Swift XRT Observations Research Articles

Swift-XRT follow-up analysis of unidentified hard X-ray sources

Context. It has been established that the sources contributing to the cosmic X-ray background (CXB) emission are mainly nearby active galactic nuclei (AGNs), in particular those that are obscured. Thus, it is important to fully identify the hard X-ray sky source population to accurately characterize the individual contribution of different AGNs to the overall CXB emission. Aims. We present a follow-up analysis of all 218 sources marked as unidentified in our previous revision of the third release of the Palermo Swift-BAT hard X-ray catalog (3PBC) based on our multifrequency classification scheme. These 218 sources were classified as unidentified in our previous analyses because they lacked an assigned low-energy counterpart. Methods. We searched for soft X-ray counterparts of these 218 3PBC sources in archival Swift-XRT observations obtained between January 1, 2005, and August 1, 2018. In particular, we found 1213 archival Swift-XRT observations for 192 of the 218 unidentified sources. Results. We find 93 possible Swift-XRT counterparts within the Swift-BAT positional uncertainty regions. They correspond to 73 3PBC sources, 60 of which have only a single Swift-XRT detection; the rest have multiple detections. We present all the detected possible counterparts of the as-of-yet-unidentified hard X-ray sources to the community as a catalog for future spectroscopic follow-up targets, together with a short catalog of our classification of the ten sources for which there were available spectra.

Origin of the Very High Energy Gamma Rays in the Low-luminosity Active Galactic Nucleus NGC 4278

NGC 4278, a low-luminosity active galactic nucleus, is generally classified as a low-ionization nuclear emission-line region (LINER). Recently, it has been reported to be associated with a very high energy γ-ray source 1LHAASO J1219+2915 in the first Large High Altitude Air Shower Observatory source catalog. However, no associated counterpart has been detected by analyzing the data collected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. By analyzing its X-ray observation data from Swift-XRT, we find that NGC 4278 is in a high-flux state on MJD 59546, with the X-ray flux more than one order of magnitude higher than that observed ∼11.7 yr earlier by Chandra. Interestingly, this Swift-XRT observation was conducted during the active phase of the γ-ray source 1LHAASO J1219+2915. We propose that the detection of very high energy γ-rays from NGC 4278 may be attributed to the presence of an active nucleus in its center. To reproduce the spectral energy distribution (SED) of NGC 4278, we employ a one-zone leptonic model, typically used for fitting broadband SEDs of BL Lacs, and find that a smaller magnetic field strength is required than that of typical TeV BL Lacs. Furthermore, NGC 4278 exhibits significantly lower luminosity in both radio and TeV bands when compared with typical TeV BL Lacs. In the radio luminosity versus Eddington ratio plane, NGC 4278 shows greater similarity to Seyfert galaxies and LINERs than to BL Lacs; however, it still roughly follows the extension toward lower luminosity seen in BL Lacs.

The more the merrier: SRG/eROSITA discovers two further galaxies showing X-ray quasi-periodic eruptions

X-ray quasi-periodic eruptions (QPEs) are a novel addition to the group of extragalactic transients. With only a select number of known sources, and many more models published trying to explain them, we are so far limited in our understanding by small number statistics. In this work, we report the discovery of two further galaxies showing QPEs, hereafter named eRO-QPE3 and eRO-QPE4, with the eROSITA X-ray telescope on board the Spectrum Roentgen Gamma observatory, followed by XMM-Newton, NICER, Swift-XRT, SALT (z = 0.024 and z = 0.044, respectively), and ATCA observations. Among the properties in common with those of known QPEs are: the thermal-like spectral shape in eruption (up to kT ∼ 110 − 120 eV) and quiescence (kT ∼ 50 − 90 eV) and its evolution during the eruptions (with a harder rise than decay); the lack of strong canonical signatures of active nuclei (from current optical, UV, infrared and radio data); and the low-mass nature of the host galaxies (logM* ≈ 9 − 10) and their massive central black holes (logMBH ≈ 5 − 7). These discoveries also bring several new insights into the QPE population: (i) eRO-QPE3 shows eruptions on top of a decaying quiescence flux, providing further evidence for a connection between QPEs and a preceding tidal disruption event; (ii) eRO-QPE3 exhibits the longest recurrence times and faintest peak luminosity of QPEs, compared to the known QPE population, excluding a correlation between the two; (iii) we find evidence, for the first time, of a transient component that is harder, albeit much fainter, than the thermal QPE spectrum in eRO-QPE4; and (iv) eRO-QPE4 displays the appearance (or significant brightening) of the quiescence disk component after the detection of QPEs, supporting its short-lived nature against a preexisting active galactic nucleus. These new properties further highlight the need to find additional QPE sources to increase the sample size and draw meaningful conclusions about the intrinsic population. Overall, the newly discovered properties (e.g., recent origin and/or transient nature of the quiescent accretion disk; lack of correlation between eruption recurrence timescales and luminosity) are qualitatively consistent with recent models that identify QPEs as extreme mass-ratio inspirals.

