arXiv subjects

Simone Migliari

Publications and source records attributed to Simone Migliari.

At least 19 recordsLinked to original sources

arXiv2024

Swift J1727.8-1613 has the Largest Resolved Continuous Jet Ever Seen in an X-ray Binary

Multi-wavelength polarimetry and radio observations of Swift J1727.8-1613 at the beginning of its recent 2023 outburst suggested the presence of a bright compact jet aligned in the north-south direction, which could not be confirmed without high angular resolution images. Using the Very Long Baseline Array and the Long Baseline Array, we imaged Swift J1727.8-1613, during the hard/hard-intermediate state, revealing a bright core and a large, two-sided, asymmetrical, resolved jet. The jet extends in the north-south direction, at a position angle of $-0.60\pm0.07\deg$ East of North. At 8.4 GHz, the entire resolved jet structure is $\sim110 (d/2.7\,\text{kpc})/\sin i$ AU long, with the southern approaching jet extending $\sim80 (d/2.7\,\text{kpc})/\sin i$ AU from the core, where $d$ is the distance to the source and $i$ is the inclination of the jet axis to the line of sight. These images reveal the most resolved continuous X-ray binary jet, and possibly the most physically extended continuous X-ray binary jet ever observed. Based on the brightness ratio of the approaching and receding jets, we put a lower limit on the intrinsic jet speed of $\beta\geq0.27$ and an upper limit on the jet inclination of $i\leq74\deg$. In our first observation we also detected a rapidly fading discrete jet knot $66.89\pm0.04$ mas south of the core, with a proper motion of $0.66\pm0.05$ mas hour$^{-1}$, which we interpret as the result of a downstream internal shock or a jet-ISM interaction, as opposed to a transient relativistic jet launched at the beginning of the outburst.

astro-ph.HE↗

arXiv2023

Chasing the break: Tracing the full evolution of a black hole X-ray binary jet with multi-wavelength spectral modeling

Black hole X-ray binaries (BH XRBs) are ideal targets to study the connection between accretion inflow and jet outflow. Here we present quasi-simultaneous, multi-wavelength observations of the Galactic black hole system MAXI J1820+070, throughout its 2018-2019 outburst. Our data set includes coverage from the radio through X-ray bands from 17 different instruments/telescopes, and encompasses 19 epochs over a 7 month time period, resulting in one of the most well-sampled multi-wavelength data sets of a BH XRB outburst to date. With our data, we compile and model the broad-band spectra of this source using a phenomenological model that includes emission from the jet, companion star, and accretion flow. This modeling allows us to track the evolution of the spectral break in the jet spectrum, a key observable that samples the jet launching region. We find that the spectral break location changes over at least $\approx3$ orders of magnitude in electromagnetic frequency over this period. Using these spectral break measurements, we link the full cycle of jet behavior, including the rising, quenching, and re-ignition, to the changing accretion flow properties as the source evolves through its different accretion states. Our analyses show a consistent jet behavior with other sources in similar phases of their outbursts, reinforcing that the jet quenching and recovery may be a global feature of BH XRB systems in outburst. Our results also provide valuable evidence supporting a close connection between the geometry of the inner accretion flow and the base of the jet.

astro-ph.HE↗

arXiv2023

Short Timescale Evolution of the Polarized Radio Jet during V404 Cygni's 2015 Outburst

We present a high time resolution, multi-frequency linear polarization analysis of Very Large Array (VLA) radio observations during some of the brightest radio flaring (~1 Jy) activity of the 2015 outburst of V404 Cygni. The VLA simultaneously captured the radio evolution in two bands (each with two 1 GHz base-bands), recorded at 5/7 GHz and 21/26 GHz, allowing for a broadband polarimetric analysis. Given the source's high flux densities, we were able to measure polarization on timescales of ~13 minutes, constituting one of the highest temporal resolution radio polarimetric studies of a black hole X-ray binary (BHXB) outburst to date. Across all base-bands, we detect variable, weakly linearly polarized emission (<1%) with a single, bright peak in the time-resolved polarization fraction, consistent with an origin in an evolving, dynamic jet component. We applied two independent polarimetric methods to extract the intrinsic electric vector position angles and rotation measures from the 5 and 7 GHz base-band data and detected a variable intrinsic polarization angle, indicative of a rapidly evolving local environment or a complex magnetic field geometry. Comparisons to the simultaneous, spatially-resolved observations taken with the Very Long Baseline Array at 15.6 GHz, do not show a significant connection between the jet ejections and the polarization state.

