Steep Spectrum Research Articles

Cosmic Baby and the Mystery of Fast Radio Bursts

With the discovery of “Pulsar” a new probe of short duration radio pulses arises to the astronomers. Searching the “tomography” of the galaxy using the short-pulses offered a new method to measure the fairly accurate distance to the new sources of pulse radiation i.e. Pulsars. Another pulse radiative object i.e. fast radio burst (FRB) remains a mysterious object till today. Analysis of observed data indicates that FRBs have multi-source origin, variation in location of the burst generation (i.e. radial distance from their central compact objects, the Magnetar’s surface) depending on model to model. Two most important puzzle creating facts of FRBs are: (a) microstructure appearance in FRBs on time-scale of ≤ 10 μs; (b) No FRBs observed at low latitudes. Recoding of observed parameters from Cosmic Baby, i.e. Swift J1818.0 – 1607, shows that it has a steep spectra at first and then flat spectra later which indicates possibly be a transient object between normal pulsars and magnetars. This unusual characteristic of radio spectrum provides a clue to rethink about the magnetar origin of FRBS. i.e. Cosmic Baby can be a probe of resolving the mangetar originated FRB problem. The author thus encourages the radio wave burst community for continuous observation through advanced radio telescopes ALMA, VLA, CHIME, UTMOST with a hope that detection of unseen FRBs at low latitudes unveil the secrets of the Universe.

Radio jets in NGC 1068 with e-MERLIN and VLA: structure and morphology

ABSTRACT We present new high-sensitivity e-MERLIN and Very Large Array (VLA) radio images of the prototypical Seyfert 2 galaxy NGC 1068 at 5, 10, and 21 GHz. We image the radio jet, from the compact components north-east (NE), C, S1, and S2 to the faint double-lobed jet structure of the NE and south-west (SW) jet lobes. We map the jet between 15 kλ and 3300 kλ spatial scales by combining enhanced-Multi Element Radio Linked Interferometer Network (e-MERLIN) and VLA data for the first time. Components NE, C, and S2 have steep spectra indicative of optically thin non-thermal emission domination between 5 and 21 GHz. Component S1, which is where the active galactic nucleus resides, has a flat radio spectrum. We report a new component, S2a, a part of the southern jet. We compare these new data with the MERLIN and VLA data observed in 1983, 1992, and 1995 and report a flux decrease by a factor of 2 in component C, suggesting variability of this jet component. With the high angular resolution e-MERLIN maps, we detect the bow shocks in the NE jet lobe that coincide with the molecular gas outflows observed with ALMA. The NE jet lobe has a jet power of $P_{\rm jet-NElobe}\, =\,$ 6.7 × 1042 erg s−1 and is considered to be responsible for driving out the dense molecular gas observed with ALMA around the same region.

Particle acceleration by sub-proton cyclotron frequency spectrum of dispersive Alfven waves in inhomogeneous solar coronal plasmas

ABSTRACT The problem of explaining observed soft X-ray fluxes during solar flares, which invokes acceleration of large fraction of electrons, if the acceleration takes places at the solar coronal loop-top, can potentially be solved by postulating that flare at loop-top creates dispersive Alfven waves (DAWs) which propagate towards the foot-points. As DAWs move in progressively denser parts of the loop (due to gravitational stratification) the large fraction of electrons is no longer needed. Here, we extend our previous results by considering f−1 frequency spectrum of DAWs and add He++ ions using fully kinetic particle-in-cell (PIC) simulations. We consider cases when transverse density gradient is in the range 4–40c/ωpe and DAW driving frequency is 0.3–0.6ωcp. We find that (i) The frequency spectrum case does not affect electron acceleration fraction in the like-to-like cases, but few times larger percentage of He++ heating is seen due to ion cyclotron resonance; (ii) In cases when counter propagating DAWs collide multiple-times, much larger electron and ion acceleration fractions are found, but the process is intermittent in time. This is because intensive heating (temperature increase) makes the-above-thermal-fraction smaller; Also more isotropic velocity distributions are seen; (iii) Development of kink oscillations occurs when DAWs collide; (iv) Scaling of the magnetic fluctuations power spectrum steepening in the higher-density regions is seen, due to wave refraction. Our PIC runs produce much steeper slopes than the orginal spectrum, indicating that the electron-scale physics has a notable effect on DAW spectrum evolution.