Swift Follow-up of Reported Radio Pulsars at Fermi 4FGL Unassociated Sources

Following the discovery of radio pulsars at the position of Fermi-LAT unassociated sources by the TRAPUM group, we conduct Swift-XRT observations of six of those 4FGL sources to determine if any pulsar-like X-ray sources are present and to confirm the reported detection of an X-ray counterpart via eROSITA at 4FGL J1803.1−6708. At two of the six targets, we detect no X-ray sources at the TRAPUM radio position, placing an upper limit on the 0.3–10.0 keV flux. At 4FGL J1803.1−6708 we find an X-ray source at the TRAPUM and eROSITA position. At 4FGL J1858.3−5424 we find a new X-ray counterpart at the TRAPUM position with signal-to-noise ratio S/N = 4.17, but also detect a distinct and separate X-ray source. At 4FGL J1823.8−3544 and 4FGL J1906.4−1757 we detect no X-ray flux at the TRAPUM positions, but we do detect separate X-ray sources elsewhere in the Fermi error ellipse. At these last two targets, our newly detected Swift sources are possible alternatives to the radio pulsar associations proposed by TRAPUM. Our findings confirm several of the discoveries reported by the TRAPUM group but suggest that further observations and investigations are necessary to confirm the low-energy counterpart of several unassociated sources.

Broad-band X-ray properties of black holes GRS 1758−258 and 1E 1740.7−2942: AstroSat and NuSTAR results

ABSTRACT We present the results on broad-band X-ray properties of persistent black hole binaries GRS 1758−258 and 1E 1740.7−2942 using AstroSat, NuSTAR, and Swift–XRT observations carried out during 2016–2022. We perform spectral modelling of both sources after eliminating the contamination in their LAXPC spectra from nearby X-ray sources. Preliminary spectral modelling using Comptonization and line emission (∼6.4 keV) models suggest that GRS 1758−258 occupies both dim-soft state (kTbb = 0.37 ± 0.01 keV, Γ ∼ 5.9, $L_{bol}=1{{\ \rm per\ cent}}$ of Eddington luminosity LEdd) and hard state (Γ = 1.64–2.22, kTe = 4–45 keV, Lbol = 1–5 per cent LEdd) that requires a multicolour disc blackbody model (kTin = 0.54 ± 0.01 keV) occasionally. 1E 1740.7−2942 instead is found only in hard state (Γ = 1.67–2.32, kTe = 5–16 keV, Lbol = 1–2 per cent LEdd). Reflection properties of both sources are studied by applying relativistic reflection model RELXILL to the broad-band spectra. Our results from AstroSat and NuSTAR consistently unveiled the presence of a Comptonizing region along with an ionized reflection region (ionization parameter log ξ = 2.7–3.8 and 2.7–4.7 erg cm s−1 in GRS 1758−258 and 1E 1740.7−2942, respectively) in both sources. Reflection modelling revealed GRS 1758−258 to have a high metal abundance ($A_{fe}=3.9^{+0.4}_{-0.3}$ times solar metal abundance) and inclination angle (i) of 61 ± 2°. In case of 1E 1740.7−2942, i is constrained to be 55 ± 1°. Finally, we discuss the implication of our findings in the context of accretion dynamics by comparing our results with the previous studies.

