Normal Outbursts Research Articles

Increasing risk of glacial lake outburst flood in Sikkim, Eastern Himalaya under climate warming

The increasing risk of Glacial Lake Outburst Floods (GLOFs) in the Eastern Himalaya is exacerbated by climate change-driven glacial ice mass loss, slowdown, and increasing infrastructure projects in the high-altitude regions. To quantify the current risk of potential future disasters we update the inventory of glacial lakes in Sikkim Himalaya, identify the most potentially dangerous glacial lakes (PDGL) and model their peak discharge in different scenarios. The updated glacial lake inventory includes 232 glacial lakes (of >0.01 km2) covering a cumulative area of 22.23 ± 0.10 km2. Our GLOF susceptibility mapping of all moraine-dammed glacial lakes using an Analytic Hierarchy Process (AHP) reveals one lake as very high risk, eight as high risk, 22 as medium risk, 56 as low risk, and 18 as very low risk. Further, we apply dam break flood simulations for the seven most dangerous lakes. Results reveal highest peak discharges of 9504 m3 s−1 and 8421 m3 s−1 in extreme case scenarios from the Khanchung and South Lhonak lakes, respectively. The lowest peak discharge of 622 m3 s−1 is estimated in a normal outburst event for Yongdi lake, with every scenario at least 447 m3 s−1 discharge is reaching to Chungthang town. We find that more than 10,000 people face direct threat of GLOF with potential large-scale infrastructure damage (∼1900 settlement, 5 bridges and 2 hydropower plants). The updated glacial lake dataset, GLOF susceptibility mapping, and modeling results demonstrate the urgent need to install an early warning system and control breaching of highly dangerous lakes.

Long-term Study of the First Galactic Ultraluminous X-Ray Source Swift J0243.6+6124 Using NICER

We present the results obtained from detailed X-ray timing and spectral studies of X-ray pulsar Swift J0243.6+6124 during its giant and normal X-ray outbursts between 2017 and 2023 observed by the Neutron star Interior Composition Explorer (NICER). We focused on a timing analysis of the normal outbursts. A distinct break is found in the power density spectra of the source. The corresponding break frequency and slopes of the power laws around the break vary with luminosity, indicating a change in the accretion dynamics with the mass accretion rate. Interestingly, we detected quasiperiodic oscillations within a specific luminosity range, providing further insights into the underlying physical processes. We also studied the neutron star spin period evolution and a luminosity variation in the pulse profile during the recent 2023 outburst. The spectral analysis was conducted comprehensively for the giant and all other normal outbursts. We identified a double transition at luminosities of ≈7.5 × 1037 and 2.1 × 1038 erg s−1 in the evolution of continuum parameters like the photon index and cutoff energy with luminosity. This indicates three distinct accretion modes experienced by the source, mainly during the giant X-ray outburst. A soft blackbody component with a temperature of 0.08–0.7 keV is also detected in the spectra. The observed temperature undergoes a discontinuous transition when the pulsar evolves from a sub- to super-Eddington state. Notably, in addition to an evolving 6–7 keV iron line complex, a 1 keV emission line was observed during the super-Eddington state of the source, implying X-ray reflection from the accretion disk or outflow material.

Mini‐outburst of black hole X‐ray binary GX 339‐4 in 2004

AbstractThe mini‐outburst of black hole X‐ray binaries (BHXBs) is a re‐bright event of light curve, which is often detected in BHXBs with the low orbital period, such as XTE J1650‐500 (), GRO J0422+32 () and Swift J1753.5‐0127 (). Because of its low X‐ray luminosity, the physics properties of mini‐outburst is still unclear. GX 339‐4 is a famous BHXBs with the orbital period and present many normal outburst and failed outburst over the past decades. However, the mini‐outburst is not still detected in GX 339‐4. In this work, using the Rossi X‐ray Timing Explorer (RXTE) observations, we investigated the energy spectrum properties of the GX 339‐4 outburst in 2004 and confirmed that the outburst in 2004 is a mini‐outburst. It is very interest that the mini‐outburst has been through a whole state transition from Low/Hard state to High/Soft state. We also give a comparison between the normal outburst in 2002 and the mini‐outburst in 2004. And we find that the two outburst present a similar Hardness‐Intensity diagram and spectral properties, which suggest that the normal outburst and mini‐outburst have a similar physics. We think that the mini‐outburst of GX 339‐4 in 2004 can be well explained by the disk instability model.

