D. W. Gardenier

Bio: D. W. Gardenier is an academic researcher from University of Amsterdam. The author has contributed to research in topics: Population & Fast radio burst. The author has an hindex of 7, co-authored 14 publications receiving 198 citations. Previous affiliations of D. W. Gardenier include ASTRON.

Inclination dependence of QPO phase lags in black hole X-ray binaries

Abstract: Quasi-periodic oscillations (QPOs) with frequencies from ∼0.05to30 Hz are a common feature in the X-ray emission of accreting black hole binaries. As the QPOs originate from the innermost accretion flow, they provide the opportunity to probe the behaviour of matter in extreme gravity. In this paper, we present a systematic analysis of the inclination dependence of phase lags associated with both type-B and type-C QPOs in a sample of 15 Galactic black hole binaries. We find that the phase lag at the type-C QPO frequency strongly depends on inclination, both in evolution with the QPO frequency and sign. Although we find that the type-B QPO soft lags are associated with high-inclination sources, the source sample is too small to confirm that this as a significant inclination dependence. These results are consistent with a geometrical origin of type-C QPOs and a different origin for type-B and type-C QPOs. We discuss the possibility that the phase lags originate from a pivoting spectral power law during each QPO cycle, while the inclination dependence arises from differences in dominant relativistic effects. We also search for energy dependences in the type-C QPO frequency. We confirm this effect in the three known sources (GRS 1915+105, H1743−322 and XTE J1550−564) and newly detect it in XTE J1859+226. Lastly, our results indicate that the unknown inclination sources XTE J1859+226 and MAXI J1543−564 are most consistent with a high inclination.

Repeating fast radio bursts with WSRT/Apertif

Abstract: Context. Repeating fast radio bursts (FRBs) present excellent opportunities to identify FRB progenitors and host environments as well as to decipher the underlying emission mechanism. Detailed studies of repeating FRBs might also hold clues as to the origin of FRBs as a population.Aims. We aim to detect bursts from the first two repeating FRBs, FRB 121102 (R1) and FRB 180814.J0422+73 (R2), and to characterise their repeat statistics. We also want to significantly improve the sky localisation of R2 and identify its host galaxy.Methods. We used the Westerbork Synthesis Radio Telescope to conduct extensive follow-up of these two repeating FRBs. The new phased-array feed system, Apertif, allows one to cover the entire sky position uncertainty of R2 with fine spatial resolution in a single pointing. The data were searched for bursts around the known dispersion measures of the two sources. We characterise the energy distribution and the clustering of detected R1 bursts.Results. We detected 30 bursts from R1. The non-Poissonian nature is clearly evident from the burst arrival times, which is consistent with earlier claims. Our measurements indicate a dispersion measure (DM) of 563.5(2) pc cm−3 , suggesting a significant increase in DM over the past few years. Assuming a constant position angle across the burst, we place an upper limit of 8% on the linear polarisation fraction for the brightest burst in our sample. We did not detect any bursts from R2.Conclusions. A single power-law might not fit the R1 burst energy distribution across the full energy range or widely separated detections. Our observations provide improved constraints on the clustering of R1 bursts. Our stringent upper limits on the linear polarisation fraction imply a significant depolarisation, either intrinsic to the emission mechanism or caused by the intervening medium at 1400 MHz, which is not observed at higher frequencies. The non-detection of any bursts from R2, despite nearly 300 h of observations, implies either a highly clustered nature of the bursts, a steep spectral index, or a combination of the two assuming that the source is still active. Another possibility is that R2 has turned off completely, either permanently or for an extended period of time.