astro-ph.HE↗

arXiv2019

A rapidly-changing jet orientation in the stellar-mass black hole V404 Cygni

Powerful relativistic jets are one of the main ways in which accreting black holes provide kinetic feedback to their surroundings. Jets launched from or redirected by the accretion flow that powers them should be affected by the dynamics of the flow, which in accreting stellar-mass black holes has shown increasing evidence for precession due to frame dragging effects that occur when the black hole spin axis is misaligned with the orbital plane of its companion star. Recently, theoretical simulations have suggested that the jets can exert an additional torque on the accretion flow, although the full interplay between the dynamics of the accretion flow and the launching of the jets is not yet understood. Here we report a rapidly changing jet orientation on a timescale of minutes to hours in the black hole X-ray binary V404 Cygni, detected with very long baseline interferometry during the peak of its 2015 outburst. We show that this can be modelled as Lense-Thirring precession of a vertically-extended slim disk that arises from the super-Eddington accretion rate. Our findings suggest that the dynamics of the precessing inner accretion disk could play a role in either directly launching or redirecting the jets within the inner few hundred gravitational radii. Similar dynamics should be expected in any strongly-accreting black hole whose spin is misaligned with the inflowing gas, both affecting the observational characteristics of the jets, and distributing the black hole feedback more uniformly over the surrounding environment.

astro-ph.HE↗

arXiv2019

Prolonged sub-luminous state of the new transitional pulsar candidate CXOU J110926.4-650224

We report on a multi-wavelength study of the unclassified X-ray source CXOU J110926.4-650224 (J1109). We identified the optical counterpart as a blue star with a magnitude of $\sim$20.1 (3300-10500 $\require{mediawiki-texvc} \AA$). The optical emission was variable on timescales from hundreds to thousands of seconds. The spectrum showed prominent emission lines with variable profiles at different epochs. Simultaneous XMM-Newton and NuSTAR observations revealed a bimodal distribution of the X-ray count rates on timescales as short as tens of seconds, as well as sporadic flaring activity. The average broad-band (0.3-79 keV) spectrum was adequately described by an absorbed power law model with photon index of $\Gamma$=1.63$\pm$0.01 (at 1$\sigma$ c.l.), and the X-ray luminosity was (2.16$\pm$0.04)$\times$10$^{34}$ erg s$^{-1}$ for a distance of 4 kpc. Based on observations with different instruments, the X-ray luminosity has remained relatively steady over the past $\sim$15 years. J1109 is spatially associated with the gamma-ray source FL8Y J1109.8-6500, which was detected with Fermi at an average luminosity of (1.5$\pm$0.2)$\times$10$^{34}$ erg s$^{-1}$ (assuming the distance of J1109) over the 0.1-300 GeV energy band between 2008 and 2016. The source was undetected during ATCA radio observations that were simultaneous with NuSTAR, down to a 3$\sigma$ flux upper limit of 18 $\mu$Jy/beam (at 7.25 GHz). We show that the phenomenological properties of J1109 point to a binary transitional pulsar candidate currently in a sub-luminous accretion disk state, and that the upper limits derived for the radio emission are consistent with the expected radio luminosity for accreting neutron stars at similar X-ray luminosities.