A comparative study on the spectra of normal outburst and mini‐outbursts of black hole X‐ray binary XTE J1550‐564

AbstractOne normal outburst and three mini‐outbursts have been detected by Rossi X‐ray Timing Explorer (RXTE) satellite after 2000 in the well‐known black hole X‐ray binary XTE J1550‐564. In our work, we explored the energy spectral properties of the four outbursts and the three mini‐outbursts retained in the Low/Hard state. The results show that the X‐ray spectra of the three mini‐outbursts are harder than that of normal outburst and have no disk component. In the correlation analysis, the anti‐relationship of the three mini‐outbursts suggested that it may be a result of the incomplete evolutionary success of Shakura‐Sunyaev disk. The whole X‐ray spectral correlation evolution shows well the state transition from the Shakura‐Sunyaev disk to the advection‐dominated accretion flow. We think that the three mini‐outbursts may originated from a small discrete accretion event.

Photometric analysis of the dwarf nova SU UMa based on TESS

We report the photometric analysis of SU UMa based on the observations of the Transiting Exoplanet Survey Satellite (TESS). TESS has released a large amount of data, which contains the light curves of a complete superoutburst and three normal outbursts of SU UMa. Based on the observations, the evolution of superhumps during the superoutburst was analyzed. By using the O - C method, the three stages were determined, and the superhump period for each stage was calculated. The mass ratio q=0.137(1) was estimated based on the stage A superhump method. A periodic oscillation with a period of 2.17(9) days was found in the superhump minimum O - C diagram, which is related to the precession of the accretion disk. We investigated the frequency information of quasiperiodic oscillations (QPOs) in the light curves. An ultralong period QPO of 0.18 days at the time of normal outburst and two ultralong periods QPOs of 0.19 days and 0.285 days in quiescence were found. No QPO was found in the superoutburst.

Outburst behavior of the dwarf nova CG Draconis

AbstractDuring the British Astronomical Association (BAA) 2022 campaign, 27,436 photometric observations of the dwarf nova (DN) CG Draconis were made, with 106 eclipses recorded. This work summarizes the new data available and provides updated ephemeris and commentary on the observed eclipse profiles. The orbital period found is . Two types of quasi‐periodic outbursts are identified: normal outbursts, of mag amplitude, and bright, of mag. The pattern resembles superoutbursts of SU UMa‐type DNe, however, no presence of superhumps characterizing these DNe was found. Given CG Dra is located above the period gap, it may represent a new intermediary subtype between SS Cyg and SU UMa‐type stars, or provide support to superoutburst models that do not rely on eccentric accretion disks.

Spectral and Timing Properties of H 1743-322 in the “Faint” 2005 Normal Outburst

H 1743-322 is a well-known black hole X-ray binary (BH XRBs) that has been observed in several outbursts over the past. In this work, we have performed the spectral and timing analysis of H 1743-322 during the “faint” 2005 outburst for the first time with the RXTE/PCA data. In this outburst, the spectral and timing parameters (e.g., Tin, Γ, Rin, rms and QPOs, etc.) presented an obvious change and a q-like pattern was found in the Hardness Intensity Diagram (HID), which often named as the hysteresis effect of BH XRBs. The radius of the innermost stable circular orbit was constrained as RISCO∼3.50 Rg, which predicts that H 1743-322 is a lower-spin black hole. We further explored the correlation between timing and spectral properties. The relation of photon index Γ and X-ray flux, F3–25keV, presented a transition between negative and positive correlation when the X-ray luminosity, L3–25keV, is above and below a critical X-ray luminosity, LX,crit≃2.55×10−3 LEdd, which can be well explained by the Shakura-Sunyaev disk–corona model (SSD-corona) and advection-dominated accretion flow (ADAF). We also found the tight linear, negative correlation between photon index Γ and the total fractional rms. Since the amount of soft photons from the accretion disk seems invariable, an increase of the number of soft photons will dilute the variability from the harder photons. Therefore, the softer the X-ray spectra will result in the smaller total fractional rms. The above results suggested that the 2005 outburst of H 1743-322 was a normal outburst and H 1743-322 represented similar properties with other black hole X-ray binaries.