THE JET KINETIC LUMINOSITIES FOR THE UTR-2 SOURCES WITH THE STEEP LOW-FREQUENCY SPECTRA

Earlier we have determined that the UTR-2 radio sources with steep linear spectra possess greater jet propagation velocity and lesser characteristic age than the UTR-2 radio sources with steep break spectra. Also examined galaxies and quasars with steep break spectra display greater mean values of the central black hole masses and mass accretion rates, than corresponding these for galaxies and quasars with steep linear spectra. Besides, as we have determined, the radio structure of the UTR-2 steep-spectrum sources is giant, its linear size has Mpc-scale. The source’s giant structure is formed by jets with enveloped radio lobes. So, this indicates on the powerful jets of the radio sources with the steep low-frequency spectra. Since the source jets are connected with the accretion disk of the source, it is important to examine relations of their physical characteristics. With this purpose we obtain estimates of the jet kinetic luminosity for the UTR-2 steep-spectrum sources on the assumption of the equality of the corresponding mass accretion rate and the jet matter flux. Using our calculated values of the jet propagation velocity and the mass accretion rate for the UTR-2 steep-spectrum galaxies and quasars, we estimate their jet kinetic luminosities. The obtained values of the jet kinetic luminosities are ~ 1045 erg/s, pointing out the great power of jets for the examined steep-spectrum sources. It is essential, that the examined objects display the relation of their kinetic luminosity and corresponding redshift (cosmological evolution).

A Search for Pulsars around Sgr A* in the First Event Horizon Telescope Data Set

In 2017 the Event Horizon Telescope (EHT) observed the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz (λ = 1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT data sets. The high observing frequency means that pulsars—which typically exhibit steep emission spectra—are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope, and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the fast folding algorithm, and single-pulse searches targeting both pulsars and burst-like transient emission. We use the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction (≲2.2%) of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region.

SAX J1810.8−2609: an outbursting neutron star X-ray binary with persistent spatially coincident radio emission

ABSTRACT Here, we report on joint X-ray and radio monitoring of the neutron star low-mass X-ray binary SAX J1810.8−2609. Our monitoring covered the entirety of its ${\sim }\, 5\,$ month outburst in 2021, revealing a temporal correlation between its radio and X-ray luminosity and X-ray spectral properties consistent with a ‘hard-only’ outburst. During the outburst, the best-fitting radio position shows significant variability, suggesting emission from multiple locations on the sky. Furthermore, our 2023 follow-up observations revealed a persistent, unresolved, steep spectrum radio source ${\sim }\, 2\,$ yr after SAX J1810.8−2609 returned to X-ray quiescence. We investigated potential origins of the persistent emission, which included an unrelated background source, long-lasting jet ejection(s), and SAX J1810 as a transitional millisecond pulsar. While the chance coincidence probability is low (${\lesssim }\, 0.16~{{\ \rm per\ cent}}$), an unrelated background source remains the most likely scenario. SAX J1810.8−2609 goes into outburst every ${\sim }\, 5\,$ yr, so monitoring of the source during its next outburst at higher sensitivities and improved spatial resolutions (e.g. with the Karl G. Jansky Very Large Array or Square Kilometre Array) should be able to identify two components (if the persistent emission originates from a background source). If only one source is observed, this would be strong evidence that the persistent emission is local SAX J1810.8−2609, and future monitoring campaigns should focus on understanding the underlying physical mechanisms, as no neutron star X-ray binary has shown a persistent radio signal absent any simultaneous X-ray emission.

G213.0−0.6, a true supernova remnant or just an H ii region?

