Optical counterparts of ULXs in NGC 1672 Allak, S; Akyuz, A; Sonbas, E ...
Monthly notices of the Royal Astronomical Society,
08/2022, Letnik:
515, Številka:
3
Journal Article
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ABSTRACT
In this work, we deploy archival data from Hubble Space Telescope, Chandra, XMM–Newton, and Swift-XRT, to probe the nature of nine candidate ultraluminous X-ray sources (ULXs) in NGC 1672. ...Specifically, our study focuses on using the precise source positions obtained via improved astrometry based on Chandra and Hubble Space Telescope observations to search for and identify potential optical counterparts for these ULXs. Unique optical counterparts are identified for two of the ULX candidates, i.e. X2 and X6; for three of the candidates, i.e. X1, X5, and X7, we found two potential counterparts for each source within the respective error radii. No optical counterparts were found for the remaining four sources. The spectral energy distribution of X2 is fitted to a blackbody spectrum with a temperature of ∼104 K and the spectral class of the source is determined to be B7−A3, a supergiant donor star. We used colour–magnitude diagrams to investigate ages of the counterparts. Of all the sources studied, X9 exhibits the most variability whereby the X-ray flux varies by a factor of ∼50 over a time period spanning 2004–2019, and also traces a partial q-curve-like feature in the hardness–intensity diagram, hinting at possible spectral transitions.
ABSTRACT
The results of a temporal analysis of observations for a sample of nine low-mass X-ray binaries (LMXBs) are presented. Of these sources, five host a neutron star (NS) primary (4U1608−52, Aql ...X-1, 4U1705−44, GX17+2-, and Cyg X-2), and four host a black hole (BH) (GX339-4, XTE J1859+226, H1743−322, and MAXI J1659−152). The NS group includes three Atolls and two Z-type sources. We utilized archival Proportional Counter Array (PCA)/RXTE data to construct high-resolution light-curves. A wavelet transform of the light curves is deployed to extract a minimal time-scale (MTS) associated with the spectral state of the sources. The MTS, together with the fractional root-mean-square (rms) and hardness ratios, is used to construct RMS–MTS and hardness–MTS diagrams that enable a direct comparison of the evolution of spectral transitions in the target sources. Observations with high fractional rms and high hardness cluster in a broad region occupied jointly by BH and NS sources. For low fractional rms observations, the Atolls exhibit large MTS whereas Z-type sources exhibit small MTS. This new feature raises the possibility of discriminating between these two types of sources. Moreover, in the hardness–MTS plane, BH sources are the sole occupiers of the low-hardness and small-MTS domain thus potentially signalling a unique property for distinguishing BH and NS hosts in LMXBs.
Abstract
We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image, ...and Neutron Star Interior Composition Explorer Mission. This energetic GRB was located relatively nearby (
z
= 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (
b
= 4.°3) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography, we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (≳10 kpc). We present analysis of the light curves and spectra at X-ray and UV–optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at
T
0
+ 4.5 ks than that from any previous GRB observed by Swift. In its rest frame, GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10
4
long GRBs were as energetic as GRB 221009A; such a large
E
γ
,iso
implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ≲1 per 1000 yr—making this a truly remarkable opportunity unlikely to be repeated in our lifetime.
ABSTRACT
Black hole transients are known to undergo spectral transitions that form q-shaped tracks on a hardness intensity diagram. In this work, we use the archival Rossi X-ray Timing Explorer data ...to extract a characteristic minimal time-scale for the spectral states in GX 339−4 for the 2002–2003 and 2010 outbursts. We use the extracted time-scale to construct an intensity variability diagram for each outburst. This new diagram is comparable to the traditional hardness intensity diagram and offers the potential for probing the underlying dynamics associated with the evolution of the relevant emission regions in black hole transients. We confirm this possibility by connecting the minimal time-scale with the inner disc radius, Rin (estimated from spectral fits), and demonstrate a positive correlation between these variables as the system evolves through its spectral transitions. Furthermore, we probe the relation between the minimal time-scale and the break frequencies extracted from the power spectral densities. Lastly, we examine a possible link between the extracted time-scale and a traditional measure of variability, i.e. the root mean square, determined directly from the power spectra.