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arXiv2018

Accretion in Strong Field Gravity with eXTP

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process.

astro-ph.HE↗

arXiv2015

Optical and Near-Infrared Spectroscopy of the Black Hole Swift J1753.5-0127

We report on a multiwavelength observational campaign of the black hole X-ray binary Swift J1753.5-0127 that consists of an ESO/X-shooter spectrum supported by contemporaneous Swift/XRT+UVOT and ATCA data. ISM absorption lines in the X-shooter spectrum allows us to determine E(B-V)=0.45+/-0.02 along the line-of-sight to the source. We also report detection of emission signatures of He II at 4686 angstrom, H alpha, and, for the first time, H I at 10906 angstrom and Paschen Beta. The double-peaked morphology of these four lines is typical of the chromosphere of a rotating accretion disk. Nonetheless, the paucity of disk features points towards a low level of irradiation in the system. This is confirmed through spectral energy distribution modeling and we find that the UVOT+X-shooter continuum mostly stems from the thermal emission of a viscous disk. We speculate that the absence of reprocessing is due to the compactness of an illumination-induced envelope that fails to reflect enough incoming hard X-ray photons back to the outer regions. The disk also marginally contributes to the Compton-dominated X-ray emission and is strongly truncated, with an inner radius about a thousand times larger than the black hole's gravitational radius. A near-infrared excess is present, and we associate it with synchrotron radiation from a compact jet. However, the measured X-ray flux is significantly higher than what can be explained by the optically thin synchrotron jet component. We discuss these findings in the framework of the radio quiet versus X-ray bright hypothesis, favoring the presence of a residual disk, predicted by evaporation models, that contributes to the X-ray emission without enhancing the radio flux.

astro-ph.HE↗

arXiv2015

The Accreting Black Hole Swift J1753.5-0127 from Radio to Hard X-Ray

(abridged) We report on multi-wavelength measurements of Swift J1753.5-0127 in the hard state at L=2.7e36 erg/s (assuming d=3 kpc) in 2014. The radio emission is optically thick synchrotron, presumably from a compact jet. We take advantage of the low extinction and model the near-IR to UV emission with a multi-temperature disk model. Assuming a BH mass of M_BH=5 Msun and a system inclination of 40 deg, the fits imply an inner radius for the disk of Rin/Rg>212 d_3 (5Msun/M_BH). The outer radius is R_out/R_g=90,000 d_3 (5Msun/M_BH), which corresponds to 6.6e10 d_3 cm, consistent with the expected size of the disk. The 0.5-240 keV spectrum measured by Swift/XRT, Suzaku, and NuSTAR is relatively well characterized by a power-law with a photon index of Gamma=1.722+/-0.003, but a significant improvement is seen when a second continuum component is added. Reflection is a possibility, but no iron line is detected, implying a low iron abundance. We are able to fit the entire SED with a multi-temperature disk component, a Comptonization component, and a broken power-law, representing the emission from the compact jet. The broken power-law cannot significantly contribute to the soft X-ray emission, and this may be related to why Swift J1753.5-0127 is an outlier in the radio/X-ray correlation. The broken power-law might dominate above 20 keV, which would constrain the break frequency to be between 2.4e10 Hz and 3.6e12 Hz. Although the fits to the full SED do not include significant thermal emission in the X-ray band, previous observations have consistently seen such a component, and we find that there is evidence at the 3.1-sigma level for a disk-blackbody component with a temperature of 150(+30)(-20) eV and an inner radius of 5-14 R_g. If this component is real, it might imply the presence of an inner accretion disk in addition to the strongly truncated (R_in>212 R_g) disk.

astro-ph.HE↗

arXiv2014

A Delayed Transition to the Hard State for 4U 1630-47 at the End of Its 2010 Outburst

