density spectra show, in addition to the harmonic components of the pulsation, strong band-limited noise with an integrated 0.1-20 Hz fractional rms of around 20% and a characteristic frequency of 1 Hz. Two large flares are observed on Dec 24 and 25 respectively, each for a duration of ~400 seconds. During the flares, the pulsed fraction increases to ~80%; the minima of the pulsation cycle increased by a factor of 2, while the maxima increased by a factor of 5. After the flare, both the average rate and the pulsed fraction went below their pre-flare values, and started a slow recovery that was not completed about 1200 seconds later, when the observation ended. A third, smaller and shorter flare was observed on Dec 27. Fits were made to the non-flare 3-25 keV PCA (PCU2) energy spectra of all observations. HEXTE spectra could not be made because of instrument problems. Using the simple model of Markwardt and Swank (ATel #679), with the iron line energy fixed to 6.4 keV, provided good fits to all spectra, but with slightly different values than reported in ATel #679. The photon index increased from ~0.9 in the first observation to ~1.3 in the last observations and the column density increased from ~4e22 cm^-2 to 5.5e22 cm^-2. The cut-off energy had values around 12 keV in all observations. Finally, the absorbed 3-25 keV flux decreased from 3.6e-9 ergs/cm^2/s to 2.4e-09 ergs/cm^2/s. All of the reported spectral changes occurred smoothly. Mild spectral hardening was observed during the flares, with the decrease in the power-law index depending on whether or not the column density was kept to the same value as found in the fit to the persistent emission. The observed flares show similarities with the bursts of the bursting pulsar GRO J1744-28, suggesting that they could be similar objects. Peculiar bursts observed in the past from the accreting X-ray pulsars LMC X-4 and SMC X-1 also showed some, but not all, of the properties observed here.

Selected Swift J1626.6-5156 light curves