Abstract G213.0−0.6 is a faint extended source situated in the anti-center region of the Galactic plane. It has been classified as a shell-type supernova remnant (SNR) based on its shell-like morphology, steep radio continuum spectrum, and high ratio of [H ii]/Hα. With new optical emission line data of Hα, [H ii], and [H ii] recently observed by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, the ratios of [H ii]/Hα and [H ii]/Hα are re-assessed. The lower values than those previously reported put G213.0−0.6 around the borderline of SNR-H ii region classification. We decompose the steep-spectrum synchrotron and the flat-spectrum thermal free-free emission in the area of G213.0−0.6 with multi-frequency radio continuum data. G213.0−0.6 is found to show a flat spectrum, in conflict with the properties of a shell-type SNR. Such a result is further confirmed by TT-plots made between the 863-MHz, 1.4-GHz, and 4.8-GHz data. Combining the evidence extracted in both optical and radio continuum, we argue that G213.0−0.6 is possibly not an SNR, but an H ii region instead. The VLSR pertaining to the Hα filaments places G213.0−0.6 approximately 1.9 kpc away in the Perseus Arm.

Damping of MHD turbulence in a partially ionized medium

ABSTRACT The coupling state between ions and neutrals in the interstellar medium plays a key role in the dynamics of magnetohydrodynamic (MHD) turbulence, but is challenging to study numerically. In this work, we investigate the damping of MHD turbulence in a partially ionized medium using 3D two-fluid (ions + neutrals) simulations generated with the athenak code. Specifically, we examine the velocity, density, and magnetic field statistics of the two-fluid MHD turbulence in different regimes of neutral-ion coupling. Our results demonstrate that when ions and neutrals are strongly coupled, the velocity statistics resemble those of single-fluid MHD turbulence. Both the velocity structures and kinetic energy spectra of ions and neutrals are similar, while their density structures can be significantly different. With an excess of small-scale sharp density fluctuations in ions, the density spectrum in ions is shallower than that of neutrals. When ions and neutrals are weakly coupled, the turbulence in ions is more severely damped due to the ion-neutral collisional friction than that in neutrals, resulting in a steep kinetic energy spectrum and density spectrum in ions compared to the Kolmogorov spectrum. We also find that the magnetic energy spectrum basically follows the shape of the kinetic energy spectrum of ions, irrespective of the coupling regime. In addition, we find large density fluctuations in ions and neutrals and thus spatially inhomogeneous ionization fractions. As a result, the neutral-ion decoupling and damping of MHD turbulence take place over a range of length-scales.

PG 1004+130: Hybrid Morphology Source or a Restarted FRII? A uGMRT Polarimetric Investigation

We present the polarization image of the hybrid morphology and broad absorption line quasar PG 1004+130 at 694 MHz obtained with the upgraded Giant Metrewave Radio Telescope. We detect linear polarization in this source’s core, jets, and lobes. The visible discontinuity in total intensity between the inner jets and the kiloparsec-scale lobes suggests that the source is restarted. The inferred poloidal magnetic (B-) field structure in the inner jet is consistent with that observed in Fanaroff–Riley (FR) type II sources, as are the B-fields aligned with the lobe edges. Moreover, archival Chandra and XMM-Newton data indicate that PG 1004+130 displays several FRII-jetlike properties in X-rays. We conclude that PG 1004+130 is a restarted quasar, with both episodes of activity being FRII type. The spectral index images show the presence of an inverted spectrum core (α = +0.30 ± 0.01) and a steep spectrum inner jet (α = −0.62 ± 0.06) surrounded by much steeper lobe emission (α ≈ −1.2 ± 0.1), consistent with the suggestion that the lobes are from a previous activity episode. The spectral age difference between the two activity episodes is likely to be small (<1.2 × 107 yr), in comparison to the lobe ages (∼3.3 × 107 yr). The inferred B-fields in the lobes are suggestive of turbulence and the mixing of plasma. This may account for the absence of X-ray cavities around this source, similar to what is observed in M87's radio halo region. The depolarization models reveal that thermal gas of mass ∼(2.4 ± 0.9) × 109 M ⊙ is mixed with the nonthermal plasma in the lobes of PG 1004+130.