Here we report on Swift and Suzaku observations near the end of an outburst from the black hole transient 4U 1630-47 and Chandra observations when the source was in quiescence. 4U 1630-47 made a transition from a soft state to the hard state ~50 d after the main outburst ended. During this unusual delay, the flux continued to drop, and one Swift measurement found the source with a soft spectrum at a 2-10 keV luminosity of L = 1.07e35 erg/s for an estimated distance of 10 kpc. While such transients usually make a transition to the hard state at L/Ledd = 0.3-3%, where Ledd is the Eddington luminosity, the 4U 1630-47 spectrum remained soft at L/Ledd = 0.008/M10% (as measured in the 2-10 keV band), where M10 is the mass of the black hole in units of 10 solar masses. An estimate of the luminosity in the broader 0.5-200 keV bandpass gives L/Ledd = 0.03/M10%, which is still an order of magnitude lower than typical. We also measured an exponential decay of the X-ray flux in the hard state with an e-folding time of 3.39+/-0.06 d, which is much less than previous measurements of 12-15 d during decays by 4U 1630-47 in the soft state. With the ~100 ks Suzaku observation, we do not see evidence for a reflection component, and the 90% confidence limits on the equivalent width of a narrow iron Kalpha emission line are <40 eV for a narrow line and <100 eV for a line of any width, which is consistent with a change of geometry (either a truncated accretion disk or a change in the location of the hard X-ray source) in the hard state. Finally, we report a 0.5-8 keV luminosity upper limit of <2e32 erg/s in quiescence, which is the lowest value measured for 4U 1630-47 to date.

astro-ph.HE↗

arXiv2013

Baryons in the relativistic jets of the stellar-mass black hole candidate 4U 1630-47

Accreting black holes are known to power relativistic jets, both in stellar-mass binary systems and at the centres of galaxies. The power carried away by the jets, and hence the feedback they provide to their surroundings, depends strongly on their composition. Jets containing a baryonic component should carry significantly more energy than electron-positron jets. While energetic considerations and circular polarisation measurements have provided conflicting circumstantial evidence for the presence or absence of baryons, the only system in which baryons have been unequivocally detected in the jets is the X-ray binary SS 433. Here we report the detection of Doppler-shifted X-ray emission lines from a more typical black hole candidate X-ray binary, 4U1630-47, coincident with the reappearance of radio emission from the jets of the source. We argue that these lines arise in a jet with velocity 0.66c, thereby establishing the presence of baryons in the jet. Such baryonic jets are more likely to be powered by the accretion disc rather than the spin of the black hole, and if the baryons can be accelerated to relativistic speeds, should be strong sources of gamma rays and neutrino emission.

astro-ph.HE↗

arXiv2012

Future Science Prospects for AMI

The Arcminute Microkelvin Imager (AMI) is a telescope specifically designed for high sensitivity measurements of low-surface-brightness features at cm-wavelength and has unique, important capabilities. It consists of two interferometer arrays operating over 13.5-18 GHz that image structures on scales of 0.5-10 arcmin with very low systematics. The Small Array (AMI-SA; ten 3.7-m antennas) couples very well to Sunyaev-Zel'dovich features from galaxy clusters and to many Galactic features. The Large Array (AMI-LA; eight 13-m antennas) has a collecting area ten times that of the AMI-SA and longer baselines, crucially allowing the removal of the effects of confusing radio point sources from regions of low surface-brightness, extended emission. Moreover AMI provides fast, deep object surveying and allows monitoring of large numbers of objects. In this White Paper we review the new science - both Galactic and extragalactic - already achieved with AMI and outline the prospects for much more.

astro-ph.IM↗

arXiv2011

The 2010 May Flaring Episode of Cygnus X-3 in Radio, X-Rays, and γ-Rays

In 2009, Cygnus X-3 (Cyg X-3) became the first microquasar to be detected in the GeV γ-ray regime, via the satellites Fermi and AGILE. The addition of this new band to the observational toolbox holds promise for building a more detailed understanding of the relativistic jets of this and other systems. We present a rich dataset of radio, hard and soft X-ray, and γ-ray observations of Cyg X-3 made during a flaring episode in 2010 May. We detect a ~3-d softening and recovery of the X-ray emission, followed almost immediately by a ~1-Jy radio flare at 15 GHz, followed by a 4.3σ γ-ray flare (E > 100 MeV) ~1.5 d later. The radio sampling is sparse, but we use archival data to argue that it is unlikely the γ-ray flare was followed by any significant unobserved radio flares. In this case, the sequencing of the observed events is difficult to explain in a model in which the γ-ray emission is due to inverse Compton scattering of the companion star's radiation field. Our observations suggest that other mechanisms may also be responsible for γ-ray emission from Cyg X-3.