Correlations between X-ray spectral parameters of Mkn 421 using long-term Swift–XRT data

ABSTRACT We have performed a detailed analysis of the X-ray spectra of the blazar Mkn 421 using Swift–XRT observations taken between 2005 and 2020, to quantify the correlations between spectral parameters for different models. In an earlier work, it has been shown that such spectral parameter correlations obtained from a single short flare of duration ∼5 d of Mkn 421, can be used to distinguish spectrally degenerate models and provide estimates of physical quantities. In this work, we show that the results from the long-term spectral parameter correlations are consistent with those obtained from the single flare. In particular, that the observed spectral curvature is due to maximum cut-off energy in the particle distribution is ruled out. Instead, models where the curvature is due to the energy dependence of escape or acceleration time-scale of the particles are favoured. The estimated values of the physical parameters for these models are similar to the ones obtained from the single flare analysis and are somewhat incompatible with the physical assumption of the models, suggesting that more complex physical models are required. The consistency of the results obtained from the long- and short-term evolution of the source, underlines the reliability of the technique to use spectral parameter correlations to distinguish physical models.

X-Ray Imaging Observations of the High-mass γ-Ray Binary HESS J0632+057

Abstract The Chandra X-Ray Observatory (CXO) imaged the high-mass γ-ray binary HESS J0632+057 with the Advanced CCD Imaging Spectrometer (ACIS). We analyzed the CXO data together with 967 ks of archival Swift-XRT observations. On arcsecond scales we find a hint of asymmetric extended emission. On arcminute scales, a region of extended emission (“the blob”), located ≈ 5 .′ 5 east of the binary, is seen in both the CXO-ACIS and the Swift-XRT images. The blob does not have a counterpart in the radio, near-IR, IR, or optical images. The ACIS spectrum of the blob fits either an absorbed power-law model with Γ ≃ 2.6, or a thermal plasma model with kT ≃ 3 keV. Since the blob’s N H is significantly higher than that of the binary we conclude that the blob and binary are not directly related. The somewhat larger, very deep XRT image suggests that the binary may be located within a shell (or cavity). The four ACIS spectra taken within the ∼20 day interval near the light-curve minimum suggest that N H varies on timescales of days, possibly due to the inhomogeneous circumbinary environment. The XRT spectra extracted from wider orbital phase intervals support significant changes in N H near the light-curve maximum/minimum, which may be responsible for the substantial systematic residuals seen near 1 keV, and provide tentative evidence for an Fe line at 6.4 keV. We find no significant periodic signal in the ACIS data up to 0.156 Hz.

Revisiting the X-ray emission of the asynchronous polar V1432 Aql

As the only eclipsing asynchronous polar, V1432 Aql provides an excellent laboratory to study the interaction between the accreted matter and the magnetic field. Here, we report an analysis of the X-ray data from the contemporaneous NuSTAR and Swift-XRT observations. The X-ray data present a profile with a low-intensity state for almost half an orbital period, a dip at 0.6 phase, and a peak at 0.75 phase, which suggests that there was only one accretion region during the observation and the claim is supported by the spectral analysis. The comparison with the previous data indicates that the X-ray data have an orbital modulation, as the case in BeppoSAX, rather than a spin one observed in ROSAT. We attribute the orbit and spin modulations to the different accretion geometries at work. The spectral analysis of the wide-band data presents a significant reflection effect, a commonly observed soft X-ray temperature, and the energy balance in V1432 Aql. Additionally, we obtained a low total accretion rate of 1.3 × 10−10 M ⊙ yr−1 and a high specific accretion rate of 3.8 g cm−2 s−1 which explains the strong reflection from the surface of the white dwarf. However, due to its complex emission, a more physical understanding of its accretion geometry is still outstanding.