Decomposition of Vertical Velocity and Its Zonal Wavenumber Kinetic Energy Spectra in the Hydrostatic Atmosphere

Abstract The spectrum of kinetic energy of vertical motions (VKE) is less well understood compared to the kinetic energy spectrum of horizontal motions (HKE). One challenge that has limited progress in describing the VKE spectrum is a lack of a unified approach to the decomposition of vertical velocities associated with the Rossby motions and inertia–gravity (IG) wave flows. This paper presents such a unified approach using a linear Rossby–IG vertical velocity normal-mode decomposition appropriate for a spherical, hydrostatic atmosphere. New theoretical developments show that for every zonal wavenumber k, the limit VKE is proportional to the total mechanical energy and to the square of the frequency of the normal mode. The theory predicts a VKE ∝ k−5 and a VKE ∝ k1/3 power law for the Rossby and IG waves, assuming a k−3 and a k−5/3 power law for the Rossby and IG HKE spectra, respectively. The Kelvin and mixed Rossby–gravity wave VKE spectra are predicted to follow k−1 and k−5 power laws, respectively. The VKE spectra for ERA5 data from August 2018 show that the Rossby VKE spectra approximately follow the predicted a k−5 power law. The expected k1/3 power law for the gravity wave VKE spectrum is found only in the SH midlatitude stratosphere for k ≈ 10–60. The inertial range IG VKE spectra in the tropical and midlatitude troposphere reflect a mixture of ageostrophic and convection-coupled dynamics and have slopes between −1 and −1/3, likely associated with too steep IG HKE spectra. The forcing by quasigeostrophic ageostrophic motions is seen as an IG VKE peak at synoptic scales in the SH upper troposphere, which gradually moves to planetary scales in the stratosphere. Significance Statement The spectrum of kinetic energy of vertical motions (VKE) is less well understood compared to the kinetic energy spectrum of horizontal motions. One challenge is a lack of a unified approach to the decomposition of vertical velocities associated with the Rossby motions and inertia–gravity (IG) wave flows. This paper presents such a unified approach using a linear Rossby–IG vertical velocity normal-mode decomposition appropriate for a spherical, hydrostatic atmosphere. It is shown that for every zonal wavenumber, the limit VKE is proportional to the total mechanical energy and to the square of the frequency of the normal mode. The theory is successfully applied to the ERA5 data. It leads the way for a more accurate computation of momentum fluxes.

Properties of the jet in M87 revealed by its helical structure imaged with the VLBA at 8 and 15 GHz

ABSTRACT We present full-track high-resolution radio observations of the jet of the galaxy M87 at 8 and 15 GHz. These observations were taken over three consecutive days in 2009 May using the Very Long Baseline Array (VLBA), one antenna of the Very Large Array (VLA), and the Effelsberg 100 m telescope. Our produced images have dynamic ranges exceeding 20 000:1 and resolve linear scales down to approximately 100 Schwarzschild radii, revealing a limb-brightened jet and a faint, steep spectrum counter-jet. We performed jet-to-counter-jet analysis, which helped estimate the physical parameters of the flow. The rich internal structure of the jet is dominated by three helical threads, likely produced by the Kelvin–Helmholtz (KH) instability developing in a supersonic flow with a Mach number of approximately 20 and an enthalpy ratio of around 0.3. We produce a clean imaging bias-corrected 8–15 GHz spectral index image, which shows spectrum flattening in regions of helical thread intersections. This further supports the KH origin of the observed internal structure of the jet. We detect polarized emission in the jet at distances of approximately 20 milliarcseconds from the core and find Faraday rotation which follows a transverse gradient across the jet. We apply Faraday rotation correction to the polarization position angle and find that the position angle changes as a function of distance from the jet axis, which suggests the presence of a helical magnetic field.

The Nature of the IMBH Candidate CXO J133815.6+043255: High-frequency Radio Emission

The ultraluminous X-ray source CXO J133815.6+043255 is a strong candidate for a bona fide intermediate-mass black hole residing in the outskirts of NGC 5252. We present 22 GHz radio observations of this source obtained serendipitously in an ongoing high-frequency imaging survey of radio-quiet Active Galactic Nuclei (AGNs), and use this new data point to construct the broadband radio spectral energy distribution (SED). We find that the SED exhibits a spectral slope of α = −0.66 ± 0.02, consistent with a steep spectrum from optically thin synchrotron emission from an unresolved jet. We also find that the L R/L X ratio is approximately 10−3, inconsistent with radio-quiet AGN and many ULXs but consistent with low-luminosity AGN and radio-loud quasars. Together, these observations support the conclusion that CXO J133815.6+043255 is an intermediate-mass black hole producing a low-mass analog of radio jets seen in classical quasars.

