Astronomy Research

Examining the Quiescent behavior of Black Hole Binary V404 Cygni!

introduction

This page will document the astronomy research I do with Dr. Will Clarkson, which is currently ongoing. This project was conceived by myself and another student as a class project during ASTR301 (Astrophysical Concepts) in Fall 2016. The assignment was to write a research proposal to the Michigan-Dartmouth-MIT (MDM) observatory. We chose the topic of V404 Cyg because it was interesting and the timing for observations of V404 Cyg was great (See "What is V404 Cyg?" below).

In August 2017, MDM granted us telescope time with the MDM 1.4m telescope in Kitt Peak, Arizona to study V404 Cyg with the goal of explaining its behavior during quiescence. Although I was not able to travel to Arizona to observe because I was on the Geology of the National Parks course, I am still studying the data that was gathered. Part of this analysis was done in Winter 2018, and I gave a talk on V404 Cyg at the Compact Objects in Michigan VI meeting on April 13, 2018. During the summer of 2018, we received two runs of observations. I traveled to Arizona to observe during the first run, and the second run was conducted remotely from Dearborn. I am still analyzing the data and preparing a submission to a peer-reviewed journal.

what is v404 cyg?

V404 Cygni (Figure 1) is a low-mass X-Ray Binary System consisting of a main-sequence star with a mass slightly less than the mass of the Sun in orbit with a black hole of about 6 solar masses. V404 Cyg is one of the most famous Soft X-Ray Transients. SXT's spend decades in quiescence, where the donor star dominates the contribution to the optical flux, punctuated by an occasional outburst, where thermal heating causes the accretion disk surrounding the compact object (black hole) to dominate the contribution to the optical flux for a few months.

Figure 1: Artist's impression of V404 Cyg. The black hole is in the center of the object to the right (with X-ray jets emitting from it, which is typical of V404 Cyg during outburst). Surrounding the black hole is an accretion disk from the donor star, which orbits it and is the object in the left portion of this figure. (Image Credit: ESA)

V404 Cyg was discovered in 1989, when it underwent an outburst and was detected by the Ginga X-Ray satellite. During the subsequent quiescent period, it was studied extensively. These studies revealed an ellipsoidal modulation, which is a variation in brightness caused by the position of the donor star relative to the accretion disk (See Figure 4). Outside of the ellipsoidal modulation, the brightness of V404 Cyg also changes rapidly on sub-orbital timescales, which may be due to stellar flaring. One study found a statistically significant variation occurring at roughly 6-hour timescales.

The main goal of our research is to compare our data with the data obtained from previous studies after the 1989 outburst. We are particularly interested in the evolution of the ellipsoidal modulation with time, as well as the presence of sub-orbital variability.

methods and observations

V404 Cyg was observed for 9 nights at MDM in August 2017, using the 1.3-meter McGraw Hill Telescope. However, when I did the analysis, I determined that only 6 nights were of good enough quality to analyze (for now). This is fine, because 6 days is close enough to V404 Cyg's orbital period to determine the ellipsoidal modulation.

The first analysis I did was to look at each image (see Figure 2 for an example of an image) and classify it by flag: 1's were images that seemed "perfect". 2's included minor obstructions, such as cosmic rays, that shouldn't have a noticeable effect on photometry. 3's were images that included major obstructions that are generally unsuitable for photometry. Most of these images were taken when a moth was physically inside the telescope, so photometry can't be done on these images (see Figure 3). 4's were images obstructed (either by clouds or the bug) to the point where no stars were visible. We found that nearly 3 out of every 4 images were suitable for photometry (flag 1 or 2).

Photometry was performed using the AstroImageJ software package. In order for the correct magnitudes to be obtained, the flux of a contaminant star (see Figure 2) had to be subtracted from the flux of V404 Cyg itself. Python routines were used for the rest of the analysis, which included fitting the ellipsoidal modulation with SciPy least squares fitting of a double sine wave, and characterizing periodicities using Lomb-Scargle periodograms.