astro-ph.HE↗

arXiv2010

Investigating accretion disk - radio jet coupling across the stellar mass scale

Relationships between the X-ray and radio behavior of black hole X-ray binaries during outbursts have established a fundamental coupling between the accretion disks and radio jets in these systems. We begin by reviewing the prevailing paradigm for this disk-jet coupling, also highlighting what we know about similarities and differences with neutron star and white dwarf binaries. Until recently, this paradigm had not been directly tested with dedicated high-angular resolution radio imaging over entire outbursts. Moreover, such high-resolution monitoring campaigns had not previously targetted outbursts in which the compact object was either a neutron star or a white dwarf. To address this issue, we have embarked on the Jet Acceleration and Collimation Probe Of Transient X-Ray Binaries (JACPOT XRB) project, which aims to use high angular resolution observations to compare disk-jet coupling across the stellar mass scale, with the goal of probing the importance of the depth of the gravitational potential well, the stellar surface and the stellar magnetic field, on jet formation. Our team has recently concluded its first monitoring series, including (E)VLA, VLBA, X-ray, optical, and near-infrared observations of entire outbursts of the black hole candidate H1743-322, the neutron star system Aquila X-1, and the white dwarf system SS Cyg. Here we present preliminary results from this work, largely confirming the current paradigm, but highlighting some intriguing new behavior, and suggesting a possible difference in the jet formation process between neutron star and black hole systems.

astro-ph.HE↗

arXiv2009

Truncation of the Inner Accretion Disk around a Black Hole at Low Luminosity

Most black hole binaries show large changes in X-ray luminosity caused primarily by variations in mass accretion rate. An important question for understanding black hole accretion and jet production is whether the inner edge of the accretion disk recedes at low accretion rate. Measurements of the location of the inner edge (Rin) can be made using iron emission lines that arise due to fluorescence of iron in the disk, and these indicate that Rin is very close to the black hole at high and moderate luminosities (near 1% of the Eddington luminosity, Ledd). Here, we report on X-ray observations of the black hole GX 339-4 in the hard state by Suzaku and the Rossi X-ray Timing Explorer (RXTE) that extend iron line studies to 0.14% Ledd and show that Rin increases by a factor of >27 over the value found when GX 339-4 was bright. The exact value of Rin depends on the inclination of the inner disk (i), and we derive 90% confidence limits of Rin > 35 Rg at i = 0 degrees and Rin > 175 Rg at i = 30 degrees. This provides direct evidence that the inner portion of the disk is not present at low luminosity, allowing for the possibility that the inner disk is replaced by advection- or magnetically-dominated accretion flows.

astro-ph.HE↗

arXiv2008

Broadband X-ray Spectra of GX 339-4 and the Geometry of Accreting Black Holes in the Hard State

A major question in the study of black hole binaries involves our understanding of the accretion geometry when the sources are in the hard state. In this state, the X-ray energy spectrum is dominated by a hard power-law component and radio observations indicate the presence of a steady and powerful compact jet. Although the common hard state picture is that the accretion disk is truncated, perhaps at hundreds of gravitational radii from the black hole, recent results for the recurrent transient GX 339-4 by Miller and co-workers show evidence for optically thick material very close to the black hole's innermost stable circular orbit. That work focused on an observation of GX 339-4 at a luminosity of about 5% of the Eddington limit and used parameters from a relativistic reflection model and the presence of a soft, thermal component as diagnostics. In this work, we use similar diagnostics, but extend the study to lower luminosities (2.3% and 0.8% Ledd) using Swift and RXTE observations of GX 339-4. We detect a thermal component with an inner disk temperature of ~0.2 keV at 2.3% Ledd. We detect broad features due to iron Kalpha that are likely related to reflection of hard X-rays off the optically thick material. If these features are broadened by relativistic effects, they indicate that optically thick material resides within 10 Rg down to 0.8% Ledd, and the measurements are consistent with the inner radius of the disk remaining at ~4 Rg down to this level. However, we also discuss an alternative model for the broadening, and we note that the evolution of the thermal component is not entirely consistent with the constant inner radius interpretation. Finally, we discuss the results in terms of recent theoretical work on the possibility that material may condense to maintain an inner optically thick disk.