X-Rays from V723 Mon are due to Optical Loading in Swift XRT

Abstract V723 Mon was recently reported to be hosting the nearest (d = 460 pc) candidate black hole by Jayasinghe et al. An X-ray detection was claimed based on deep Swift XRT observations with a luminosity of L X ≈ 8 × 1029 erg s−1 or L X/L Edd ≈ 10−9. Here we show that the X-ray photons detected by Swift XRT are due to optical loading from the bright (V = 8.3 mag) star. Using events in the 2–10 keV energy band, where optical loading does not strongly contribute, we find a 3σ upper-limit on the unabsorbed 2–10 keV X-ray flux of F X < 2.8 × 10−14 erg cm−2 s−1, corresponding to a luminosity L X < 7.2 × 1029 erg s−1 at a distance of 460 pc or L X/L Edd ≲ 2 × 10−9 for a M ≈ 3M ⊙ black hole.

Minute-timescale Variability in the X-ray Emission of the Highest Redshift Blazar*

Abstract We report on two Chandra observations of the quasar PSO J0309+27, the most distant blazar observed so far (z = 6.1), performed eight months apart, in 2020 March and November. Previous Swift-XRT observations showed that this object is one of the brightest X-ray sources beyond redshift 6.0 ever observed so far. This new dataset confirmed the high flux level and unveiled a spectral change that occurred on a very short timescale (250 s rest frame), caused by a significant softening of the emission spectrum. This kind of spectral variability, on such a short interval, has never been reported in the X-ray emission of a flat-spectrum radio quasar. A possible explanation for this is given by the emission produced by the inverse Compton scatter of the quasar UV photons by the cold electrons present in a fast shell moving along the jet. Although this bulk Comptonization emission should be an unavoidable consequence of the standard leptonic jet model, this would be the first time that it has been observed.

NICER, NuSTAR, and Swift follow-up observations of the γ-ray flaring blazar BL Lacertae in 2020 August–October

ABSTRACT During a period of strong γ-ray flaring activity from BL Lacertae, we organized Swift, Neutron star Interior Composition Explorer (NICER), and Nuclear Spectroscopic Telescope Array (NuSTAR) follow-up observations. The source has been monitored by Swift-XRT (X-ray Telescope) between 2020 August 11 and October 16, showing a variability amplitude of 65, with a flux varying between 1.0 × 10−11 and 65.3 × 10−11 erg cm−2 s−1. On 2020 October 6, Swift-XRT has observed the source during its historical maximum X-ray flux. A softer-when-brighter behaviour has been observed by XRT, suggesting an increasing importance of the synchrotron emission in the X-ray part of the spectrum covered by XRT during this bright state. Rapid variability in soft X-rays has been observed with both the Swift-XRT and NICER observations with a minimum variability time-scale of 60 and 240 s, and a doubling time-scale of 274 and 1008 s, respectively, suggesting very compact emitting regions (1.1 × 1014 and 4.0 × 1014 cm). At hard X-rays, a minimum variability time-scale of ∼5.5 ks has been observed by NuSTAR. We report the first simultaneous NICER and NuSTAR observations of BL Lacertae during 2020 October 11–12. The joint NICER and NuSTAR spectra are well fitted by a broken power law with a significant difference of the photon index below (2.10) and above (1.60) an energy break at ∼2.7 keV, indicating the presence of two different emission components (i.e. synchrotron and inverse Compton) in the broad-band X-ray spectrum. Leaving the total hydrogen column density towards BL Lacertae free to vary, a value of NH,tot = (2.58 ± 0.09) × 1021 cm−2 has been estimated.

Unfolding the X-ray spectral curvature of Mkn 421 for further clues

ABSTRACT The X-ray observations of Mkn 421 show significant spectral curvature that can be reproduced by a log-parabola function. The spectra can also be fitted by an analytical model considering synchrotron emission from an electron distribution that is accelerated at a shock front with an energy-dependent diffusion (EDD model). The spectral fit of NuSTAR and Swift-XRT observations using EDD model during different flux states reveals that the model parameters are strongly correlated. We perform a detailed investigation of this correlation to decipher the information hidden underneath. The model predicts the synchrotron peak energy to be correlated with the peak spectral curvature, which is consistent with the case of Mkn 421. Expressing the energy dependence of the diffusion in terms of the magnetohydrodynamic turbulence energy index, it appears that the turbulence shifts from Kolmogorov/Kraichnan type to Bohm limit during high flux states. Further, the correlation between the best-fitting parameters of EDD model lets us derive an expression for the product of source magnetic field(B) and jet Doppler factor(δ) in terms of synchrotron and Compton peak energies. The synchrotron peak energy is obtained using the simultaneous Swift-XRT–NuSTAR observations, whereas the Compton peak energy is estimated by performing a linear regression analysis of the archival spectral peaks. The deduced δB varies over a wide range; however, it satisfies reasonably well with the values estimated solely from the spectral peak energies independent of the EDD model. This highlights the plausible connection between the microscopic description of the electron diffusion with the macroscopic quantities deciding the broad-band spectrum of Mkn 421.

