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Register a new userLOFAR discovery of a quiet emission mode in PSR B0823+26
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PSR B0823+26, a 0.53-s radio pulsar, displays a host of emission phenomena over timescales of seconds to (at least) hours, including nulling, subpulse drifting, and mode-changing. Studying pulsars like PSR B0823+26 provides further insight into the relationship between these various emission phenomena and what they might teach us about pulsar magnetospheres. Here we report on the LOFAR discovery that PSR B0823+26 has a weak and sporadically emitting quiet (Q) emission mode that is over 100 times weaker (on average) and has a nulling fraction forty-times greater than that of the more regularly-emitting bright (B) mode. Previously, the pulsar has been undetected in the Q-mode, and was assumed to be nulling continuously. PSR B0823+26 shows a further decrease in average flux just before the transition into the B-mode, and perhaps truly turns off completely at these times. Furthermore, simultaneous observations taken with the LOFAR, Westerbork, Lovell, and Effelsberg telescopes between 110 MHz and 2.7 GHz demonstrate that the transition between the Q-mode and B-mode occurs within one single rotation of the neutron star, and that it is concurrent across the range of frequencies observed.
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We present results of the analysis of interstellar scintillation in PSR B0823+26. Observations were conducted at a frequency of 1.7 GHz using the 32-m Torun Centre for Astronomy radio telescope. More than 50 observing sessions, lasting on average 10 h, were conducted between 2003 and 2006. We found interstellar scintillation parameters by means of dynamic spectrum analysis as well as structure function analysis of the flux density variations. We identified two distinctive time-scales, which we believe to be the time-scales of diffractive and refractive scintillation. Our results show that at the given frequency the diffractive time-scale in PSR B0823+26 is $tau_{diss} = 19.3^{+1.7}_{-1.6}$ min, the refractive time-scale is $tau_{riss} = 144 pm 23$ min and the decorrelation bandwidth is $B_{iss} = 81 pm 3$ MHz.
Simultaneous observations of PSR B0823+26 with ESAs XMM-Newton, the Giant Metrewave Radio Telescope and international stations of the Low Frequency Array revealed synchronous X-ray/radio switching between a radio-bright (B) mode and a radio-quiet (Q) mode. During the B mode we detected PSR B0823+26 in 0.2$-$2 keV X-rays and discovered pulsed emission with a broad sinusoidal pulse, lagging the radio main pulse by 0.208 $pm$ 0.012 in phase, with high pulsed fraction of 70$-$80%. During the Q mode PSR B0823+26 was not detected in X-rays (2 $sigma$ upper limit a factor ~9 below the B-mode flux). The total X-ray spectrum, pulse profile and pulsed fraction can globally be reproduced with a magnetized partially ionized hydrogen atmosphere model with three emission components: a primary small hot spot ($T$$sim$3.6$times10^6$ K, $R$$sim$17 m), a larger cooler concentric ring ($T$$sim$1.1$times10^6$ K, $R$$sim$280 m) and an antipodal hot spot ($T$$sim$1.1$times10^6 $ K, $R$$sim$100 m), for the angle between the rotation axis and line of sight direction $sim66^circ$. The latter is in conflict with the radio derived value of $(84pm0.7)^circ$. The average X-ray flux within hours-long B-mode intervals varied by a factor $pm$20%, possibly correlated with variations in the frequency and lengths of short radio nulls or short durations of weak emission. The correlated X-ray/radio moding of PSR B0823+26 is compared with the anti-correlated moding of PSR B0943+10, and the lack of X-ray moding of PSR B1822-09. We speculate that the X-ray/radio switches of PSR B0823+26 are due to variations in the rate of accretion of material from the interstellar medium through which it is passing.