VLBI Observations of a sample of Palomar-Green quasars II: characterizing the parsec-scale radio emission

ABSTRACT This study uses multifrequency very long baseline interferometry (VLBI) to study the radio emission from 10 radio-quiet quasars (RQQs) and four radio-loud quasars (RLQs). The diverse morphologies, radio spectra, and brightness temperatures observed in the VLBI images of these RQQs, together with the variability in their GHz spectra and VLBI flux densities, shed light on the origins of their nuclear radio emission. The total radio emission of RQQs appears to originate from non-thermal synchrotron radiation due to a combination of active galactic nuclei and star formation activities. However, our data suggest that the VLBI-detected radio emission from these RQQs is primarily associated with compact jets or corona, with extended emissions such as star formation and large-scale jets being resolved by the high resolution of the VLBI images. Wind emission models are not in complete agreement the VLBI observations. Unlike RLQs, where the parsec-scale radio emission is dominated by a relativistically boosted core, the radio cores of RQQs are either not dominant or are mixed with significant jet emission. RQQs with compact cores or core-jet structures typically have more pronounced variability, with flat or inverted spectra, whereas jet-dominated RQQs have steep spectra and unremarkable variability. Future high-resolution observations of more RQQs could help to determine the fraction of different emission sources and their associated physical mechanisms.

Searching for Radio Outflows from M31* with VLBI Observations

As one of the nearest and most dormant supermassive black holes (SMBHs), M31* provides a rare but promising opportunity for studying the physics of black hole accretion and feedback at the quiescent state. Previous Karl G. Jansky Very Large Array (VLA) observations with an arcsecond resolution have detected M31* as a compact radio source over centimeter wavelengths, but the steep radio spectrum suggests optically thin synchrotron radiation from an outflow driven by a hot accretion flow onto the SMBH. Aiming to probe the putative radio outflow, we conducted milliarcsecond-resolution very long baseline interferometric (VLBI) observations of M31* in 2016, primarily at 5 GHz and combining the Very Long Baseline Array, Tianma 65 m, and Shanghai 25 m radio telescopes. Despite the unprecedented simultaneous resolution and sensitivity achieved, no significant (≳3σ) signal is detected at the putative position of M31* given an rms level of 5.9 μJy beam−1, thus ruling out a pointlike source with a peak flux density comparable to that (∼30 μJy beam−1) measured by the VLA observations taken in 2012. We disfavor the possibility that M31* has substantially faded since 2012, in view that a 2017 VLA observation successfully detected M31* at a historically high peak flux density (∼75 μJy beam−1 at 6 GHz). Instead, the nondetection of the VLBI observations is best interpreted as the arcsecond-scale core being resolved out at the milliarcsecond scale, suggesting an intrinsic size of M31* at 5 GHz larger than ∼300 times the Schwarzschild radius. Such extended radio emission may originate from a hot wind driven by the weakly accreting SMBH.

The Core Starbursts of the Galaxy NGC 3628: Radio Very Long Baseline Interferometry and X-Ray Studies

We present radio very long baseline interferometry (VLBI) and X-ray studies of the starburst galaxy NGC 3628. The VLBI observation at 1.5 GHz reveals seven compact (0.7−7 pc) radio sources in the central ∼250 pc region of NGC 3628. Based on their morphology, high radio-brightness temperatures (105–107 K), and steep radio spectra, none of these seven sources can be associated with active galactic nuclei (AGNs); instead, they can be identified as supernova remnants (SNRs), with three of them appearing consistent with partial shells. Notably, one of them (W2) is likely a nascent radio supernova (SN) and appears to be consistent with the star formation rate of NGC 3628 when assuming a canonical initial mass function. The VLBI observation provides the first precise measurement of the diameter of the radio sources in NGC 3628, which allow us to fit a well-constrained radio surface brightness—diameter (Σ–D) correlation by including the detected SNRs. Furthermore, future VLBI observations can be conducted to measure the expansion velocity of the detected SNRs. In addition to our radio VLBI study, we analyze Chandra and XMM-Newton spectra of NGC 3628. The spectral fitting indicates that the SNR activities could well account for the observed X-ray emissions. Along with the Chandra X-ray image, it further reveals that the X-ray emission is likely maintained by the galactic-scale outflow triggered by SN activities. These results provide strong evidence that SN-triggered activities play a critical role in generating both radio and X-ray emissions in NGC 3628.