Discovery of a Short-period and Unusually Helium-deficient Dwarf Nova KSP-OT-201701a by the KMTNet Supernova Program

Abstract We present the first ever discovery of a short-period and unusually helium-deficient dwarf nova KSP-OT-201701a by the Korea Microlensing Telescope Network Supernova Program. The source shows three superoutbursts, each led by a precursor outburst, and several normal outbursts in BVI during the span of ∼2.6 yr with supercycle and normal cycle lengths of about 360 and 76 days, respectively. Spectroscopic observations near the end of a superoutburst reveal the presence of strong double-peaked H i emission lines together with weak He i emission lines. The helium-to-hydrogen intensity ratios measured by He i λ5876 and Hα lines are 0.10 ± 0.01 at a quiescent phase and 0.26 ± 0.04 at an outburst phase, similar to the ratios found in long-period dwarf novae, while significantly lower than those in helium cataclysmic variables (He CVs). Its orbital period of 51.91 ± 2.50 minutes, which is estimated based on time-series spectroscopy, is a bit shorter than the superhump period of 56.52 ± 0.19 minutes, as expected from the gravitational interaction between the eccentric disk and the secondary star. We measure its mass ratio to be 0.37 − 0.21 + 0.32 using the superhump period excess of 0.089 ± 0.053. The short orbital period, which is under the period minimum, the unusual helium deficiency, and the large mass ratio suggest that KSP-OT-201701a is a transition object evolving to an He CV from a long-period dwarf nova with an evolved secondary star.

Anomalous nature of outbursts of black hole candidate 4U 1630−472

ABSTRACT The Galactic black hole candidate (BHC) 4U 1630−472 has gone through several outbursts (13 to be particular) in the last two and a half decades starting from the RXTE era till date. Like the outbursts of other transient BHCs, the outbursts of this source show variations in duration, peak numbers, highest peak flux, etc. However, unlike other soft X-ray transients, this source showed outbursts of two types, such as normal and super. The normal outbursts of duration ∼100−200 d are observed with an average recurrence or quiescence period of ∼500 d. The super outbursts of duration ∼1.5−2.5 yr, consist of more than one normal outburst and one mega outburst. We make an effort to separate the flux contributions of the normal and mega components from the super outbursts, and try to understand the nature of evolution of both types (normal and mega) of outbursts, based on the quiescent period prior to the outbursts. Archival data of RXTE or ASM from 1996 January to 2011 June, and MAXI or GSC from 2009 August to 2020 July are used for our study. Based on the variations of various properties of both the normal and mega outbursts, we think that this BHC is a special source which may contain two companions. Two companions might be responsible for two types of outbursts.