We report on a detailed analysis of the radio emission during the different modes of the pulsar J0826+2637 (B0823+26), observed using the Giant Meterwave Radio Telescope at 306-339 MHz observing frequencies. The pulsar profile has a postcursor and interpulse emission in addition to the main pulse. The single pulses showed the presence of nulling, periodic fluctuation in the emission as well as two prominent modes. In addition the pulsar also showed the presence of a null state where no emission was seen for roughly an hour which was immediately followed by a short duration ($sim$5 minutes) bright state termed the Q-bright state. The nulling varied significantly in the two modes, from a few percent nulls in B-mode to more than 90 percent nulling during the Q-mode. Additionally, the pulsar showed the presence of low level emission in both the interpulse and postcursor components when the main pulse nulled in B-mode. We detected periodic fluctuations in both the main pulse and postcursor during B-mode which were most likely a form of periodic amplitude modulation unrelated to subpulse drifting. We have also detected the appearance of periodicity during the transitions from the null to the burst states in the Q-mode, which was longer than the B-mode modulations. Our analysis further revealed a significant increase in the main pulse and post-cursor intensity during the transition from the Q-mode to the short duration Q-bright mode. On the other hand no commensurate variation was visible in the interpulse intensity.
We present broadband, low-frequency (25-80 MHz and 110-190 MHz) LOFAR observations of PSR B0943+10, with the goal of better illuminating the nature of its enigmatic mode-switching behaviour. This pulsar shows two relatively stable states: a Bright (B) and Quiet (Q) mode, each with different characteristic brightness, profile morphology, and single-pulse properties. We model the average profile evolution both in frequency and time from the onset of each mode, and highlight the differences between the two modes. In both modes, the profile evolution can be well explained by radius-to-frequency mapping at altitudes within a few hundred kilometres of the stellar surface. If both B and Q-mode emission originate at the same magnetic latitude, then we find that the change of emission height between the modes is less than 6%. We also find that, during B-mode, the average profile is gradually shifting towards later spin phase and then resets its position at the next Q-to-B transition. The observed B-mode profile delay is frequency-independent (at least from 25-80 MHz) and asymptotically changes towards a stable value of about 0.004 in spin phase by the end of mode instance, much too large to be due to changing spin-down rate. Such a delay can be interpreted as a gradual movement of the emission cone against the pulsars direction of rotation, with different field lines being illuminated over time. Another interesting explanation is a possible variation of accelerating potential inside the polar gap. This explanation connects the observed profile delay to the gradually evolving subpulse drift rate, which depends on the gradient of the potential across the field lines.
The physical origin of radio emission in Radio Quiet Active Galactic Nuclei (RQ AGN) remains unclear, whether it is a downscaled version of the relativistic jets typical of Radio Loud (RL) AGN, or whether it originates from the accretion disk. The correlation between 5 GHz and X-ray luminosities of RQ AGN, which follows $L_R = 10^{-5}L_X$ observed also in stellar coronae, suggests an association of both X-ray and radio sources with the accretion disk corona. Observing RQ AGN at higher (mm-wave) frequencies, where synchrotron self absorption is diminished, and smaller regions can be probed, is key to exploring this association. Eight RQ AGN, selected based on their high X-ray brightness and variability, were observed at 95 GHz with the CARMA and ATCA telescopes. All targets were detected at the $1-10$ mJy level. Emission excess at 95~GHz of up to $times 7$ is found with respect to archival low-frequency steep spectra, suggesting a compact, optically-thick core superimposed on the more extended structures that dominate at low frequencies. Though unresolved, the 95 GHz fluxes imply optically thick source sizes of $10^{-4}-10^{-3}$ pc, or $sim 10 - 1000$ gravitational radii. The present sources lie tightly along an $L_R$ (95 GHz) = $10^{-4}L_X$ (2$-$10 keV) correlation, analogous to that of stellar coronae and RQ AGN at 5 GHz, while RL AGN are shown to have higher $L_R / L_X$ ratios. The present observations argue that simultaneous mm-wave and X-ray monitoring of RQ AGN features a promising method for understanding accretion disk coronal emission.
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