Swift-XRT follow-up of gravitational wave triggers during the third aLIGO/Virgo observing run

ABSTRACT The Neil Gehrels Swift Observatory followed up 18 gravitational wave (GW) triggers from the LIGO/Virgo collaboration during the O3 observing run in 2019/2020, performing approximately 6500 pointings in total. Of these events, four were finally classified (if real) as binary black hole (BH) triggers, six as binary neutron star (NS) events, two each of NSBH and Mass Gap triggers, one an unmodelled (Burst) trigger, and the remaining three were subsequently retracted. Thus far, four of these O3 triggers have been formally confirmed as real gravitational wave events. While no likely electromagnetic counterparts to any of these GW events have been identified in the X-ray data (to an average upper limit of 3.60 × 10−12 erg cm−2 s−1 over 0.3–10 keV), or at other wavelengths, we present a summary of all the Swift-XRT observations performed during O3, together with typical upper limits for each trigger observed. The majority of X-ray sources detected during O3 were previously uncatalogued; while some of these will be new (transient) sources, others are simply too faint to have been detected by earlier survey missions such as ROSAT. The all-sky survey currently being performed by eROSITA will be a very useful comparison for future observing runs, reducing the number of apparent candidate X-ray counterparts by up to 95 per cent.

High-latitude emission from the structured jet of γ-ray bursts observed off-axis

The X-ray emission of γ-ray burst (GRBs) is often characterized by an initial steep decay followed by a nearly constant emission phase (so-called “plateau”) which can extend up to thousands of seconds. While the steep decay is usually interpreted as the tail of the prompt γ-ray flash, the long-lasting plateau is commonly associated to the emission from the external shock sustained by energy injection from a long-lasting central engine. A recent study proposed an alternative interpretation, ascribing both the steep decay and the plateau to high-latitude emission (HLE) from a “structured jet” whose energy and bulk Lorentz factor depend on the angular distance from the jet symmetry axis. In this work we expand on this idea and explore more realistic conditions: (a) the finite duration of the prompt emission, (b) the angular dependence of the optical depth, and (c) the dependence of the light curve on the observer viewing angle. We find that, when viewed highly off-axis, the structured jet HLE light curve is smoothly decaying with no clear distinction between the steep and flat phases, as opposed to the on-axis case. For a realistic choice of physical parameters, the effects of a latitude-dependent Thomson opacity and finite duration of the emission have a marginal effect on the overall light-curve evolution. We discuss the possible HLE of GW170817, showing that the emission would have faded away long before the first Swift-XRT observations. Finally, we discuss the prospects for the detection of HLE from off-axis GRBs by present and future wide-field X-ray telescopes and X-ray surveys, such as eROSITA and the mission concept THESEUS.