TACOS: TESS AM CVn Outbursts Survey

ABSTRACT Using Transiting Exoplanet Survey Satellite (TESS), we are doing a systematic study of outbursting AM CVn systems to place some limits on the current outbursts models. We present the TESS light curve (LC) for 9 AM CVns showing both superoutbursts (SOs) and normal outbursts (NOs). The continuous coverage of the outbursts with TESS allows us to place stringent limits on the duration and structures of the SOs and the NOs. We present evidence that in at least some of the systems enhanced mass transfer (EMT) has to be taken into account to explain the observed LC of the SOs and rebrighthening phase after the SOs. For others, the colour evolution from simultaneous observations in g and r with ZTF differs from previously reported colour evolution of longer period AM CVns where EMT is responsible for the SO. We also find that due to the lack of sufficiently high cadence coverage the duration of many systems might have been overestimated in previous ground-based surveys. We report the SO duration for 6 AM CVns. We also found that precursors are a common feature of SOs in AM CVns and are seen in the LC of 5 of the 6 reported SOs. Finally, the 10-min and 2-min cadence LCs from TESS also allowed us to find two new candidate orbital periods of AM CVns, both of which are in reasonably good agreement with the predictions for their periods based on their past outburst histories.

A Study on the Hysteresis Effect and Spectral Evolution in the Mini-Outbursts of Black Hole X-Ray Binary XTE J1550-564

One normal outburst and three mini-outbursts have been detected by Rossi X-ray Timing Explorer satellite after 2000 in the well-known black hole X-ray binary XTE J1550-564. In this work, we explore the hysteresis effect of the four outbursts, which is a phenomenon that a similar spectral state transition occurs at different luminosity in an outburst of black hole X-ray binary. A q-like track was found in the hardness-intensity diagram of the normal outburst in 2000 but not in the three mini-outbursts that only occur in the Low/Hard state. The results demonstrate that the hysteresis effect is not apparent in the three mini-outbursts and the X-ray spectra are harder than that of the normal outburst at the same photon count rate. Furthermore, the results of the correlation analysis show that the Γ − F2−10keV correlation of mini-outburst maintain negative in the Low/Hard state with the harder spectra than that of the normal outburst at the same X-ray flux. The X-ray spectral evolution can be well-explained by the state-transition model from the Shakura–Sunyaev disk to the advection-dominated accretion flow, which implies that the three mini-outbursts of XTE J1550-564 might originate from a smaller discrete accretion event.

MASTER OT J172758.09+380021.5: a peculiar ER UMa-type dwarf nova, probably a missed nova in the recent past

A CCD photometry of the dwarf nova MASTER OT J172758.09 +380021.5 was carried out in 2019 during 134 nights. Observations covered three superoutbursts, five normal outbursts and quiescence between them. The available ASASSN and ZTF data for 2014-2020 were also examined. Spectral observations were done in 2020 when the object was in quiescence. Spectra and photometry revealed that the star is an H-rich active ER UMa-type dwarf nova with a highly variable supercycle of ~50-100 d that implies a high and variable mass-transfer rate. This object demonstrated peculiar behaviour: short-lasted superoutbursts (a week); a slow superoutburst decline and cases of rebrightenings; low frequency (from none to a few) of the normal outbursts during the supercycle. In 2019 a mean period of positive superhumps was found to be 0.05829 d during the superoutbursts. Late superhumps with a mean period of 0.057915 d which lasted about ~20 d after the end of superoutburst and were replaced by an orbital period of 0.057026 d or its orbital-negative superhump beat period were detected. An absence of eclipse in the orbital light curve and its moderate amplitude are consistent with the orbital inclination of about 40 degr found from spectroscopy. The blue peaks of the V-Ic and B-Rc of superhumps during the superoutburst coincided with minima of the light curves, while B-Rc of the late superhumps coincided with a rising branch of the light curves. We found that a low mass ratio q=0.08 could explain most of the peculiarities of this dwarf nova. The mass-transfer rate should be accordingly higher than what is expected from gravitational radiation only, this assumes the object is in a post-nova state and underwent a nova eruption relatively recently -- hundreds of years ago. This object would provide probably the first observational evidence that a nova eruption can occur even in CVs near the period minimum.