ABSTRACT
We report the results of new transit observations for the three hot Jupiter-like planets, HATP-36b, HATP-56b, and WASP-52b, respectively. Transit timing variations (TTVs) are presented for ...these systems based on observations that span the period 2016–2020. The data were collected with the 0.6-m telescope at Adiyaman University (ADYU60, Turkey) and the 1.0 m telescope at TÜBİTAK National Observatory (TUG, Turkey). Global fits were performed to the combined light curves for each system along with the corresponding radial velocity (RV) data taken from the literature. The extracted parameters (for all three systems) are found to be consistent with the values from previous studies. Through fits to the combined mid-transit times data from our observations and the data available in the literature, an updated linear ephemeris is obtained for each system. Although a number of potential outliers are noted in the respective O-C diagrams, the majority of the data are consistent within the 3σ confidence level implying a lack of convincing evidence for the existence of additional objects in the systems studied.
ABSTRACT
Archival XMM–Newton, Chandra, and Hubble Space Telescope(HST) data have been used to study the X-ray and optical properties of two candidate ultraluminous X-ray sources in NGC 4536. In order ...to search for potential optical counterparts, relative astrometry between Chandra and HST was improved, and as a result, optical counterparts were detected for both X-ray sources. To complement our findings (based on the archival data), ground-based optical spectra of the counterparts were obtained with the 6-m BTA (Big Telescope Alt-azimuth) Telescope located at the Special Astrophysical Observatory. The calculated redshift (z = 0.4391 ± 0.0010) for one of the sources (X-3) indicates that the source is, in fact, a background active galactic nucleus. Two possible optical counterparts (s1 and s2) were found for X-2. Whether s1 is point-like or an extended source is unclear: If it is point-like and the emission is dominated by the donor, its spectral type indicates O–B star. The second source (s2) is point-like and is consistent with the colours and absolute magnitudes of a red supergiant.
ABSTRACT
We present the results of a temporal and spectral analysis of the transient source ULX-4 in the galaxy M51. The data used were drawn from Chandra, XMM–Newton, and Swift-XRT archives, ...spanning the years 2000–2019.. The X-ray flux of the source is seen to vary by two orders of magnitudes within a month but a short-term variability was not observed over the time intervals of 100–2000 s in the 0.3–10 keV energy band. We find some evidence for the existence of bi-modality feature in the flux distribution of ULX-4. We identified two optical sources as possible counterparts within an error radius of 0${_{.}^{\prime\prime}}$18 at 95 ${{\ \rm per\ cent}}$ confidence level for ULX-4 based on the archival Hubble Space Telescope (HST)/ACS and HST/WFC3 data. Blackbody fits of the spectral energy distributions (SEDs) indicate the spectral type to be B-type stars. One of these counterparts exhibits a low-amplitude optical periodicity of 264 ± 37 d in the F606W filter; if we assume this apparent periodicity is associated with the orbital motion of the donor, then it is more likely that the donor is a red supergiant satisfying the long periodicity and accretion via Roche lobe overflow. Consequently, the SED would then have to be interpreted as a superposition of emissions from a cold donor and a hot flow component, most likely from an accretion disc. If, on the other hand, the periodicity is superorbital in nature i.e. due to possible interactions of the compact object with a circumstellar disc, the donor could then be a Be/X star hosting a neutron star.
ABSTRACT
Similar to black hole X-ray binary transients, hysteresis-like state transitions are also seen in some neutron-star X-ray binaries. Using a method based on wavelets and light curves ...constructed from archival Rossi X-ray Timing Explorer observations, we extract a minimal timescale over the complete range of transitions for 4U 1608-52 during the 2002 and 2007 outbursts and the 1999 and 2000 outbursts for Aql X-1. We present evidence for a strong positive correlation between this minimal timescale and a similar timescale extracted from the corresponding power spectra of these sources.
A recent study of a small sample of X-ray binaries (XRBs) suggests a significant softening of spectra of neutron star (NS) binaries as compared to black hole (BH) binaries in the luminosity range ...1034-1037 erg s−1. This softening is quantified as an anticorrelation between the spectral index and the 0.5-10 keV X-ray luminosity. We extend the study to significantly lower luminosities (i.e., ∼a few × 1030 erg s−1) for a larger sample of XRBs. We find evidence for a significant anticorrelation between the spectral index and the luminosity for a group of NS binaries in the luminosity range 1032-1033 erg s−1. Our analysis suggests a steep slope for the correlation i.e., −2.12 0.63. In contrast, BH binaries do not exhibit the same behavior. We examine the possible dichotomy between NS and BH binaries in terms of a Comptonization model that assumes a feedback mechanism between an optically thin hot corona and an optically thick cool source of soft photons. We gauge the NS-BH dichotomy by comparing the extracted corona temperatures, Compton-y parameters, and the Comptonization amplification factors: the mean temperature of the NS group is found to be significantly lower than the equivalent temperature for the BH group. The extracted Compton-y parameters and the amplification factors follow the theoretically predicted relation with the spectral index.
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