Repeating fast radio bursts with WSRT/Apertif

Abstract: Repeating fast radio bursts (FRBs) present excellent opportunities to identify FRB progenitors and host environments, as well as decipher the underlying emission mechanism. Detailed studies of repeating FRBs might also hold clues to the origin of FRBs as a population. We aim to detect the first two repeating FRBs: FRB 121102 (R1) and FRB 180814.J0422+73 (R2), and characterise their repeat statistics. We also want to significantly improve the sky localisation of R2. We use the Westerbork Synthesis Radio Telescope to conduct extensive follow-up of these two repeating FRBs. The new phased-array feed system, Apertif, allows covering the entire sky position uncertainty of R2 with fine spatial resolution in one pointing. We characterise the energy distribution and the clustering of detected R1 bursts. We detected 30 bursts from R1. Our measurements indicate a dispersion measure of 563.5(2) pc cm$^{-3}$, suggesting a significant increase in DM over the past few years. We place an upper limit of 8% on the linear polarisation fraction of the brightest burst. We did not detect any bursts from R2. A single power-law might not fit the R1 burst energy distribution across the full energy range or widely separated detections. Our observations provide improved constraints on the clustering of R1 bursts. Our stringent upper limits on the linear polarisation fraction imply a significant depolarisation, either intrinsic to the emission mechanism or caused by the intervening medium, at 1400 MHz that is not observed at higher frequencies. The non-detection of any bursts from R2 implies either a highly clustered nature of the bursts, a steep spectral index, or a combination of both. Alternatively, R2 has turned off completely, either permanently or for an extended period of time.

A bright, high rotation-measure FRB that skewers the M33 halo

Abstract: We report the detection of a bright fast radio burst, FRB 191108, with Apertif on the Westerbork Synthesis Radio Telescope. The interferometer allows us to localize the FRB to a narrow 5 arcsec × 7 arcmin ellipse by employing both multibeam information within the Apertif phased-array feed beam pattern, and across different tied-array beams. The resulting sightline passes close to Local Group galaxy M33, with an impact parameter of only 18 kpc with respect to the core. It also traverses the much larger circumgalactic medium (CGM) of M31, the Andromeda Galaxy. We find that the shared plasma of the Local Group galaxies could contribute ∼10 per cent of its dispersion measure of 588 pc cm-3. FRB 191108 has a Faraday rotation measure (RM) of +474 $\pm \, 3$ rad m-2, which is too large to be explained by either the Milky Way or the intergalactic medium. Based on the more moderate RMs of other extragalactic sources that traverse the halo of M33, we conclude that the dense magnetized plasma resides in the host galaxy. The FRB exhibits frequency structure on two scales, one that is consistent with quenched Galactic scintillation and broader spectral structure with Δν ≍ 40 MHz. If the latter is due to scattering in the shared M33/M31 CGM, our results constrain the Local Group plasma environment. We found no accompanying persistent radio sources in the Apertif imaging survey data.

Beaming as an explanation of the repetition/width relation in FRBs

Abstract: It is currently not known if repeating fast radio bursts (FRBs) are fundamentally different from those that have not been seen to repeat. One striking difference between repeaters and apparent non-repeaters in the CHIME sample is that the once-off events are typically shorter in duration than sources that have been detected two or more times. We offer a simple explanation for this discrepancy based on a selection effect due to beamed emission, in which highly-beamed FRBs are less easily observed to repeat, but are abundant enough to detect often as once-off events. The explanation predicts that there is a continuous distribution of burst duration---not a static bimodal one---with a correlation between repetition rate and width. Pulse width and opening angle may be related by relativistic effects in shocks, where short-duration bursts have small solid angles due to a large common Lorentz factor. Alternatively, the relationship could be a geometric effect where narrow beams sweep past the observer more quickly, as with pulsars. Our model has implications for the FRB emission mechanism and energy scale, volumetric event rates, and the application of FRBs to cosmology.