Evidence that short-period AM CVn systems are diverse in outburst behaviour

ABSTRACT We present results of our analysis of up to 15 yr of photometric data from eight AM CVn systems with orbital periods between 22.5 and 26.8 min. Our data have been collected from the GOTO, ZTF, Pan-STARRS, ASAS-SN, and Catalina all-sky surveys and amateur observations collated by the AAVSO. We find evidence that these interacting ultracompact binaries show a similar diversity of long-term optical properties as the hydrogen accreting dwarf novae. We found that AM CVn systems in the previously identified accretion disc instability region are not a homogenous group. Various members of the analysed sample exhibit behaviour reminiscent of Z Cam systems with long superoutbursts (SOs) and standstills, SU UMa systems with regular, shorter SOs, and nova-like systems that appear only in a high state. The addition of TESS full frame images of one of these systems, KL Dra, reveals the first evidence for normal outbursts appearing as a precursor to SOs in an AM CVn system. Our results will inform theoretical modelling of the outbursts of hydrogen deficient systems.

Evolution of the Quiescent Disk Surrounding a Superoutburst of the Dwarf Nova TW Virginis

Abstract In this paper, we investigate portions of the Kepler K2 Short Cadence light curve of the dwarf nova (DN) TW Vir at quiescence, using light-curve modeling. The light curve was separated into 24 sections, each with a data length of ∼0.93 days, comprising 4 sections before, and 20 after a superoutburst (SO). Due to morphological differences, the quiescent orbital modulation is classified into three types. Using a fixed disk radius and the two component stellar parameters, all 24 synthetic disk models from these sections show a consistent configuration, consisting of a disk and two hotspots: one at the vertical side of the edge of the disk and the other on the surface of the disk. Before the SO, the disk and a ringlike surface-hotspot are suddenly enhanced, triggering a precursor, and then the SO. At the end of the quiescent period following the SO and before the first normal outburst, the edge-hotspot becomes hotter, while the surface-hotspot switches into a “coolspot” with a coverage of nearly half of the disk’s surface. During quiescence, the surface-hotspot is always located at the outer part of the disk, with a constant radial width. A flat radial temperature distribution of the disk is found, and appears flatter when approaching the outburst. Like many U Gem-type DN with orbital periods of 3–5 hr, the mass transfer rate is significantly lower than the predictions of the standard/revised models of CV evolution.

Humps and Superhumps in the SU UMa-Type Dwarf Nova System IRXS J161659.5+620014

Photometric studies of the SU UMa-type dwarf nova IRXS J161659.5+620014 have been made on the 2.6-m and 1.25-m telescopes at the Crimean Astrophysical Observatory during 2017 and 2018 in the quiescent state of the system, during normal outbursts, and during superoutbursts. It is found that the interval between adjacent normal outbursts was about 11 days during 2017. Observations in the quiescent state during 2017 yielded an orbital period of 0.06888(2) days. Observations during 2018 manifested good reproducibility of the characteristics of the superoutbursts compared to those of previous years. A refined period of 0.071044(15) days was obtained for the positive superhumps in stage “C” and their color indices were determined.

Variations in the Period of Negative Superhumps in SU UMa-Type Dwarf Novae. I. Mn Dra (2012-2017)

This is a photometric study of the dwarf nova MN Dra made during 2012-2017 on nine telescopes over 152 nights. Overall, the observations covered 4 superoutbursts, 7 normal outbursts and the quiescent state. The interval between neighboring superoutbursts in 2017 was 65 days, and between neighboring normal bursts, 15 days. During the superoutbursts of 2012 and 2017, positive superhumps with a period of 0.10558(6) and 0.10500(2) days, respectively, were observed and in the quiescent state, negative superhumps with an average period of 0.095921(3) days, It is shown that the period of the negative superhumps varied cyclically between normal outbursts: sharply decreasing during an outburst and gradually increasing toward the onset of the next outburst. This feature of the variation in the period of the negative superhumps may correspond to a rapid increase in the radius of the accretion disk during the time of an outburst followed by a slow decrease, in agreement with the theory of thermal-tidal instability.