Figure 2: This is a typical image in the dataset (957 of 1259). This was flagged as a "2" due to the cosmic ray to the top of the image. North is up; East is right. V404 Cyg is indicated by the yellow marker, while the unresolved contaminant is indicated by the green marker. "C4" is the contaminant star used for photometry, which is the same star used in the literature.

Figure 3: image (flagged as a "4") where the bug is prominent.

indications: ellipsoidal modulation

Figure 4: Ellipsoidal modulation, which characterizes the difference in brightness due to the orientation of the disk vs. the donor. The actual magnitude data is folded on the orbital ephemeris given in the literature to produce the correct phase. The top panel of this figure shows the orientation of the system for each phase. The data are color-coded by night of observation; the first night of observation is the second series on the figure.

After we obtained our ellipsoidal modulation, we immediately thought of comparing it to the previous ellipsoidal modulations obtained in the literature. We decided to plot the years since outburst against the ellipsoidal (Figure 5). We believe our data looks the most similar to 1992, which is to be expected if the ellipsoidal modulation varies in a way that depends on how long it has been since outburst (which is a hypothesis we have that still needs much more data to be validated!)

Figure 5: ellipsoidal modulation vs. time since outburst for time following both the 1989 (literature data) and 2015 (our data) outbursts.

indications: sub-orbital variations

Before we plotted our nightly sub-orbital variations, we looked at the sub-orbital variations in the literature. They look like this:

Figure 6: sub-orbital variations from six different years from the literature (Zurita et al. 2004). Stellar flaring was reported in each of those years. Each box represents an individual night of data.

We expected our data to look something like the above figure. We were quite surprised when we found the results look like this:

Figure 7: Like Figure 6, but for our data.

It may be difficult to see on this web page, but we are confident that this behavior is not the same as the behavior observed following the 1989 outburst! We are currently determining if we have enough evidence to claim that V404 Cyg's behavior is significantly different post-2015 outburst. We have tried Lomb-Scargle periodograms, but so far we have only been able to determine the periodicity is significantly different than that of plain white noise.

Much more work needs to be done on v404 Cyg, and it will be done! In the meantime, we created a poster out of these indications and presented it at the 2018 Natural Sciences Poster Session at UM-Dearborn. I also gave a talk at the 6th annual Compact Objects in Michigan meeting at UM-Ann Arbor.

2018 OBservations

We had two separate observing sessions at MDM during 2018. The first came in late May and continued into early June. We were awarded 5 nights at the 2.4-meter telescope. I made the trip to Arizona (see the pictures below), and we were rewarded with great seeing each night: At times, the contaminant could almost be seen separately from V404 Cyg!

Figure 8: Left: three of the monitors used to control the 2.4m, from the control room. This image was taken when the telescope was pointed at V404 Cyg and data was coming in. Right: Myself with the 2.4m in the background as the dome was being opened for the night.

In August 2018, we were awarded eight nights on the 1.3m. Instead of travelling to Arizona, we decided to observe remotely from Dearborn. This allowed us to use UM-Dearborn's 0.4m telescope to take data on V404 Cyg as well as the 1.3m. However, bad luck finally caught up to us: We never got a full night's worth of data due to a combination of bad weather in Arizona and the campus telescope not working correctly. At the very least, we should have enough data for an ellipsoidal modulation (using data from both telescopes) and a couple nights may have enough coverage to search for flaring.

future work

There is still much left to do on V404 Cyg. The analysis of the data from 2018 must be refined more before we can achieve true results, but it is looking like V404 Cyg may have returned to large nightly flaring. If this is true, however, we are left with the question of whether V404 Cyg's flaring activity had ceased for a period of time after the 2015 outbursts.

We reached out to two of the authors of previous papers on V404 Cyg, and they have generously agreed to give us the data that produced what is Figure 6 on this webpage. We plan on using their data in statistical analyses to determine if our sampling was simply unlucky.

We also submitted a proposal for an observing run in 2019, and it was successful. I, however, was not a part of the observing team as I have moved on to the PhD program at Purdue. Despite this, I still plan to have a role with the analysis of the observing data and future submission of this research to a peer-reviewed scientific journal.

Google Sites

Report abuse