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Register a new userConstraints on RRAT Emission Mechanisms from RXTE/PCA Observations of RRAT J1819-1458
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Authors
Robert E. Rutledge
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We derive the second and most stringent limit to date of the X-ray/radio flux ratio (F_x/F_R) for the radio bursts associated with the recently identified source class, the Rotating Radio Transients (RRATs). We analyze 20.1 hr of rxte/PCA observations of RRAT J1819-1458 -- a period during which 350ppm23 RRAT radio bursts occurred, based on the previously observed average radio burst rate. No X-ray bursts were detected, implying an upper-limit on the X-ray flux for RRAT-bursts of <1.5e-8 ergs cm-2 s-1 (2-10 keV) or a luminosity <2.3e37 (d/3.6kpc)^2 ergs s-1. The time-average burst flux is <2e-13 ergs cm-2 s-1 (0.5-8 keV) -- a factor of 10 below that of the previously identified persistent X-ray counterpart. Thus, X-ray bursts from the RRAT are energetically unimportant compared with the persistent X-ray emission. From the previously observed burst radio flux, we derive an upper-limit F_x/F_R< 4.2e-12 erg cm-2 s-1 mJy-1 for the radio bursts from this RRAT, the most stringent to date, due to the high radio flux of bursts from this source. The F_x/F_R ratio is a factor approximately 80 larger than that of the millisecond pulsar PSR B1821-24; thus emission processes of X-ray/radio efficiency comparable to MSP pulses cannot be ruled out. However, if the RRAT burst emission mechanism is identical to the msec bursts of magnetars, then the msec bursts of magnetars should be easily detected with radio instrumentation; yet none have been reported to date.
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The Rotating RAdio Transient (RRAT) J1819-1458 exhibits ~3 ms bursts in the radio every ~3 min, implying that it is visible for only ~1 per day. Assuming that the optical light behaves in a similar manner, long exposures of the field would be relatively insensitive due to the accumulation of sky photons. A much better way of detecting optical emission from J1819-1458 would then be to observe with a high-speed optical camera simultaneously with radio observations, and co-add only those optical frames coincident with the dispersion-corrected radio bursts. We present the results of such a search, using simultaneous ULTRACAM and Lovell Telescope observations. We find no evidence for optical bursts in J1819-1458 at magnitudes brighter than i=19.3 (5-sigma limit). This is nearly 3 magnitudes fainter than the previous burst limit, which had no simultaneous radio observations.
We present an analysis of regular timing observations of the high-magnetic-field Rotating Radio Transient (RRAT) J1819$-$1458 obtained using the 64-m Parkes and 76-m Lovell radio telescopes over the past five years. During this time, the RRAT has suffered two significant glitches with fractional frequency changes of $0.6times10^{-6}$ and $0.1times10^{-6}$. Glitches of this magnitude are a phenomenon displayed by both radio pulsars and magnetars. However, the behaviour of J1819$-$1458 following these glitches is quite different to that which follows glitches in other neutron stars, since the glitch activity resulted in a significant long-term net decrease in the slow-down rate. If such glitches occur every 30 years, the spin-down rate, and by inference the magnetic dipole moment, will drop to zero on a timescale of a few thousand years. There are also significant increases in the rate of pulse detection and in the radio pulse energy immediately following the glitches.
(abridged) The high radio-flux brightness temperature of the recently discovered class of sources known as Rotating RAdio Transients (RRATs) motivates detailed study in the X-ray band. We describe analyses of historical X-ray data, searching for X-ray phenomena (sources, behaviors), finding no sources or behaviors which may unequivocally be associated with RRAT J1911+00. We put forward a candidate X-ray counterpart to RRAT J1911+00, discovered in a Chandra observation in Feb 2001, which fades by a factor >5 prior to April 2004. The X-ray flux and optical (F_X/F_R>12) and near infra-red (F_X/F_J>35) limits, as well as the X-ray flux itself, are consistent with an AGN origin, unrelated to RRAT J1911+00. Searches for msec X-ray bursts found no evidence for such a signal, and we place the first observational upper-limit on the X-ray to radio flux ratio of RRAT bursts: F_X/F_{radio} <6e-11 ergs cm-2 s-1 mJy-1. The upper-limit on the X-ray burst flux (corresponding to <2.2e37 (d/3.3 kpc)^2 erg s-1, 2-10 keV) requires a limit on the spectral energy density power-law slope of alpha<-0.3 between the radio and X-ray bands. We place a limit on the time-average X-ray burst luminosity, associated with radio bursts, of < 3.4e30 (d/3.3 kpc)^2 erg s-1.
We present the results of simultaneous radio and X-ray observations of PSR J1819-1458. Our 94-ks XMM-Newton observation of the high magnetic field 5*10^13 G pulsar reveals a blackbody spectrum (kT~130 eV) with a broad absorption feature, possibly composed of two lines at ~1.0 and ~1.3 keV. We performed a correlation analysis of the X-ray photons with radio pulses detected in 16.2 hours of simultaneous observations at 1-2 GHz with the Green Bank, Effelsberg, and Parkes telescopes, respectively. Both the detected X-ray photons and radio pulses appear to be randomly distributed in time. We find tentative evidence for a correlation between the detected radio pulses and X-ray photons on timescales of less than 10 pulsar spin periods, with the probability of this occurring by chance being 0.46%. This suggests that the physical process producing the radio pulses may also heat the polar-cap.
We present the first blind interferometric detection and imaging of a millisecond radio transient with an observation of transient pulsar J0628+0909. We developed a special observing mode of the Karl G. Jansky Very Large Array (VLA) to produce correlated data products (i.e., visibilities and images) on a time scale of 10 ms. Correlated data effectively produce thousands of beams on the sky that can localize sources anywhere over a wide field of view. We used this new observing mode to find and image pulses from the rotating radio transient (RRAT) J0628+0909, improving its localization by two orders of magnitude. Since the location of the RRAT was only approximately known when first observed, we searched for transients using a wide-field detection algorithm based on the bispectrum, an interferometric closure quantity. Over 16 minutes of observing, this algorithm detected one transient offset roughly 1 from its nominal location; this allowed us to image the RRAT to localize it with an accuracy of 1.6. With a priori knowledge of the RRAT location, a traditional beamforming search of the same data found two, lower significance pulses. The refined RRAT position excludes all potential multiwavelength counterparts, limiting its optical luminosity to L_i<1.1x10^31 erg/s and excluding its association with a young, luminous neutron star.
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