Possible periodic activity in the repeating FRB 121102

Abstract: The discovery that at least some Fast Radio Bursts (FRBs) repeat has ruled out cataclysmic events as the progenitors of these particular bursts. FRB~121102 is the most well-studied repeating FRB but despite extensive monitoring of the source, no underlying pattern in the repetition has previously been identified. Here, we present the results from a radio monitoring campaign of FRB~121102 using the 76-m Lovell telescope. Using the pulses detected in the Lovell data along with pulses from the literature, we report a detection of periodic behaviour of the source over the span of five years of data. We predict that the source is currently `off' and that it should turn `on' for the approximate MJD range $59002-59089$ (2020-06-02 to 2020-08-28). This result, along with the recent detection of periodicity from another repeating FRB, highlights the need for long-term monitoring of repeating FRBs at a high cadence. Using simulations, we show that one needs at least 100 hours of telescope time to follow-up repeating FRBs at a cadence of 0.5--3 days to detect periodicities in the range of 10--150 days. If the period is real, it shows that repeating FRBs can have a large range in their activity periods that might be difficult to reconcile with neutron star precession models.

Repeating behaviour of FRB 121102: periodicity, waiting times, and energy distribution

Abstract: Detections from the repeating fast radio burst FRB 121102 are clustered in time, noticeable even in the earliest repeat bursts. Recently, it was argued that the source activity is periodic, suggesting that the clustering reflected a not-yet-identified periodicity. We performed an extensive multi-wavelength campaign with the Effelsberg telescope, the Green Bank telescope and the Arecibo Observatory to shadow the Gran Telescope Canaria (optical), NuSTAR (X-ray) and INTEGRAL (gamma-ray). We detected 36 bursts with Effelsberg, one with a pulse width of 39\,ms, the widest burst ever detected from FRB 121102. With one burst detected during simultaneous NuSTAR observations, we place a 5-$\sigma$ upper limit of $5\times10^{47}$ erg on the 3--79\,keV energy of an X-ray burst counterpart. We tested the periodicity hypothesis using 165-hr of Effelsberg observations and find a periodicity of 161$\pm$5 days. We predict the source to be active from 2020-07-09 to 2020-10-14 and subsequently from 2020-12-17 to 2021-03-24. We compare the wait times between consecutive bursts within a single observation to Weibull and Poisson distributions. We conclude that the strong clustering was indeed a consequence of a periodic activity and show that if the few events with millisecond separation are excluded, the arrival times are Poisson distributed. We model the bursts' cumulative energy distribution with energies from ${\sim}10^{38}$-$10^{39}$ erg and find that it is well described by a power-law with slope of $\gamma=-1.1\pm 0.2$. We propose that a single power-law might be a poor descriptor of the data over many orders of magnitude.

Spectral States of XTE J1701-462: Link between Z and Atoll Sources

Abstract: We have analyzed 866 Rossi X-ray Timing Explorer observations of the 2006-2007 outburst of the accreting neutron star XTE J1701 –462, during which the source evolves from super-Eddington luminosities to quiescence. The X-ray color evolution first resembles the Cyg X-2 subgroup of Z sources, with frequent excursions on the horizontal and normal branches (HB/NB). The source then decays and evolves to the Sco X-1 subgroup, with increasing focus on the flaring branch (FB) and the lower vertex of the "Z." Finally, the FB subsides, and the source transforms into an atoll source, with the lower vertex evolving to the atoll soft state. Spectral analyses suggest that the atoll stage is characterized by a constant inner disk radius, while the Z stages exhibit a luminosity-dependent expansion of the inner disk, which we interpret as effects related to the local Eddington limit. Contrary to the view that the mass accretion rate changes along the Z, we find that changes in are instead responsible for the secular evolution of the Z and the subclasses. Motion along the Z branches appears to be caused by three different mechanisms that may operate at roughly constant . For the Sco X-1-like Z stage, we find that the FB is an instability track that proceeds off the lower vertex when the inner disk radius shrinks from the value set by the X-ray luminosity toward the value measured for the atoll soft state. Excursions up the NB occur when the apparent size of the boundary layer increases while the disk exhibits little change. The HB is associated with Comptonization of the disk emission. The Z branches for the Cyg X-2-like stage are more complicated, and their origin is unclear. Finally, our spectral results lead us to hypothesize that the lower and upper Z vertices correspond to a standard thin disk and a slim disk, respectively.