Multiwavelength study of high-redshift blazars

ABSTRACT High-redshift blazars are among the most powerful objects in the Universe. The spectral and temporal properties of 33 distant blazars (z > 2.5) detected in the high-energy γ-ray band are investigated by analysing the Fermi-LAT and Swift Ultraviolet and Optical Telescope/X-ray Telescope (UVOT/XRT) data. The considered sources have soft time-averaged γ-ray spectra (Γγ ≥ 2.2) whereas those that have been observed in the X-ray band have hard X-ray spectra (ΓX = 1.01−1.86). The γ-ray flux of high-redshift blazars ranges from 4.84 × 10−10 to 1.50 × 10−7 photon cm−2 s−1 and the luminosity is within (0.10−5.54) × 1048 erg s−1 which during the γ-ray flares increases up to (0.1−1) × 1050 erg s−1. In the X-ray band, only the emission of PKS 0438−43, B2 0743+25, and TXS 0222+185 is found to vary in different Swift XRT observations whereas in the γ-ray band, the emission is variable for fourteen sources: the flux of B3 1343+451 and PKS 0537−286 changes in sub-day scales, that of PKS 0347−211 and PKS 0451−28 in day scales, while the γ-ray variability of the others is in week or month scales. The properties of distant blazar jets are derived by modelling the multiwavelength spectral energy distributions within a one-zone leptonic scenario assuming that the X-ray and γ-ray emissions are produced from inverse Compton scattering of synchrotron and dusty torus photons. From the fitting, the emission region size is found to be ≤0.05 pc and the magnetic field and the Doppler factor are correspondingly within 0.10−1.74 G and 10.0−27.4. By modelling the optical–UV excess, we found that the central black hole masses and accretion disc luminosities are within Ld ≃ (1.09−10.94) × 1046 erg s−1 and (1.69−5.35) × 109 M⊙, respectively.

Multi-wavelength observations of the BL Lac object Fermi J1544-0649: One year after its awakening

We report observations of a transient source FermiJ1544-0649 from radio to γ-rays. FermiJ1544-0649 was discovered by the Fermi-LAT in May 2017. Follow-up Swift-XRT observations revealed three flaring episodes through March 2018, and the peak X-ray flux is about 103 higher than the ROSAT all-sky survey (RASS) flux upper limit. Optical spectral measurements taken by the Magellan 6.5-m telescope and the Lick-Shane telescope both show a largely featureless spectrum, strengthening the BL Lac interpretation first proposed by Bruni et al. (2018). The optical and mid-infrared (MIR) emission goes to a higher state in 2018, when the flux in high energies goes down to a lower level. Our RATAN-600 m measurements at 4.8 GHz and 8.2 GHz do not indicate any significant radio flux variation over the monitoring seasons in 2017 and 2018, nor deviate from the archival NVSS flux level. During GeV flaring times, the spectrum is very hard (Γγ∼1.7) in the GeV band and at times also very hard ((ΓX≲2) in the X-rays, similar to a high-synchrotron-peak (or even an extreme) BL Lac object, making FermiJ1544-0649 a good target for ground-based Cherenkov telescopes.

The SUrvey for Pulsars and Extragalactic Radio Bursts – IV. Discovery and polarimetry of a 12.1-s radio pulsar

ABSTRACT We report the discovery of PSR J2251−3711, a radio pulsar with a spin period of 12.1 s, the second longest currently known. Its timing parameters imply a characteristic age of 15 Myr, a surface magnetic field of 1.3 × 1013 G, and a spin-down luminosity of 2.9 × 1029 erg s−1. Its dispersion measure of 12.12(1) pc cm−3 leads to distance estimates of 0.5 and 1.3 kpc according to the NE2001 and YMW16 Galactic free electron density models, respectively. Some of its single pulses show an uninterrupted 180-deg sweep of the phase-resolved polarization position angle, with an S-shape reminiscent of the rotating vector model prediction. However, the fact that this sweep occurs at different phases from one pulse to another is remarkable and without straightforward explanation. Although PSR J2251−3711 lies in the region of the $P-\dot{P}$ parameter space occupied by the X-ray isolated neutron stars (XINS), there is no evidence for an X-ray counterpart in our Swift XRT observation; this places a 99 per cent-confidence upper bound on its unabsorbed bolometric thermal luminosity of $1.1 \times 10^{31}~(d / 1~\mathrm{kpc})^2~\mathrm{erg\, s}^{-1}$ for an assumed temperature of 85 eV, where d is the distance to the pulsar. Further observations are needed to determine whether it is a rotation-powered pulsar with a true age of at least several Myr, or a much younger object such as an XINS or a recently cooled magnetar. Extreme specimens like PSR J2251−3711 help bridge populations in the so-called neutron star zoo in an attempt to understand their origins and evolution.