Long-Term Activity of a Little Known Dwarf Nova DT Octantis

We analyze the long-term activity of a little known dwarf nova (DN) DT Oct. We use photographic observations from the Bamberg Observatory (Southern Patrol Photographic Sky Survey), CCD data from ASAS-3 survey, and AAVSO data (both CCD and visual). We determined the recurrence times of the superoutbursts, TCso, and of the normal outbursts, TCno, to be approximately 311 d and about 30 d, respectively. Although the length of TCso evolves gradually and undergoes only small variations (increases and decreases) of the nearby or neighbouring events (on the timescale of a few epochs), the evolution of TCso displays considerably larger changes on the longer timescales. They suggest a dominant trend of a decrease of TCso on the timescale of tens of years. The possibly magnetized white dwarf does not prevent DT Oct to show the long-term activity of the SU UMa systems. We place the time evolution of the superoutbursts of DT Oct in the context of DNe of various sub-types.

FUSE and IUE Spectroscopy of the Prototype Dwarf Nova ER Ursa Majoris During Quiescence

Abstract ER Ursae Majoris is the prototype for a subset of SU UMa-type dwarf novae characterized by short cycle times between outburst, high outburst frequency, and negative superhumps. It suffers superoutbursts every 43 days, lasting 20 days, normal outbursts every 4 days, and has an outburst amplitude of 3 mag. We have carried out a far-ultraviolet (FUV) spectral analysis of ER UMa in quiescence, by fitting Far Ultraviolet Spectroscopic Explorer and International Ultraviolet Explorer spectra with model accretion disks and high-gravity photosphere models. Using the Gaia parallax distance and an orbital inclination of 50°, we find that during the brief quiescence of only four days, the accretion rate is 7.3 × 10−11 M ⊙ yr−1, with the ER UMa white dwarf contributing 55% of the FUV flux and the accretion disk contributing the remaining 45% of the flux. The white dwarf in ER UMa is markedly hotter (32,000 K) than the other white dwarfs in dwarf novae below the cataclymic variable (CV) period gap, which have typical temperatures of ∼15,000 K. For higher inclinations of 60°–75°, the accretion rates that we derive are roughly an order of magnitude higher, (1–3) × 10−10 M ⊙ yr−1.

KSP-OT-201611a: A Distant Population II Dwarf Nova Candidate Discovered by the KMTNet Supernova Program

Abstract We present a multicolor, high-cadence photometric study of a distant dwarf nova KSP-OT-201611a discovered by the Korea Microlensing Telescope Network Supernova Program. From 2016 October to 2017 May, two outbursts, which comprise a super/long outburst followed by a normal/short outburst separated by ∼91 days, were detected in the BVI bands. The shapes and amplitudes of the outbursts reveal the nature of KSP-OT-201611a to be an SU UMa- or U Gem-type dwarf nova. Color variations of periodic humps in the super/long outburst possibly indicate that KSP-OT-201611a is an SU UMa-type dwarf nova. The super and normal outbursts show distinctively different color evolutions during the outbursts due most likely to the difference in time when the cooling wave is formed in the accretion disk. The outburst peak magnitudes and the orbital period of the dwarf nova indicate that it is at a large Galactocentric distance (∼13.8 kpc) and height (∼1.7 kpc) from the Galactic plane. KSP-OT-201611a, therefore, may provide a rare opportunity to study the accretion disk process of Population II dwarf novae.

Insights from multi-wavelength observations during high and low states of non-magnetic CVs

Close binaries containing a white dwarf accreting from a disk that receives material from a late stellar companion show an interesting variety of behaviors that depend on their accretion rates and the changes in this rate. The highest accretion rates are evident during the bright states of novalikes, as well as the superoutbursts of the shortest period dwarf novae followed by normal outbursts and the Z Cam systems. While the normal outbursts and superoutbursts of dwarf novae can be understood from the standpoint of disk and tidal instabilities, the changes in rates in novalikes when they enter low brightness states and the cause of extremely high rates for the systems with orbital periods between 3 and 4 h remain elusive. This paper highlights some recent insights and continuing problems found from X-ray, UV and optical observations of these high and low states, as well as the prospects for increased understanding from the anticipated future ground and space missions.