The spectral index-flux density relation for extragalactic radio sources selected at metre and decametre wavelengths

We use the recent releases of sensitive VLA/LOFAR large-area surveys at 340 MHz and 54 MHz, in conjunction with the 1.4 GHz NVSS, to accurately determine the ‘spectral index-flux density relation’ (α − S) for extragalactic radio sources selected at metre and decametre wavelengths, with the latter explored for the first time. This newly determined α − S340 MHz relation shows a progressive flattening of αmedian towards lower flux densities, starting from its steepest value (peak) occurring near S340 MHz ∼ 1 − 2 Jy. This resolves the controversy extant in the literature since the 1980s. The α − S54 MHz relation also shows a spectral index flattening with decreasing flux density which is, however, significantly milder and the relation less sharply peaked than that found at 340 MHz. A possible reason for the difference could be that the 54 MHz sample has a distinctly stronger and more conspicuous presence ( at ∼20% level) of very steep spectrum sources having α541400 &lt; −1.3, most of which are probably associated with clusters of galaxies.

Compact Steep Spectrum Radio Sources with Enhanced Star Formation Are Smaller Than 10 kpc

Compact steep spectrum (CSS) radio sources are active galactic nuclei (AGN) that have radio jets propagating only on galactic scales, defined as having projected linear size (LS) of up to 20 kpc. CSS sources are generally hosted by massive early-type galaxies with little ongoing star formation; however, a small fraction are known to have enhanced star formation. Using archival data from the Faint Images of the Radio Sky at Twenty cm survey, the Very Large Array Sky Survey, and the Sloan Digital Sky Survey, we identify a volume-limited sample of 166 CSS sources at z < 0.2 with L 1.4 GHz > 1024 W Hz−1. Comparing the star formation rates and linear sizes of these CSS sources, we find that the ≈14% of CSS sources with specific star formation rates above 0.01 Gyr−1 all have LS < 10 kpc. We discuss the possible mechanisms driving this result, concluding that it is likely the excess star formation in these sources occurred in multiple bursts and ceased prior to the AGN jet being triggered.

HESS J1809−193: A halo of escaped electrons around a pulsar wind nebula?

Context. HESS J1809−193 is an unassociated very-high-energy γ-ray source located on the Galactic plane. While it has been connected to the nebula of the energetic pulsar PSR J1809−1917, supernova remnants and molecular clouds present in the vicinity also constitute possible associations. Recently, the detection of γ-ray emission up to energies of ∼100 TeV with the HAWC observatory has led to renewed interest in HESS J1809−193. Aims. We aim to understand the origin of the γ-ray emission of HESS J1809−193. Methods. We analysed 93.2 h of data taken on HESS J1809−193 above 0.27 TeV with the High Energy Stereoscopic System (H.E.S.S.), using a multi-component, three-dimensional likelihood analysis. In addition, we provide a new analysis of 12.5 yr of Fermi-LAT data above 1 GeV within the region of HESS J1809−193. The obtained results are interpreted in a time-dependent modelling framework. Results. For the first time, we were able to resolve the emission detected with H.E.S.S. into two components: an extended component (modelled as an elongated Gaussian with a 1-σ semi-major and semi-minor axis of ∼0.62° and ∼0.35°, respectively) that exhibits a spectral cutoff at ∼13 TeV, and a compact component (modelled as a symmetric Gaussian with a 1-σ radius of ∼0.1°) that is located close to PSR J1809−1917 and shows no clear spectral cutoff. The Fermi-LAT analysis also revealed extended γ-ray emission, on scales similar to that of the extended H.E.S.S. component. Conclusions. Our modelling indicates that based on its spectrum and spatial extent, the extended H.E.S.S. component is likely caused by inverse Compton emission from old electrons that form a halo around the pulsar wind nebula. The compact component could be connected to either the pulsar wind nebula or the supernova remnant and molecular clouds. Due to its comparatively steep spectrum, modelling the Fermi-LAT emission together with the H.E.S.S. components is not straightforward.