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arXiv2006

Jet-dominated advective systems: radio and X-ray luminosity dependence on the accretion rate

We present a novel method to measure the accretion rate of radio emitting X-ray binaries (XRBs) and active galactic nuclei (AGN) independently of the X-ray luminosity. The radio emission of the jet is used as a tracer for the accretion rate and is normalised using sources of known accretion rates: island state neutron stars and efficiently radiating black holes close to a state transition. We show that the radio power in black holes and neutron stars is comparable for a given mass accretion rate and verify empirically the assumed analytic scaling of the radio luminosity with accretion rate. As our accretion measure is independent of the X-ray luminosities, we can search for radiatively inefficient accretion in black holes by comparing the X-ray luminosities with the accretion rate in XRBs and AGN. While the X-ray luminosity of efficiently radiating objects scales linearly with accretion rate, the scaling of hard state black holes is quadratical, in agreement with theoretical models. We show that the turnover from the inefficient quadratic scaling to the linear scaling has to occur at accretion rates of 1-10 % Eddington both in XRBs and AGN. The comparison of both accretion states supports the idea that in a black hole in the hard state some accretion power is advected into the black hole while the jet power exceeds the X-ray luminosity: these are therefore jet-dominated advective systems.

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arXiv2003

Evidence of a decrease of kHz QPO peak separation towards low frequencies in 4U 1728-34 (GX 354-0)

We have produced the colour-colour diagram of all the observations of 4U 1728-34 available in the Rossi X-ray Timing Explorer public archive (from 1996 to 2002) and found observations filling in a previously reported 'gap' between the island and the banana X-ray states. We have made timing analysis of these gap observations and found, in one observation, two simultaneous kHz quasi-periodic oscillations (QPOs). The timing parameters of these kHz QPOs fit in the overall trend of the source. The 'lower' kHz QPO has a centroid frequency of ~308 Hz. This is the lowest 'lower' kHz QPO frequency ever observed in 4U 1728-34. The peak frequency separation between the 'upper' and the 'lower' kHz QPO is 274+/-11 Hz, significantly smaller than the constant value of ~350 Hz found when the 'lower' kHz QPO frequency is between ~500 and 800 Hz. This is the first indication in this source for a significant decrease of kHz QPO peak separation towards low frequencies. We briefly compare the result to theoretical models for kHz QPO production.

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arXiv2003

Disc-jet coupling in an atoll-type neutron star X-ray binary: 4U 1728-34 (GX 354-0)

We have analysed 12 simultaneous radio (VLA) and X-ray (RXTE) observations of the atoll-type X-ray binary 4U 1728-34, performed in two blocks in 2000 and 2001. We have found that the strongest and most variable emission seems to be associated with repeated transitions between hard (island) and softer (lower banana) X-ray states, while weaker, persistent radio emission is observed when the source is steadily in the hard X-ray state. There is a significant positive ranking correlation between the radio flux density at 8.46 GHz and the 2-10 keV X-ray flux. Moreover, significant positive ranking correlations between radio flux density and X-ray timing features (i.e. break and low-frequency Lorentzian frequencies) have been found. These correlations represent the first evidence for a coupling between disc and jet in an atoll-type X-ray binary. Furthermore, drawing an analogy between the hard (island) state and the low/hard state of black hole binaries, we confirm previous findings that accreting neutron stars are a factor of ~30 less `radio loud' than black holes.

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