Probing the circumnuclear absorbing medium of the buried AGN in NGC 1068 through NuSTAR observations

ABSTRACT We present the results of the latest NuSTAR monitoring campaign of the Compton-thick Seyfert 2 galaxy NGC 1068, composed of four ∼50 ks observations performed between 2017 July and 2018 February to search for flux and spectral variability on time-scales from 1 to 6 months. We detect one unveiling and one eclipsing event with time-scales less than 27 and 91 d, respectively, ascribed to Compton-thick material with NH = (1.8 ± 0.8) × 1024 and ≥ (2.4 ± 0.5) × 1024 cm−2 moving across our line of sight. This gas is likely located in the innermost part of the torus or even further inward, thus providing further evidence of the clumpy structure of the circumnuclear matter in this source. Taking advantage of simultaneous Swift–XRT observations, we also detected a new flaring ULX, at a distance d ∼ 30 arcsec (i.e. ∼2 kpc) from the nuclear region of NGC 1068, with a peak X-ray intrinsic luminosity of (3.0 ± 0.4) × 1040 erg s−1 in the 2–10 keV band.

Quasi-simultaneous radio and X-ray observations of Aql X-1 : probing low luminosities

ABSTRACT Aql X-1 is one of the best-studied neutron star low-mass X-ray binaries. It was previously targeted using quasi-simultaneous radio and X-ray observations during at least seven different accretion outbursts. Such observations allow us to probe the interplay between accretion inflow (X-ray) and jet outflow (radio). Thus far, these combined observations have only covered one order of magnitude in radio and X-ray luminosity range; this means that any potential radio–X-ray luminosity correlation, LR ∝ LXβ, is not well constrained (β ≈ 0.4–0.9, based on various studies) or understood. Here we present quasi-simultaneous Very Large Array and Swift-XRT observations of Aql X-1’s 2016 outburst, with which we probe one order of magnitude fainter in radio and X-ray luminosity compared to previous studies (6 × 1034 erg s−1 < LX <3 × 1035 erg s−1, i.e. the intermediate to low-luminosity regime between outburst peak and quiescence). The resulting radio non-detections indicate that Aql X-1’s radio emission decays more rapidly at low X-ray luminosities than previously assumed – at least during the 2016 outburst. Assuming similar behaviour between outbursts, and combining all available data in the hard X-ray state, this can be modelled as a steep β =$1.17^{+0.30}_{-0.21}$ power-law index or as a sharp radio cut-off at LX ≲ 5 × 1035 erg s−1 (given our deep radio upper limits at X-ray luminosities below this value). We discuss these results in the context of other similar studies.

Discovery and Early Evolution of ASASSN-19bt, the First TDE Detected by TESS

Abstract We present the discovery and early evolution of ASASSN-19bt, a tidal disruption event (TDE) discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of d ≃ 115 Mpc and the first TDE to be detected by TESS. As the TDE is located in the TESS Continuous Viewing Zone, our data set includes 30 minute cadence observations starting on 2018 July 25, and we precisely measure that the TDE begins to brighten ∼8.3 days before its discovery. Our data set also includes 18 epochs of Swift UVOT and XRT observations, 2 epochs of XMM-Newton observations, 13 spectroscopic observations, and ground data from the Las Cumbres Observatory telescope network, spanning from 32 days before peak through 37 days after peak. ASASSN-19bt thus has the most detailed pre-peak data set for any TDE. The TESS light curve indicates that the transient began to brighten on 2019 January 21.6 and that for the first 15 days, its rise was consistent with a flux ∝t 2 power-law model. The optical/UV emission is well fit by a blackbody spectral energy distribution, and ASASSN-19bt exhibits an early spike in its luminosity and temperature roughly 32 rest-frame days before peak and spanning up to 14 days, which has not been seen in other TDEs, possibly because UV observations were not triggered early enough to detect it. It peaked on 2019 March 4.9 at a luminosity of L ≃ 1.3 × 1044 erg s−1 and radiated E ≃ 3.2 × 1050 erg during the 41 day rise to peak. X-ray observations after peak indicate a softening of the hard X-ray emission prior to peak, reminiscent of the hard/soft states in X-ray binaries.