The Planck clusters in the LOFAR sky

Context. Diffuse cluster-scale synchrotron radio emission is discovered in an increasing number of galaxy clusters in the form of radio haloes, probing the presence of relativistic electrons and magnetic fields in the intra-cluster medium (ICM). The favoured scenario to explain their origin is that they trace turbulent regions that are generated during cluster-cluster mergers, where particles are re-accelerated. In this framework, radio haloes are expected to probe cluster dynamics and are predicted to be more frequent in massive systems, in which more energy becomes available for the re-acceleration of relativistic electrons. For these reasons, statistical studies of galaxy cluster samples have the power to derive fundamental information on the radio haloes populations and on their connection with cluster dynamics, and hence to constrain theoretical models. Furthermore, low-frequency cluster surveys have the potential to shed light on the existence of radio haloes with very steep radio spectra, which are a key prediction of turbulent models and are thought to be generated in less energetic merger events and thus be more common in the Universe. Aims. The main question we address is whether we can explain the observed properties of the radio halo population within the framework of current models. Methods. We study the occurrence and properties of radio haloes from clusters of the second catalogue of Planck Sunyaev Zel’dovich-detected sources that lie within the 5634 deg2 that are covered by the second data release (DR2) of the LOFAR Two-meter Sky Survey. We derive their integral number, flux density, and redshift distributions. We compare these observations with expectations of theoretical models. We also study the connection between radio haloes and cluster mergers by using cluster morphological parameters derived through Chandra and/or XMM-Newton data. Results. We find that the number of observed radio haloes, their radio flux density, and their redshift distributions agree with what is expected in the framework of the re-acceleration scenario. In line with model expectations, the fraction of clusters with radio haloes increases with the cluster mass, confirming the leading role of the gravitational process of cluster formation in the generation of radio haloes. These models predict a large fraction of radio haloes with very steep spectra in the DR2 Planck sample. This will be tested in future studies, but a comparison of the occurrence of haloes in GMRT and LOFAR samples indeed shows a more frequent occurrence of haloes at lower frequencies, suggesting the presence of a population of haloes with very steep spectra that is preferentially detected by LOFAR. Using morphological information, we confirm that radio haloes are preferentially located in merging systems, and that the fraction of newly LOFAR-discovered radio haloes is larger in less strongly disturbed systems.

Geoeffectiveness of Interplanetary Alfvén Waves. II. Spectral Characteristics and Geomagnetic Responses

Using multipoint observations over 10 yr near 1 au, we investigate the spectra (5 minutes to 2 hr) of interplanetary Alfvén waves and the responses in the geomagnetic activities. We compute the two-point correlations of the wave magnetic field between the ACE and the THEMIS spacecraft, which are separated by ∼200 Earth radius (R E) in the solar wind. Alfvén waves associated with high two-point correlations exhibit steep spectra (spectra index ∼−1.63). Such Alfvén waves occur mostly in slow-speed streams. By contrast, Alfvén waves with low two-point correlations exhibit flatter spectra (spectra index ∼−1.51) with a relative enhancement of power above 2 × 10−4 Hz. The occurrence of Alfvén waves with low two-point correlations is more equally distributed between high-speed and low-speed streams. In general, interplanetary Alfvén waves show correlations with moderate geomagnetic responses in symmetric ring-current intensity, SuperMAG electrojet (SME), and Kp indices. Statistical analyses indicate that the Alfvén waves with flat spectra correspond to stronger responses in the geomagnetic indices than those with steep spectra, suggesting the importance of the tens of minutes (30–90 minutes) Alfvénic power spectra in the generation of SME/Auroral Electrojets. These observations may shed light on the response of the magnetosphere to fluctuating interplanetary magnetic field B z .